IMPLEMENTATION OF REACTION FIELD METHODS IN QUANTUM-CHEMISTRY COMPUTER CODES

The embedding of a quantum mechanically described subsystem by classic al representations of its surroundings is reviewed. The choices for a distributed monopole representation and a distributed (group) polariza bility representation, as well as the continuum approach to model bulk effects, are discussed. Focus is on the practical implementation of t he classical description in quantum chemistry codes (in particular, HO NDO8.1). Expressions are given for the self-consistent coupling betwee n the classical partitions (dipole polarizabilities and boundary surfa ce dipoles and charges) and for the coupling between classical and qua ntum partitions. The latter is mediated through expanded, rather than exact, potentials and fields. In this way, the computation of only a l imited number of formal interactions between unit charge distributions located at the expansion centers suffices to evaluate the reaction fi eld contributions. The electronic part of the coupling can be included in the Hamiltonian via the Fock matrix. The field operators, as well as the one- and two-electron matrix elements over the basis functions, are simple. The expressions for these are given explicitly. Nonequili brium potentials and Monte Carlo sampling over classical degrees of fr eedom have been added to better mimic experimental conditions. (C) 199 5 by John Wiley and Sons, Inc.