Dr. Chirstophe Chipot
Equipe de dynamique des assemblages membranaires, Unité mixte de recherche 7565
Nancy Université, CNRS
Vandoeuvre-les-Nancy, France

Monday, December 17, 2007
2:00 pm (CT)
3269 Beckman Institute

Abstract

Improvement in the description of intermolecular interactions in statistical mechanics simulations, going beyond the restrictive limitations of the pairwise additive approximation, has triggered the development of novel approaches for the design of tractable and reliable sets of polarization parameters. One possible route for the development of a polarizable potential energy function relies on the use of atomic polarizabilities derived from the induction energy mapped around the molecule. In the framework of optimally partitioned electric properties (OPEP), implicitly interacting polarizability models have been proposed. Whereas such polarizable models are expected to reproduce the signature induction energy with an appreciable accuracy, it is far from clear whether they will perform equally well in the context of intermolecular interactions. To address this issue, while keeping in mind the requirement of facile incorporation in a molecular mechanics platform, polarizability models determined quantum mechanically and consisting of atomic isotropic dipole plus charge-flow polarizabilities were combined with the classical, nonpolarizable Charmm force field. Performance of the models was probed in the challenging test cases of cation-π binding and the association of a divalent calcium ion with water, where induction effects are known to be considerable. Since brute force comparison of the binding energies estimated from the polarizable and the classical Charmm potential energy functions is not justified, the individual electrostatic and induction contributions of the force field were confronted with the corresponding terms of a symmetry-adapted perturbation theory (SAPT) expansion carried out with the 6-311++G(d, p) basis set. While the quantum-mechanical and the molecular-mechanical electrostatic and damped induction contributions agree reasonably well, overall reproduction of the binding energies is plagued by an underestimated repulsion that underlines the necessity of de novo parametrization of the classical 6-12 form of the van derWaals potential. Based on the SAPT expansion, new Lennard-Jones parameters were optimized, which, combined with the remainder of the polarizable force field, yield an improved reproduction of the target binding energies.