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15 Dec 2000

Volume 113, Issue 23, pp. 10429-10822

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back to top Theoretical Methods and Algorithms

High order finite difference algorithms for solving the Schrödinger equation in molecular dynamics. II. Periodic variables

Raul Guantes and Stavros C. Farantos

J. Chem. Phys. 113, 10429 (2000); http://dx.doi.org/10.1063/1.1324004 (9 pages) | Cited 9 times

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Variable high order finite difference methods are applied to calculate the action of molecular Hamiltonians on the wave function using centered equi-spaced stencils, mixed centered and one-sided stencils, and periodic Chebyshev and Legendre grids for the angular variables. Results from one-dimensional model Hamiltonians and the three-dimensional spectroscopic potential of SO2 demonstrate that as the order of finite difference approximations of the derivatives increases the accuracy of pseudospectral methods is approached in a regular manner. The high order limit of finite differences to Fourier and general orthogonal polynomial discrete variable representation methods is analytically and numerically investigated. © 2000 American Institute of Physics.
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31.15.xf Finite-difference schemes
03.65.Ge Solutions of wave equations: bound states
34.20.Gj Intermolecular and atom-molecule potentials and forces
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
02.70.Bf Finite-difference methods

Concerted electron and proton transfer: Transition from nonadiabatic to adiabatic proton tunneling

Yuri Georgievskii and Alexei A. Stuchebrukhov

J. Chem. Phys. 113, 10438 (2000); http://dx.doi.org/10.1063/1.1323723 (13 pages) | Cited 21 times

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A concerted electron–proton transfer reaction is discussed, in which proton tunneling occurs simultaneously with electronic transition. It is assumed that the potential in which the proton moves is formed by two electronic states, which in the absence of their interaction would cross in the region between the two minima of the proton adiabatic potential. The proton tunneling between the two wells is, therefore, coupled to a switch between the two electronic states. The later occurs only when the proton is in the tunneling region under the barrier. A simple analytical expression for the tunneling matrix element TDA is derived, which is uniformly correct for small and large values of the electronic coupling. For small electronic coupling our expression coincides with that obtained in the nonadiabatic theory of proton-coupled electron transfer reactions. For large electronic coupling the expression is reduced to that obtained in the Born–Oppenheimer approximation. The transition from nonadiabatic to adiabatic tunneling is governed by the magnitude of the Landau–Zener parameter defined for the tunneling process. The obtained result is discussed in the context of the proton tunneling time. © 2000 American Institute of Physics.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)

Scaling reduction of the perturbative triples correction (T) to coupled cluster theory via Laplace transform formalism

Pere Constans, Philippe Y. Ayala, and Gustavo E. Scuseria

J. Chem. Phys. 113, 10451 (2000); http://dx.doi.org/10.1063/1.1324989 (8 pages) | Cited 25 times

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A reformulation of the perturbative triples correction to coupled cluster singles and doubles (CCSD) based on the numerical Laplace transform of the energy denominator is presented. Rearranged equations reduce the O(N7) canonical scaling to O(N6), where N is a size measure of the electronic system. Two to three quadrature points is adequate for chemical predictions. The Laplace ansatz permits simple, noniterative expressions in noncanonical orbital representations. Furthermore, substituting canonical by generalized CCSD natural orbitals, the Laplace ansatz exhibits scaling close to O(N5), while retaining accuracy and providing crossover with respect to canonical triples for small size systems. A developing atomic orbital formulation is also introduced. © 2000 American Institute of Physics.
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31.15.bw Coupled-cluster theory
02.30.Uu Integral transforms
02.30.Vv Operational calculus
31.15.xp Perturbation theory

Ewald sums for Yukawa potentials

Gwenaël Salin and Jean-Michel Caillol

J. Chem. Phys. 113, 10459 (2000); http://dx.doi.org/10.1063/1.1326477 (5 pages) | Cited 21 times

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The numerical simulation of systems involving Yukawa interaction y(r) = exp(−αr)/r (e.g., colloids, dusty plasmas,…) needs some caution in the case where the potential cannot be neglected on the boundaries of the cubic simulation cell [i.e., when y(L/2)−L side of the cube — is larger than the absolute uncertainties wanted for the thermal average of the energy]. In that case the usual minimum image convention fails and it is necessary to introduce Ewald sums similar to those used for Coulomb systems. In this study we derive the expression of the Ewald sums associated to Yukawa interactions and discuss the numerical errors induced by their truncation. © 2000 American Institute of Physics.
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82.70.Dd Colloids
52.27.Lw Dusty or complex plasmas; plasma crystals
52.65.-y Plasma simulation

Optimal charge-shaping functions for the particle–particle—particle–mesh (P3M) method for computing electrostatic interactions in molecular simulations

Philippe H. Hünenberger

J. Chem. Phys. 113, 10464 (2000); http://dx.doi.org/10.1063/1.1324713 (13 pages) | Cited 30 times

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The application of the particle–particle—particle–mesh (P3M) method for computing electrostatic interactions in molecular simulations relies on the use of a charge-shaping function to split the potential into two contributions, evaluated in real and reciprocal space, respectively. Although the charge-shaping function is traditionally taken to be a Gaussian, many other choices are possible. In the present study, we investigate the accuracy of the P3M method employing, as charge-shaping functions, polynomials truncated to a finite spacial range (TP functions). We first discuss and test analytical estimates of the P3M root-mean-square force error for both types of shaping functions. These estimates are then used to find the optimal values of the free parameters defining the two types of charge-shaping function (width of the Gaussian or coefficients of the TP function). Finally, we compare the accuracy properties of these optimized functions, using both analytical estimates and numerical results for a model ionic system. It is concluded that the use of specific TP functions instead of the traditional Gaussian function leads to improvements in terms of computational speed, simplicity of use, and accuracy of results. © 2000 American Institute of Physics.
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34.20.Gj Intermolecular and atom-molecule potentials and forces
02.10.De Algebraic structures and number theory

On the role of coherence in the transition from kinetics to dynamics: Theory and application to femtosecond unimolecular reactions

Klaus B. Møller, Niels E. Henriksen, and Ahmed H. Zewail

J. Chem. Phys. 113, 10477 (2000); http://dx.doi.org/10.1063/1.1323729 (9 pages) | Cited 13 times

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We consider the relation between observed pump–probe signals in the femtosecond regime and the kinetics of unimolecular reactions, that is, the exponential decay of reactants and the exponential rise of the product population, respectively. It is shown that the signals cannot be fully accounted for within standard approaches of unimolecular decay, conventionally used in the past, since interference effects between the quasi-bound vibrational states within the bandwidth of the pump laser cannot be neglected. When these effects are included, all features of the signals can be accounted for. We apply this theoretical treatment of coherent interference to examine the dynamics and kinetics of the quasi-bound transition configurations, and relate them to the decay rates of individual quasi-bound resonance states. The signals show multi-exponential behavior, reflecting the different decay rates of the resonance states, with an average rate constant (within the bandwidth of the pump laser) which can be extracted directly from the signals. The persistence of coherence is evident in the observed signals. The predissociation of NaI is used as a prototype for numerical illustration. © 2000 American Institute of Physics.
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82.30.-b Specific chemical reactions; reaction mechanisms
82.53.-k Femtochemistry
82.20.Pm Rate constants, reaction cross sections, and activation energies
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.50.Df Potential energy surfaces for excited electronic states

Density-functional study of intramolecular ferromagnetic interaction through m-phenylene coupling unit (II): Examination of functional dependence

Masaki Mitani, Daisuke Yamaki, Yu Takano, Yasutaka Kitagawa, Yasunori Yoshioka, and Kizashi Yamaguchi

J. Chem. Phys. 113, 10486 (2000); http://dx.doi.org/10.1063/1.1290008 (19 pages) | Cited 24 times

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As a first step toward examination of ferromagnetic polymers and dendrimers by ab initio crystal orbital methods, we elucidated candidates for monomer units with the high-spin ground states in the previous study of Part I [J. Chem. Phys. 113, 4035 (2000)] by employing density-functional (DFT) methods using Becke’s and Becke’s three parameter exchanges with Lee–Yang–Parr correlation or Hartree–Fock (HF) molecular orbital and post HF approximations. However, it was found that further computations applying other DFT functionals should be carried out to clarify the level of approximations which appropriately describe the electronic structures of magnetic molecules. In this part II, we present details of numerical results concerning magnetic properties and electronic structures for m-phenylene molecules with three neutral and one cation radicals by spin-polarized density functional methods using variety of local and nonlocal functionals and unrestricted molecular orbital methods including Møller–Plesset and coupled-cluster (CC) correlation corrections. The dependence of total, exchange and correlation energies, and spin densities on various approximated functionals is investigated thoroughly. The effective exchange integrals in the Heisenberg model are calculated by local and nonlocal DFT methods, and they are compared with those of complete active space (CAS) CI, CASSCF, and CASPT2. It is concluded that nonlocal DFT with density-gradient corrections can be used as a practical alternative to UCCSD(T) and CASPT2. The broken-symmetry Unrestricted Hartree–Fock (UHF) and DFT calculations of m-phenylene polyradicals with polar substituents are carried out to elucidate roles of superexchange interactions arising from the significant mixing of charge-transfer (CT) configurations. The resonance of covalent structures with CT or zwitterionic structures entails antiferromagnetic exchange interactions even in polyradicals with m-phenylene bridges; for example, substituted nitroxide polyradicals. Stable ferromagnetic polymers and dendrimers are designed on the basis of the theoretical grounds. © 2000 American Institute of Physics.
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75.30.Et Exchange and superexchange interactions
75.50.Dd Nonmetallic ferromagnetic materials
71.20.Rv Polymers and organic compounds
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
75.50.Xx Molecular magnets
75.10.Jm Quantized spin models, including quantum spin frustration
71.70.Gm Exchange interactions
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons

Quantitative harmonization of the three molecular orbital, valence bond, and broken symmetry approaches to the exchange coupling constant: Corrections and discussion

Jean-Marie Mouesca

J. Chem. Phys. 113, 10505 (2000); http://dx.doi.org/10.1063/1.1323262 (7 pages) | Cited 14 times

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Three current methods, used to evaluate exchange coupling constants in molecular magnetism, i.e., the molecular orbital (MO) model [Hay et al., J. Am. Chem. Soc. 94, 4884 (1975)], the valence bond (VB) model [Kahn and Briat, J. Chem. Soc. Trans. II 72, 268 (1976)], and the broken symmetry (BS) model [Noodleman, J. Chem. Phys. 74, 5737 (1981)], have been revisited. In effect, the three published antiferromagnetic contributions seem mutually inconsistent, as far as their magnitudes are concerned. As it turns out, the VB term −2ΔS, where Δ is the singly occupied MO gap in the triplet state and S the (natural) magnetic orbital overlap, is shown to be overestimated by a factor of 2 (the VB ferromagnetic term, supposedly small, is actually of the order of ΔS>0). Moreover, Noodleman’s explicit condition derived from the variational optimization of the BS state energy results in fact from the implicit neglect of the VB ionic contribution right from the start of his methodology. Alternative (both rigorous and approximate) expressions are proposed in both VB and BS cases. The MO approach, although not being the best physically appropriate for the study of weakly interacting monomers (i.e., defined by S2≪1), is left untouched at this level of the theory. © 2000 American Institute of Physics.
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31.15.xw Valence bond calculations

Critical assessment of the performance of the semiempirical divide and conquer method for single point calculations and geometry optimizations of large chemical systems

Arjan van der Vaart, Dimas Suárez, and Kenneth M. Merz

J. Chem. Phys. 113, 10512 (2000); http://dx.doi.org/10.1063/1.1323257 (12 pages) | Cited 13 times

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We present a detailed analysis of the performance of the semiempirical divide and conquer method as compared with standard semiempirical MO calculations. The influence of different subsetting schemes involving dual buffer regions on the magnitude of the errors in energies and computational cost of the calculations are discussed. In addition, the results of geometry optimizations on several protein systems (453 to 4088 atoms) driven by a quasi-Newton algorithm are also presented. These results indicate that the divide and conquer approach gives reliable energies and gradients and suggest that protein geometry optimization using semiempirical methods can be routinely feasible using current computational resources. © 2000 American Institute of Physics.
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31.15.bu Semi-empirical and empirical calculations (differential overlap, Hückel, PPP methods, etc.)
87.15.B- Structure of biomolecules

Rovibrational Hamiltonians for general polyatomic molecules in spherical polar parametrization. III. Global vs local axis system and angular coordinates

Mirjana Mladenović

J. Chem. Phys. 113, 10524 (2000); http://dx.doi.org/10.1063/1.1319876 (11 pages) | Cited 12 times

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We study different parametrizations of the angular space of polyatomic molecules for an orthogonal description of the molecular geometry. Kinetic energy operators for pentatomic molecules, given by compact and computationally useful forms in a global and a local formulation of the axis system, are compared and discussed. A new decomposition of math for sequentially bonded pentatomic molecules in conjunction with a basis of Wigner and associated Legendre functions provides kinetic energy matrix elements which are free of singularities. Practical problems caused by an unusual volume element for a description involving only intervector (bending) angles are addressed. The corresponding rovibrational kinetic energy operators are derived for the two-vector body-fixed formulation. © 2000 American Institute of Physics.
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33.20.Vq Vibration-rotation analysis
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

First overtone helium nanodroplet isolation spectroscopy of molecules bearing the acetylenic CH chromophore

C. Callegari, A. Conjusteau, I. Reinhard, K. K. Lehmann, and G. Scoles

J. Chem. Phys. 113, 10535 (2000); http://dx.doi.org/10.1063/1.1324003 (16 pages) | Cited 45 times

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High-resolution helium nanodroplet isolation spectra of the first overtone (2ν1) of the acetylenic stretch of several substituted acetylenes (RC ≡ C–H) at T = 0.38 K, have been observed for the first time. A tunable 1.5 μm laser is coupled, using a power buildup cavity, to a beam of He droplets seeded with the molecule to be studied. Absorption spectra are recorded by monitoring the beam depletion as a function of laser frequency with a thermal detector. The spectra of hydrogen cyanide (HCN), monodeuteroacetylene (DCCH), cyanoacetylene (NCCCH), propyne (CH3CCH), trifluoropropyne (CF3CCH), 3,3-dimethylbutyne ((CH3)3CCCH), and trimethylsilylacetylene ((CH3)3SiCCH) have been recorded. Due to the superfluid nature of the droplet, rotational resolution is achieved despite the presence of some solvent-induced broadening. The spectroscopic constants have been extracted by means of spectral simulations. The resulting rotational constants are smaller than for the bare molecule by a factor which depends on the molecule nonsphericity and its gas-phase moment of inertia. The linewidths are found to be at least twice as large as those of the corresponding fundamental (ν1) transitions observed in a helium droplet by Nauta et al. [Faraday Discuss. Chem. Soc. 113, 261 (1999) and references therein]. The helium-induced spectral shifts are found to be very small, but cannot be easily rationalized. © 2000 American Institute of Physics.
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33.20.Ea Infrared spectra
33.70.Jg Line and band widths, shapes, and shifts

Energy disposal in the two-photon laser-assisted reaction in xenon and chlorine gas mixtures

J. Kohel and J. W. Keto

J. Chem. Phys. 113, 10551 (2000); http://dx.doi.org/10.1063/1.1324976 (9 pages) | Cited 1 time

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The two-photon laser assisted reaction (LAR) in low pressure xenon and chlorine gas mixtures has been studied over a broad range of excitation wavelengths in order to characterize the role of the entrance channel in determining the vibrational state distribution of the reaction products. We measure a high degree of vibrational excitation in the XeCl∗ product, confirming observations from previous studies of the LAR of Xe+Cl2 collision pairs [Ku et al., J. Phys. Chem. 87, 2989 (1983)], and in distinct contrast with the vibrationally cold excimer observed following laser excitation in molecular beam experiments [Boivineau et al., Chem. Phys. Lett. 128, 528 (1986)]. The mean vibrational energy in the XeCl excimer depends strongly on laser wavelength, increasing with decreasing wavelength. Moreover, an increasing fraction of the available energy from the reaction is observed as vibration in the XeCl∗ product as the laser is tuned toward shorter wavelengths. The reaction outcomes are interpreted in terms of a selectivity of initial conditions on the ionic potential surface that mediates the reactive collision. © 2000 American Institute of Physics.
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82.50.-m Photochemistry
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
32.80.Wr Other multiphoton processes
33.80.Wz Other multiphoton processes

Adiabatic and vertical ionization energies of 1,4-diazabicyclo[2,2,2]-octane measured by zero electron kinetic energy spectroscopy and Rydberg extrapolation

Mark J. Watkins and Martin C. R. Cockett

J. Chem. Phys. 113, 10560 (2000); http://dx.doi.org/10.1063/1.1328379 (12 pages) | Cited 13 times

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We have used a combination of high-n Rydberg spectroscopy and zero electron kinetic energy (ZEKE) spectroscopy to measure the adiabatic and vertical ionization energies of 1,4-diazabicyclo[2,2,2]-octane to high precision. A comparison of the two methods revealed that where extended and unperturbed Rydberg series can be observed, the precision with which the ionization energy of a molecule can be measured is up to six times better via Rydberg extrapolation than with high resolution ZEKE spectroscopy. The difference in precision derives, not from any inherent shortcoming in the resolution achievable with ZEKE spectroscopy, but from errors that arise in evaluating the field ionization redshift. Accurate vertical ionization energies of 59 049.0±0.1 and 58 889.9±0.1 cm−1 were obtained for ionization to the 24+1 and 25+1 vibrational levels in the ion. A value for the adiabatic ionization energy of 58 033.3±0.3 cm−1 has been measured by ZEKE spectroscopy for the first time. The precision with which we have been able to account for the field ionization redshift has been improved by calibrating the ZEKE spectrum against the ν25 Rydberg spectrum. The ZEKE spectra presented here were recorded using a double inverted pulse sequence with field strengths as low as 140 mV cm−1. The result was exceptionally well-resolved spectra revealing the rotational contour of each ZEKE band. For the 24+1 and 25+1 bands the spectra revealed a strong central Q branch, with weaker P and R branches, consistent with a propensity for angular momentum transfer to the Rydberg electron rather than within the core. In contrast to what is commonly observed in ZEKE spectroscopy, where rotational autoionization often results in branches associated with negative ΔJ appearing with enhanced intensity, the R branch appears with significantly enhanced intensity compared to the very weak P branch. © 2000 American Institute of Physics.
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33.60.+q Photoelectron spectra
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.40.+f Multiple resonances (including double and higher-order resonance processes, such as double nuclear magnetic resonance, electron double resonance, and microwave optical double resonance)
33.80.Eh Autoionization, photoionization, and photodetachment

Third-order derivatives of the dipole moment function for the ozone molecule

O. N. Sulakshina, Yu. G. Borkov, Vl. G. Tyuterev, and A. Barbe

J. Chem. Phys. 113, 10572 (2000); http://dx.doi.org/10.1063/1.1290614 (11 pages) | Cited 7 times

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The third-order contributions to the dipole moment function of ozone has been evaluated from available experimental values of the transition moment parameters for the second overtone and ternary combination vibration–rotation bands. The calculations are based on the formalism of effective dipole moment operators. The purely vibrational part of the transformed transition moment operators for three-quanta bands is presented in the form suitable for an iterative programming. It allows one to determine the values and to make the optimal choice of signs of the third derivatives of the dipole moment function using the transition moment parameters deduced from experimental spectra. The estimation of the errors have been done by an error propagation of uncertainties in anharmonicity parameters of the potential function and previously determined first- and secondorder dipole moment derivatives. © 2000 American Institute of Physics.
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33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.70.Fd Absolute and relative line and band intensities

Intramolecular vibrational redistribution in aromatic molecules. I. Eigenstate resolved CH stretch first overtone spectra of benzene

A. Callegari, U. Merker, P. Engels, H. K. Srivastava, K. K. Lehmann, and G. Scoles

J. Chem. Phys. 113, 10583 (2000); http://dx.doi.org/10.1063/1.1319875 (14 pages) | Cited 17 times

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We have used infrared–infrared double resonance spectroscopy to record a rovibrational eigenstate resolved spectrum of benzene in the region of the CH stretch first overtone. This experiment is the first of a series aimed at investigating intramolecular vibrational energy redistribution (IVR) in aromatic molecules. The experiment has been carried out in a supersonic molecular beam apparatus using bolometric detection. A tunable resonant cavity was used to enhance the on-beam intensity of the 1.5 μm color center laser used to pump the overtone, and a fixed frequency [R(30)] 13CO2 laser was used to saturate the coinciding ν18 rQ(2) transition of benzene. After assigning the measured lines of the highly IVR fractionated spectrum to their respective rotational quantum number J, analysis of the data reveals that the dynamics occurs on several distinct time scales and is dominated by anharmonic coupling with little contribution from Coriolis coupling. After the fast (∼100 fs) redistribution of the energy among the previously observed “early time resonances” [R. H. Page, Y. R. Shen, and Y. T. Lee, J. Chem. Phys. 88, 4621 (1988) and 88, 5362 (1988)], a slower redistribution (10–20 ps) takes place, which ultimately involves most of the symmetry allowed vibrational states in the energy shell. Level spacing statistics reveal that IVR produces a highly mixed, but nonstatistical, distribution of vibrational excitation, even at infinite time. We propose that this nonintuitive phenomenon may commonly occur in large molecules when the bright state energy is localized in a high-frequency mode. © 2000 American Institute of Physics.
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33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Vq Vibration-rotation analysis
33.40.+f Multiple resonances (including double and higher-order resonance processes, such as double nuclear magnetic resonance, electron double resonance, and microwave optical double resonance)

Dissociation dynamics of the water molecule on the  1B1 electronic surface

X. F. Yang, D. W. Hwang, J. J. Lin, and X. Ying

J. Chem. Phys. 113, 10597 (2000); http://dx.doi.org/10.1063/1.1285899 (8 pages) | Cited 27 times

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Photodissociation of H2O, D2O, and HOD on the  1B1′ surface through 157.6 nm excitation has been studied using the H(D) atom Rydberg tagging time-of-flight technique. Vibrational state distribution has been measured for the OH/OD product from the photodissociation of the H2O, D2O, and HOD molecules. Comparisons of our results with previous theoretical calculations and experimental results obtained using the laser induced fluorescence (LIF) technique have been made. Experimental results in this work indicate that the relative populations for vibrationally excited OH(v ≥ 2) products measured using LIF are significantly underestimated, suggesting that LIF as a technique to quantitatively measure vibrational distributions of reaction product OH is seriously flawed. The experimental results presented here are in rather good agreement with previous theoretical calculations. However, our results indicate that the calculated vibrational populations for the higher vibrational states of OH are still somewhat overestimated. Relative branching ratio of H and D productions from HOD has also been estimated. This complete set of data for the photodissociation of H2O should provide an excellent testing ground for improving the theoretical potential energy surface of the first electronically excited state of the water molecule. © 2000 American Institute of Physics.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
33.50.Dq Fluorescence and phosphorescence spectra
82.20.Tr Kinetic isotope effects including muonium
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
31.50.Df Potential energy surfaces for excited electronic states
31.30.Gs Hyperfine interactions and isotope effects
07.57.-c Infrared, submillimeter wave, microwave and radiowave instruments and equipment
07.60.-j Optical instruments and equipment
82.20.Rp State to state energy transfer

Multiple configuration quantum/classical treatments of reaction dynamics

Lichang Wang, William J. Meurer, and Anne B. McCoy

J. Chem. Phys. 113, 10605 (2000); http://dx.doi.org/10.1063/1.1317549 (10 pages) | Cited 10 times

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The accuracy of quantum/classical approaches for studies of reaction dynamics is investigated through simulations of the collinear and J = 0 dynamics of the O(3P)+HCl reaction on two potential surfaces. The results of classical and two types of quantum/classical treatments of this reaction are compared to the results of quantum wave packet simulations. It is found that the accuracy of the single configuration quantum/classical treatment is sensitive to features of the potential surface. Most of this sensitivity is removed when a second configuration is introduced. For collision energies below 0.8 eV, the multiple configuration quantum/classical treatment provides reaction probabilities and product state distributions that are in good agreement with the results of the corresponding quantum simulation. The agreement deteriorates at higher collision energies, but here the agreement between the quantum and classical results is quite good. © 2000 American Institute of Physics.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Kh Potential energy surfaces for chemical reactions
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Inherent structures and nonequilibrium dynamics of one-dimensional constrained kinetic models: A comparison study

A. Crisanti, F. Ritort, A. Rocco, and M. Sellitto

J. Chem. Phys. 113, 10615 (2000); http://dx.doi.org/10.1063/1.1324994 (20 pages) | Cited 35 times

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We discuss the relevance of the Stillinger and Weber approach to the glass transition investigating the nonequilibrium behavior of models with nontrivial dynamics, but with simple equilibrium properties. We consider a family of 1D constrained kinetic models, which interpolates between the asymmetric chain introduced by Jäckle and Eisinger [Z. Phys. B 84, 115 (1991)] and the symmetric chain introduced by Fredrickson and Andersen [Phys. Rev. Lett 53, 1244 (1984)], and the 1D version of the Backgammon model [Phys. Rev. Lett. 75, 1190 (1995)]. We show that the configurational entropy obtained from the inherent structures is the same for all models irrespective of their different microscopic dynamics. We present a detailed study of the coarsening behavior of these models, including the relation between fluctuations and response. Our results suggest that any approach to the glass transition inspired by mean-field ideas and resting on the definition of a configurational entropy must rely on the absence of any growing characteristic coarsening pattern. © 2000 American Institute of Physics.
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64.70.P- Glass transitions of specific systems
64.70.Q- Theory and modeling of the glass transition
65.20.-w Thermal properties of liquids
65.40.gd Entropy

Mass and size dependence of single ion dynamics in molten monohalides

Olga Alcaraz and Joaquim Trullàs

J. Chem. Phys. 113, 10635 (2000); http://dx.doi.org/10.1063/1.1323978 (7 pages) | Cited 5 times

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This work is concerned with four molten monohalides with different ionic radii ratios (RbCl, NaI, AgCl, and CuCl) and ideal isotopic systems of these salts with different ionic mass ratios. The velocity autocorrelation functions of the two ionic species in each melt have been studied by both a theoretical approximation and molecular dynamics simulations. It is found that their main features may be qualitatively predicted by considering suitable combinations of the second and fourth frequency moments of their spectra. The analysis of these two parameters allows us to determine how the structure (strongly dependent on the ionic size difference) and the ionic masses contribute to the shape of the velocity autocorrelation functions. The results show that the averaged microscopic motion of the small ions is mainly determined by the first neighboring shell of unlike ions, whereas the nearest shell of like ions also affects the dynamics of the large ions. This effect is more pronounced as the size difference is greater. Furthermore, it is concluded that the size differences encourage the rattling motion of the large ions, whereas the mass difference encourages the backscattering and oscillations of the velocity autocorrelation function of the light ions. A simple rule is derived to determine the interplay between these two effects. Comparison between the mass and nearest distance ratios enables the prediction as to which species will experience a more pronounced backscattering motion. The size difference effects prevail in the hydrodynamics regime and the self-diffusion coefficient of the small ions is higher than that of the large ones. The difference between the self-diffusion coefficient increases as the size differences increases. © 2000 American Institute of Physics.
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61.20.Ja Computer simulation of liquid structure
66.10.C- Diffusion and thermal diffusion

A two-chain path integral model of positronium

L. Larrimore, R. N. McFarland, P. A. Sterne, and Amy L. R. Bug

J. Chem. Phys. 113, 10642 (2000); http://dx.doi.org/10.1063/1.1323979 (9 pages) | Cited 5 times

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We have used a path integral Monte Carlo technique to simulate positronium (Ps) in a cavity. The primitive propagator is used, with a pair of interacting chains representing the positron and electron. We calculate the energy and radial distribution function for Ps enclosed in a hard, spherical cavity, and the polarizability of the model Ps in the presence of an electrostatic field. We find that the positron distribution near the hard wall differs significantly from that for a single particle in a hard cavity. This leads to systematic deviations from predictions of free-volume models which treat Ps as an effective, single particle. A virial-type estimator is used to calculate the kinetic energy of the particle in the presence of hard walls. This estimator is found to be superior to a kinetic-type estimator given the interaction potentials, cavity sizes, and chain lengths considered in the current study. © 2000 American Institute of Physics.
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36.10.Dr Positronium
32.30.-r Atomic spectra
34.20.Cf Interatomic potentials and forces
02.50.Ng Distribution theory and Monte Carlo studies

Computing the classical mechanical vibrational echo with the fluctuating frequency approximation

Ryan B. Williams and Roger F. Loring

J. Chem. Phys. 113, 10651 (2000); http://dx.doi.org/10.1063/1.1324992 (12 pages) | Cited 14 times

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The vibrational photon echo is an infrared nonlinear spectroscopic measurement probing the time scales of dynamical processes that underlie a linear absorption spectrum. The challenges posed by the quantum mechanical treatment of large anharmonic systems motivate the consideration of this observable within classical mechanics. The rigorous calculation of the classical mechanical vibrational echo requires going beyond a conventional molecular dynamics simulation of trajectories to propagate stability matrix elements, which quantify the sensitivity of classical trajectories to small changes in initial conditions. As an alternative to this procedure, we present an approximate theory of the vibrational echo that avoids the numerical calculation of stability matrix elements. This approach, the fluctuating frequency approximation (FFA), generalizes a well established treatment of linear spectroscopy that models a driven anharmonic oscillator as a harmonic system with a fluctuating frequency. The FFA compares well with numerically exact calculations of the echo for a solvated anharmonic oscillator. © 2000 American Institute of Physics.
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42.50.Md Optical transient phenomena: quantum beats, photon echo, free-induction decay, dephasings and revivals, optical nutation, and self-induced transparency
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
07.57.Ty Infrared spectrometers, auxiliary equipment, and techniques

Resolving the hydrogen bond dynamics conundrum

Alenka Luzar

J. Chem. Phys. 113, 10663 (2000); http://dx.doi.org/10.1063/1.1320826 (13 pages) | Cited 99 times

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This paper analyzes dynamic properties of hydrogen bonds in liquid water. We use molecular dynamics simulation to calculate different probability densities that govern the time evolution of the formation and rupture of hydrogen bonds. We provide analytical connections between these functions. Excellent agreement with our simulation results is observed. We prove transition state theory rate constant to be identical to the inverse of the associated mean first passage time (hydrogen bond lifetime). Hence, the analysis establishes its Arrhenius temperature dependence. We give the explicit relation between reactive flux correlation function for the relaxation dynamics of hydrogen bonds, and their first passage time probability densities. All the different observations in the existing literature, associated with various estimates of hydrogen bonding times in liquid water that are affected (or not affected) by particular bond criteria, as well as by different definitions of hydrogen bond lifetimes applied in simulation, can be easily reconciled within the framework of reactive flux correlation function approach. © 2000 American Institute of Physics.
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61.20.Ja Computer simulation of liquid structure
33.15.Fm Bond strengths, dissociation energies
31.15.xv Molecular dynamics and other numerical methods
71.15.Pd Molecular dynamics calculations (Car-Parrinello) and other numerical simulations

Ab initio molecular dynamics simulation of LiBr association in water

Sergei Izvekov and Michael R. Philpott

J. Chem. Phys. 113, 10676 (2000); http://dx.doi.org/10.1063/1.1311965 (9 pages) | Cited 3 times

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A computationally economical scheme which unifies the density functional description of an ionic solute and the classical description of a solvent was developed. The density functional part of the scheme comprises Car–Parrinello and related formalisms. The substantial saving in the computer time is achieved by performing the ab initio molecular dynamics of the solute electronic structure in a relatively small basis set constructed from lowest energy Kohn–Sham orbitals calculated for a single anion in vacuum, instead of using plane wave basis. The methodology permits simulation of an ionic solution for longer time scales while keeping accuracy in the prediction of the solute electronic structure. As an example the association of the Li+–Br− ion-pair system in water is studied. The results of the combined molecular dynamics simulation are compared with that obtained from the classical simulation with ion-ion interaction described by the pair potential of Born–Huggins–Mayer type. The comparison reveals an important role played by the polarization of the Br− ion in the dynamics of ion pair association. © 2000 American Institute of Physics.
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61.25.-f Studies of specific liquid structures
82.30.Nr Association, addition, insertion, cluster formation
31.15.A- Ab initio calculations
61.20.Ja Computer simulation of liquid structure
31.15.E- Density-functional theory
31.70.Dk Environmental and solvent effects
back to top Surfaces, Interfaces, and Materials

Microscopic treatment of substrate effects on linear and quadratic optical response of model Langmuir–Blodgett multilayers

M. in het Panhuis and R. W. Munn

J. Chem. Phys. 113, 10685 (2000); http://dx.doi.org/10.1063/1.1323725 (6 pages) | Cited 8 times

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Microscopic calculations are reported of linear and quadratic nonlinear optical response in model Langmuir–Blodgett films on a substrate, which is treated by the method of images. The effect of the substrate is significant in the first two layers, and is greatest for tilted molecules with their head groups adjacent to the substrate. The main qualitative effect is to lower the symmetry relative to a free-standing film. Calculations for stearic acid films show that the substrate effect is most important for molecules with nonuniform response on a metallic substrate. © 2000 American Institute of Physics.
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68.18.-g Langmuir-Blodgett films on liquids
78.66.-w Optical properties of specific thin films
42.65.-k Nonlinear optics

Analysis of linear and quadratic optical response of mixed Langmuir–Blodgett films of stearic acid and 5-CT

M. in het Panhuis and R. W. Munn

J. Chem. Phys. 113, 10691 (2000); http://dx.doi.org/10.1063/1.1323724 (6 pages) | Cited 5 times

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Published experimental optical second-harmonic generation (SHG) results for mixed films formed between stearic acid and the mesogen 5-CT (4″-n-pentyl-4-cyano-p-terphenyl) are reanalyzed. Experimental refractive index results for the pure films are used to deduce molecular polarizabilities in order to calculate local fields. At low and high fractions of 5-CT, a consistent interpretation of the SHG is obtained using a mean molecular response based on a fixed axial hyperpolarizability βLLL for the mesogen, but at high mesogen fractions molecular tilt makes small off-diagonal components βLLM ≈ −βLLL/40 essential. At intermediate fractions the treatment can interpret the SHG only by requiring molecular tilt not found in the experiments, implying that a more detailed structural model is required. © 2000 American Institute of Physics.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
42.25.Gy Edge and boundary effects; reflection and refraction
68.18.-g Langmuir-Blodgett films on liquids
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
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