Biasing the Center of Charge in Molecular Dynamics Simulations with Empirical Valence Bond Models: Free Energetics of an Excess Proton in a Water Droplet
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- 作者：Jü ; rgen Kö ; finger ; Christoph Dellago
- 刊名：Journal of Physical Chemistry B
- 出版年：2008
- 出版时间：February 28, 2008
- 年：2008
- 卷：112
- 期：8
- 页码：2349 - 2356
- 全文大小：172K
- 年卷期：v.112,no.8(February 28, 2008)
- ISSN：1520-5207
文摘
Multistate empirical valence bond (EVB) models provide an accurate description of the energetics of protontransfer and solvation in complex molecular systems and can be efficiently used in molecular dynamicscomputer simulations. Within such models, the location of the moving protonic charge can be specified bythe so-called center of charge, defined as a weighted average over the diabatic states of the EVB model. Inthis paper, we use first-order perturbation theory to calculate the molecular forces that arise if a bias potentialis applied to the center of charge. Such bias potentials are often necessary when molecular dynamics simulationsare used to determine free energies related to proton transfer and not all relevant proton positions are sampledwith sufficient frequency during the available computing time. The force expressions we derive are easy toevaluate and do not create any significant computational cost compared with unbiased EVB simulations. Asan illustration of the method, we study proton transfer in a small liquid water droplet consisting of 128 watermolecules plus an excess proton. Contrary to predictions of continuum electrostatics, but in agreement withprevious computer simulations of similar systems, we observe that the excess proton is predominantly locatedat the surface of the droplet. Using the formalism developed in this paper, we calculate the reversible workrequired to carry the protonic charge from the droplet surface to its core, finding a value of roughly 4 kBT.