The charge density and potential for five parametrical variations of the IP model. The parameters for the five models are given in Table I. The left panels show the charge density and potential for the models with center fixed at the origin (the black solid and dashed lines are identical in these panels). The right panels show the charge density and potential for an interfacial system with boundaries at . For point charge models the potential is positive for models with greater than and negative for models with greater. Models C and D have Gaussian distributed radial charge distributions with finite width . Model D includes a contribution that corresponds to the neutral atom potential. Models A, A2, and B correspond to the point charge limit (i.e., ). The molecule density of the monomer is set to . For models A, B, C and D the molecule density is a step function equal to between the boundaries and zero otherwise. The interface width characterized by the change in is finite for model A2.
Shown are the molecule density, charge density, and potential along the interface normal of the vacuum-methane system. The molecule density is plotted in the top left panel, the charge density is plotted in the top right panel and the interface potential is plotted in the bottom panel. The data for the CHARMM-METH model correspond to a simulation, using the CHARMM force field, of a liquid methane slab comprised of 500 molecules surrounded by vacuum. The IP-METH model is the simple model of methane with parameters translated from the CHARMM model of methane.
The potential drop along the interface normal for three IP models of the TIP3P water force field. The center of the IP-TIP3Pa corresponds to the position of the oxygen atom. The center of the IP-TIP3Pb model is the geometric center of the molecule and the center of the IP-TIP3Pc model is along the H–O–H bisector closest to the hydrogen atoms. The potential in the bulk region of the interface model is negative with respect to the vacuum for all three models. Also included are the interfacial potential computed from simulation of neat liquid water in PBCs with the TIP3P force field. Image atoms are replaced by vacuum leaving only the molecules of the reference unit cell. Boundary conditions are applied to positions on the molecule that are analogous to the choice for molecular center in the IP models.
Shown are the molecule density, charge density, and potential along the interface normal of the vacuum-water system. The molecule density is plotted in the top left panel, the charge density is plotted in the top right panel and the interface potential is plotted in the bottom panel. The data for the TIP3P model are from a simulation of a liquid water slab comprised of 500 molecules surrounded by vacuum. The IP-TIP3Pb model is the IP model translated from the TIP3P water force field with molecular center chosen to coincide with the geometric center of the molecule. The IP- model includes an atomic potential contribution.
Parameters for the IP models.
Parameters for the IP models of methane and water. The IP- model includes a nonvanishing atomic potential with charge and a width . All other models are point charge based.
The solvation-free enegy difference between monovalent sodium and a negatively charged analog in liquid methane. The free energy is computed for two systems: a sodiumlike ion solvated in the center of a spherical liquid drop and under periodic boundary conditions (PBCs). includes a correction for the missing liquid to vacuum interface potential (i.e., ) where from the methane-vacuum simulation is and is to change a sodiumlike ion to .
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