Assessing the Performance of the Diffusion Monte Carlo Method As Applied to the Water Monomer, Dimer, and Hexamer
| Autor: | Vladimir A. Mandelshtam, Joel D. Mallory, Sandra E. Brown |
|---|---|
| Rok vydání: | 2015 |
| Předmět: |
Dimer
Binding energy Population Analytical chemistry FOS: Physical sciences Random hexamer 010402 general chemistry 01 natural sciences Molecular physics Isotopomers chemistry.chemical_compound Physics - Chemical Physics TheoryofComputation_ANALYSISOFALGORITHMSANDPROBLEMCOMPLEXITY ComputingMethodologies_SYMBOLICANDALGEBRAICMANIPULATION 0103 physical sciences Physics::Atomic and Molecular Clusters Water model Physical and Theoretical Chemistry education Chemical Physics (physics.chem-ph) education.field_of_study 010304 chemical physics Chemistry 0104 chemical sciences Monomer Diffusion Monte Carlo MathematicsofComputing_DISCRETEMATHEMATICS |
| Zdroj: | Mallory, JD; Brown, SE; & Mandelshtam, VA. (2015). Assessing the Performance of the Diffusion Monte Carlo Method As Applied to the Water Monomer, Dimer, and Hexamer. Journal of Physical Chemistry A, 119(24), 6504-6515. doi: 10.1021/acs.jpca.5b02511. UC Irvine: Retrieved from: http://www.escholarship.org/uc/item/6ss3h619 |
| ISSN: | 1520-5215 1089-5639 |
| Popis: | The Diffusion Monte Carlo (DMC) method is applied to the water monomer, dimer, and hexamer, using q-TIP4P/F, one of the most simple, empirical water models with flexible monomers. The bias in the time step ($\Delta\tau$) and population size ($N_w$) is investigated. For the binding energies, the bias in $\Delta\tau$ cancels nearly completely, while a noticeable bias in $N_w$ still remains. However, for the isotope shift, (e.g, in the dimer binding energies between (H$_2$O)$_2$ and (D$_2$O)$_2$) the systematic errors in $N_w$ do cancel. Consequently, very accurate results for the latter (within $\sim 0.01$ kcal/mol) are obtained with relatively moderate numerical effort ($N_w\sim 10^3$). For the water hexamer and its (D$_2$O)$_6$ isotopomer the DMC results as a function of $N_w$ are examined for the cage and prism isomers. For a given isomer, the issue of the walker population leaking out of the corresponding basin of attraction is addressed by using appropriate geometric constraints. The population size bias for the hexamer is more severe, and in order to maintain accuracy similar to that of the dimer, the population size $N_w$ must be increased by about two orders of magnitude. Fortunately, when the energy difference between cage and prism is taken, the biases cancel, thereby reducing the systematic errors to within $\sim 0.01$ kcal/mol when using a population of $N_w=4.8\times 10^5$ walkers. Consequently, a very accurate result for the isotope shift is also obtained. Notably, both the quantum and the isotope effects for the prism-cage energy difference are small. Comment: 11 pages, 5 figures, 36 references. Submitted to the Journal of Physical Chemistry |
| Databáze: | OpenAIRE |
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