Calculating Nonbonded Potentials for Classical Simulations of Atoms in Molecules and Metal Surfaces

Classical simulations of real molecules require realistic
nonbonded
interactions between constituent atoms, which have traditionally been
adjusted for good agreement with liquid properties and copied extensively
among similar atom types. In this work, we propose ab initio methods
to compute both the C12 short-range repulsion
and the C6 dispersive attraction between
atoms. We relate the repulsion to the distance at which the electron
density near an atom falls below a certain threshold, chosen to match
radii for atoms in the OPLS force field. We compute the dispersive
attraction by applying the McLachlan integral formula to the polarizability
contributions of each atom in a molecule as a function of imaginary
frequency. These polarizability contributions can be computed by time-dependent
Hartree–Fock methods in GAMESS, which conveniently partitions
the total polarizability among bonds and lone pairs. Our method produces
values for both C12 and C6 parameters in good agreement with existing OPLS values
when applied to nearly 200 atom types in over 100 organic molecules
from the virtualchemistry.org archive. We verify that different instances
of OPLS atom types have nearly identical polarizabilities, lending
credence both to our method and to atom types based on local chemical
environments. We extend our frequency-integral method for computing
dispersive interactions to atoms in molecules near metal surfaces,
which screen nearby fluctuating fields, with a frequency response
limited by the plasma frequency. The screening is equivalent to a
fluctuating image dipole with which the atom interacts, giving rise
to a 1/z3 interaction with a metal half-space.
This interaction can be conveniently represented as a conventional
1/r6 interaction with each metal atom,
summed over the half-space.