Simulations of light induced processes in water based on ab initio path integrals molecular dynamics. I. Photoabsorption

Svoboda, Ondřej; Ončák, Milan; Slavíček, Petr

doi:doi:10.1063/1.3649942

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Sizes of large He droplets

Helium droplets spanning a wide size range, NHe = 103–1010, were formed in a continuous-nozzle beam expansion at different nozzle temperatures and a constant stagnation pressure of 20 bars. The average sizes of the droplets have been obtained by attenuation of the droplet beam through collisions with argon and helium gases at room temperature. The results obtained are in good agreement with previous measurements in the size range NHe = 105–107. Moreover, the measurements give the average sizes i...

Simulations of light induced processes in water based on ab initio path integrals molecular dynamics. II. Photoionization

We have applied ab initio based reflection principle to simulate photoelectron spectra of small water clusters, ranging from monomer to octamer. The role of quantum and thermal effects on the structure of the water photoelectron spectra is discussed within the ab initio path integral molecular dynamics (PIMD) framework. We have used the PIMD method with up to 40 beads to sample the ground state quantum distribution at temperature T = 180 K. We have thoroughly tested the performance of various de...

We have performed large-scale simulations of UV absorption spectra of water clusters (monomer to octamer) using a combination of ab initio path-integral molecular dynamics with reflection principle. The aim of the present work is four-fold: (1) To explore the transition from isolated molecules to bulk water from the perspective of UV photoabsorption. (2) To investigate quantum nuclear and thermal effects on the shape of the water UV spectra. (3) To make an assessment of the density functional theory functionals to be used for water excited states. (4) To check the applicability of the QM/MM schemes for a description of the UV absorption. Within the path integral molecular dynamics (PIMD)/reflection principle approach both the thermal and quantum vibrational effects including anharmonicities are accounted for. We demonstrate that shape of the spectra is primarily controlled by the nuclear quantum effects. The excited states and transition characteristics of the water clusters were calculated with the time-dependent density functional theory and equation-of-motion coupled clusters singles and doubles methods. Based on our benchmark calculations considering the whole UV spectrum we argue that the BHandHLYP method performs best among the 6 functionals tested (B3LYP, BHandHLYP, BNL, CAM-B3LYP, LC-ωPBE, and M06HF). We observe a gradual blueshift of the maximum of the first absorption peak with the increasing cluster size. The UV absorption spectrum for the finite size clusters (i.e., the peak centers, peak widths, and photoabsorption cross section) essentially converges into the corresponding bulk water spectrum. The effect of distant molecules accounted for within the polarizable continuum model is shown to be almost negligible. Using the natural transition orbitals we demonstrate that the first absorption band is formed by localized excitations while the second band includes delocalized excited states. Consequently, the QM/MM electrostatic embedding scheme can only be used for the modeling of the low energy part of the spectrum.

Article Outline

INTRODUCTION
METHODS
1. Calculation of cross sections via the reflection principle
2. Electronic structure methods
RESULTS AND DISCUSSION
1. Analysis of hydrogen bonding patterns for water clusters
2. Character of the excited states in the water clusters
3. Assessment of electronic structure methods for water photoabsorption
4. Quantum and thermal effects on photoabsorption spectra
5. The performance of electrostatic embedding scheme
6. Water UV absorption spectra: From the gas phase to bulk

EDITORIALLY RELATED

Simulations of light induced processes in water based on ab initio path integrals molecular dynamics. II. Photoionization
Ondřej Svoboda et al.
J. Chem. Phys. 135, 154302 (2011)JCPSA6000135000015154302000001

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KEYWORDS and PACS

Keywords

ab initio calculations, coupled cluster calculations, density functional theory, excited states, molecular clusters, molecular dynamics method, photoexcitation, spectral line shift, ultraviolet spectra, vibrational states, water

PACS

36.40.Cg

Electronic and magnetic properties of clusters
36.40.Mr

Spectroscopy and geometrical structure of clusters
33.20.Tp

Vibrational analysis
33.20.Lg

Ultraviolet spectra
31.15.ee

Time-dependent density functional theory
31.15.at

Molecule transport characteristics; molecular dynamics; electronic structure of polymers

ARTICLE DATA

Digital Object Identifier

http://dx.doi.org/10.1063/1.3649942

PUBLICATION DATA

ISSN

0021-9606 (print)

1089-7690 (online)

Publisher

$abstract.society.value is a Member of CrossRef

American Institute of Physics

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