Molecular simulation using recent force fields provide a powerful framework for predicting phase equilibrium properties. Molecules with partial charges or induced dipoles require special algorithms to determine the long-ranged electrostatic interactions. The most common method is the Ewald summation, which, however, is a computationally demanding part of molecular simulations.
With the Wolf Summation [1,2], which is a spherically truncated direct r-1 summation, the electrostatic interactions can be handled with a lower computational cost than the Ewald summation. For an improved description of the electrostatic interactions the self-interaction term has been modified. This term is important for systems of fluctuating molecule numbers. We also apply the Wolf scheme for calculating electrostatic interactions to a spherically truncation method of neutral groups.
This study assesses the modified Wolf summation and the spherically truncation method of neutral groups for molecules with partial charges as well as for polarizable molecules with induced dipoles by systematic comparison to the Ewald summation. For liquid-vapor coexistence properties of pure components and mixtures we find excellent agreement of the modified Wolf Summation methods to results from the Ewald Summation.
To solve polarizable systems with induced dipoles in a self-consistent manner, the electric field is needed. In this study we furthermore assess the need of a shifted force approach which ensures that the force is smooth at the cut-off radius.
Phase equilibria are calculated using a Monte Carlo procedure in the grand canonical ensemble {μVT}. To determine liquid-vapor phase coexistence properties, the Transition Matrix method [3] is applied. Multiple histograms for varying temperature and molecule numbers N to N+∆N are collected in order to cover the entire phase envelope using histogram reweighting [4].
References
[1] D. Wolf, P. Keblinski, S. R. Phillpot, J. Eggebrecht. J. Chem. Phys. 110, 8254-8282, 1999.
[2] C. J. Fennell, J. D. Gezelter. J. Chem. Phys. 124, 234104, 2006
[3] A. S. Paluch, V. K. Shen, J. R. Errington. Ind. Eng. Chem. Res. 47, 4533-4541, 2008.
[4] A. Z. Panagiotopoulos. J. Phys.: Condens. Matter 12, R25-R52, 2000.
