ChBE Seminar Series: John Weeks
Tuesday, May 4, 2010
11:00 a.m.-12:15 p.m.
Room 2110, Chemical and Nuclear Engineering Bldg.
Professor F. Joseph Schork
Taming Coulomb Interactions in Models for Ionic Solutions and Water
Presented by John Weeks
Distinguished University Professor
CMPS-Institute for Physical Science & Technology
University of Maryland College Park
Coulomb interactions from the dipoles in water or in classical models for ionic solutions cause major problems because of their slow decay, requiring, e.g., special and expensive treatment of periodic boundary conditions in computer simulations using Ewald sums. But they also cause problems at small length scales, where they can be so strong that they compete efficiently with the other strong molecular core interactions. We describe a new theoretical approach for such systems by exactly separating the point charge Coulomb interaction into a slowly-varying long-ranged component that arises from a rigid Gaussian charge distribution with width sigma, and the remainder, which is short-ranged and can be added to the other short-ranged core interactions. When sigma is properly chosen, we show that one can account very accurately for the locally averaged effects of the long-ranged components in terms of an effective single particle potential, or "local molecular field" (LMF), making consistent use of a simple mean field approximation.
The general theory is a mapping that relates the properties of a nonuniform system with long ranged intermolecular interactions in a given external field (arising, e.g., from charged solutes or walls) to those of a simpler "mimic system" with short-ranged intermolecular interactions in an effective or restructured field. The theory simplifies greatly when applied only to Coulomb interactions, where it provides a new view of classical electrostatics. The slowly-varying component of the effective field is shown to satisfy Poissons equation, but with a Gaussian-smoothed total charge density.
Characteristic phenomena such as ion pairing or the dielectric and electrostatic properties of the Simple (Extended) Point Charge (SPC/E) Model for water are quantitatively captured in the simpler mimic system. We describe a new and very efficient way to solve the self-consistent LMF equation for the restructured field.