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28 Feb 2012

Volume 136, Issue 8 (partial)

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J. Chem. Phys. 136, 084102 (2012); http://dx.doi.org/10.1063/1.3685604 (13 pages)

Junchao Xia, Chen Huang, Ilgyou Shin, and Emily A. Carter
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Communication: Fundamental measure theory for hard disks: Fluid and solid

Roland Roth, Klaus Mecke, and Martin Oettel

J. Chem. Phys. 136, 081101 (2012); http://dx.doi.org/10.1063/1.3687921 (4 pages)

Online Publication Date: 22 February 2012

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Two-dimensional hard-particle systems are rather easy to simulate but surprisingly difficult to treat by theory. Despite their importance from both theoretical and experimental points of view, theoretical approaches are usually qualitative or at best semi-quantitative. Here, we present a density functional theory based on the ideas of fundamental measure theory for two-dimensional hard-disk mixtures, which allows for the first time an accurate description of the structure of the dense fluid and the equation of state for the solid phase within the framework of density functional theory. The properties of the solid phase are obtained by freely minimizing the functional.
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64.10.+h General theory of equations of state and phase equilibria
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
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back to top Theoretical Methods and Algorithms

Mapping quantum-classical Liouville equation: Projectors and trajectories

Aaron Kelly, Ramses van Zon, Jeremy Schofield, and Raymond Kapral

J. Chem. Phys. 136, 084101 (2012); http://dx.doi.org/10.1063/1.3685420 (14 pages)

Online Publication Date: 22 February 2012

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The evolution of a mixed quantum-classical system is expressed in the mapping formalism where discrete quantum states are mapped onto oscillator states, resulting in a phase space description of the quantum degrees of freedom. By defining projection operators onto the mapping states corresponding to the physical quantum states, it is shown that the mapping quantum-classical Liouville operator commutes with the projection operator so that the dynamics is confined to the physical space. It is also shown that a trajectory-based solution of this equation can be constructed that requires the simulation of an ensemble of entangled trajectories. An approximation to this evolution equation which retains only the Poisson bracket contribution to the evolution operator does admit a solution in an ensemble of independent trajectories but it is shown that this operator does not commute with the projection operators and the dynamics may take the system outside the physical space. The dynamical instabilities, utility, and domain of validity of this approximate dynamics are discussed. The effects are illustrated by simulations on several quantum systems.
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03.65.Ta Foundations of quantum mechanics; measurement theory
03.65.Ud Entanglement and quantum nonlocality (e.g. EPR paradox, Bell's inequalities, GHZ states, etc.)
05.45.Xt Synchronization; coupled oscillators
02.30.Rz Integral equations

Can orbital-free density functional theory simulate molecules?

Junchao Xia, Chen Huang, Ilgyou Shin, and Emily A. Carter

J. Chem. Phys. 136, 084102 (2012); http://dx.doi.org/10.1063/1.3685604 (13 pages)

Online Publication Date: 22 February 2012

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Orbital-free density functional theory (OFDFT), with its attractive linearly scaling computation cost and low prefactor, is one of the most powerful first principles methods for simulating large systems (∼104–106 atoms). However, approximating the electron kinetic energy with density functionals limits the accuracy and generality of OFDFT compared to Kohn-Sham density functional theory (KSDFT). In this work, we test whether the Huang-Carter (HC) kinetic energy density functional (KEDF), which contains the physics to properly describe covalently bonded semiconductor materials, can also be used to describe covalent bonds in molecules. In particular, we calculate a variety of homonuclear diatomic molecules with the HC functional within OFDFT. The OFDFT bond dissociation energy, equilibrium bond length, and vibrational frequency of these dimers are in remarkably good agreement with benchmark KSDFT results, given the lack of orbitals in the calculation. We vary the two parameters λ (controlling the reduced density gradient contribution to the nonlocal kernel) and β (the exponent of the density in the nonlocal term) present in the HC KEDF and find that the optimal λ correlates with the magnitude of the highest occupied molecular orbital - lowest unoccupied molecular orbital energy gap. Although the HC KEDF represents a significant improvement over previous KEDFs in describing covalent systems, deficiencies still exist. Despite the similar overall shape of the KSDFT and OFDFT ground state electron densities, the electron density within the bonding region is still quite different. Furthermore, OFDFT is not yet able to give reasonable description of magnetic states. The energy orderings of the triplet and singlet states of Si2 and Al family dimers are not consistent with KSDFT or experimental results and the spin polarization distributions also differ widely between the two theories.
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31.15.E- Density-functional theory
33.20.Tp Vibrational analysis
33.15.Fm Bond strengths, dissociation energies
33.15.Dj Interatomic distances and angles
31.15.ae Electronic structure and bonding characteristics
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

A set of molecular models for alkali and halide ions in aqueous solution

Stephan Deublein, Jadran Vrabec, and Hans Hasse

J. Chem. Phys. 136, 084501 (2012); http://dx.doi.org/10.1063/1.3687238 (10 pages)

Online Publication Date: 22 February 2012

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This work presents new molecular models for alkali and halide ions in aqueous solution. The force fields were parameterized with respect to the reduced liquid solution density at 293.15 K and 1 bar, considering all possible ion combinations simultaneously. The experimental target data are reproduced with a high accuracy over a wide range of salinity. The ion models predict structural properties of electrolyte solutions well, such as pair correlation functions and hydration numbers. The force fields provide good predictions of the properties studied here in combination with different models for water.
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61.20.Ne Structure of simple liquids
61.20.Gy Theory and models of liquid structure
82.30.Nr Association, addition, insertion, cluster formation
back to top Surfaces, Interfaces, and Materials

Sensitivity of nucleation phenomena on range of interaction potential

Rakesh S. Singh, Mantu Santra, and Biman Bagchi

J. Chem. Phys. 136, 084701 (2012); http://dx.doi.org/10.1063/1.3685835 (8 pages)

Online Publication Date: 22 February 2012

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Theoretical and computational investigations of nucleation have been plagued by the sensitivity of the phase diagram to the range of the interaction potential. As the surface tension depends strongly on the range of interaction potential and as the classical nucleation theory (CNT) predicts the free energy barrier to be directly proportional to the cube of the surface tension, one expects a strong sensitivity of nucleation barrier to the range of the potential; however, CNT leaves many aspects unexplored. We find for gas-liquid nucleation in Lennard-Jones system that on increasing the range of interaction the kinetic spinodal (KS) (where the mechanism of nucleation changes from activated to barrierless) shifts deeper into the metastable region. Therefore the system remains metastable for larger value of supersaturation and this allows one to explore the high metastable region without encountering the KS. On increasing the range of interaction, both the critical cluster size and pre-critical minima in the free energy surface of kth largest cluster, at respective kinetic spinodals, shift towards smaller cluster size. In order to separate surface tension contribution to the increase in the barrier from other non-trivial factors, we introduce a new scaling form for surface tension and use it to capture both the temperature and the interaction range dependence of surface tension. Surprisingly, we find only a weak non-trivial contribution from other factors to the free energy barrier of nucleation.
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64.60.Q- Nucleation
68.03.Cd Surface tension and related phenomena
81.30.Dz Phase diagrams of other materials
61.20.-p Structure of liquids
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