In previous work [JCP, 131 (15):154103 (2009)] the Image Charge Solvation Model (ICSM) was introduced as an accurate and fast way to model the electrostatic interactions and simulate bulk properties of water, and then subsequently made available to be used in TINKER [CPC, 184 (1):19-26 (2013)]. In spite of the success of the ICSM in simulating a pure water system and dilute ionic solutions, the periodic boundary conditions created spurious imaged ions that cause unphysical correlations. Reducing these correlations by expanding the system size is computationally less efficient than using the well-accepted PME method with periodic boundaries. However, in this work, finite boundary conditions were developed that correct the problem at its origin. Finite boundary conditions are used with an effective van der Waals short-range force combined with adaptive-constraint forces in the buffer layer region to represent bulk solvent outside the cavity. While the idea of implementing boundary constraint forces is not new, the implementation details are not trivial to maintain bulk properties of water to be in agreement with PME bulk properties. As described in the talk, the source of difficulties is from the long-range nature of the electric field due to local structure and fluctuations of the structure of water at the boundary. Subtle structural differences in the water layer affect the bulk properties of water with surprising hypersensitivity. Nevertheless, with appropriate self-governing constraint forces, the bulk properties of water are maintained in a much larger usable region than the original ICSM. The new ICSM maintains high accuracy on static and diffusive properties of water, performs much more efficiently, removes the primary cause of spatial correlations between ions and can be used for NVT and NPT simulations.

Image-charge reaction field method with non-periodic boundary conditions applied to water using molecular dynamics simulation

248th National Meeting (2014)

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