Steven Rick of the University of New Orleans and Susan Rempe of Sandia National Laboratory are supported by an award from the Chemical Theory, Models and Computational Methods program in the Chemistry Division to develop charge transfer (CT) models for ions and protein chains, building on previous work for water and to examine the effects of charge transfer on the properties of interfaces, water transport though nanochannels, ion solvation, and biological ion channels. Two often opposing principles, a need for efficiency and a need for accuracy, guide the development of interaction potentials for atomistic simulations. One strategy is to use quantum mechanical calculations or experimental results to determine the interaction types that are significant and then develop efficient methods for treating these interactions. Experimental and theoretical studies have demonstrated that charge transfer can play an important role in intermolecular interactions. Specifically, for two molecules in contact, the electron density is not equally shared between the two molecules. Thus, molecules can have small transient charges and ions can have non-integer charges. This amount of charge transfer may have important and largely unexamined effects on the thermodynamics and kinetics of condensed-phase systems. Studies of ions in non-aqueous systems, including ethylene carbonate and ionomers, are also planned. All these projects involve quantum mechanical calculations both to parameterize the CT models and to validate the results of the models.
This award supports theoretical and computational research aimed to develop and use simulation models with a new level of sophistication, without sacrificing computational efficiency. The understanding of materials at the molecular level is greatly enhanced by the use of computer simulations, which require a mathematical description of the interactions between particles. Charge transfer is a class of interactions commonly excluded in molecular simulations but which quantum theoretical calculations indicate is important. The quantum calculations reveal that charge can be spread out from a central molecule or ion to nearby particles. As an example, in water a chloride ion, with a formal charge of minus one, shares this excess charge with its neighbors and has a charge that is reduced by about 20%. The charge reduction of this and other ions is strongly environmentally sensitive, as its nearest neighbors change, impacting how ions interact and flow through various materials. This charge variation could have important consequences on the behavior of electrolytes as, for example, used in batteries, and in design of water desalination filters. This work would develop and use charge transfer models for the study of the structure and dynamics of ions and water in a variety of materials, including biological ion channels, water purification membranes, and battery materials.
