Methods for constraining zero-point energy in classical Monte Carlo transition-state theory

Two microcanonical sampling methods for constraining zero-point energy (ZPE) within classical Monte Carlo transition-state theory (MCTST) are described. Each is based on the efficient microcanonical sampling method [H. W. Schranz, S. Nordholm, and G., Nyman, J., Chern. Phys. 94, 1487 !(1991)], with exclusion of phase space paints not satisfying imposed ZPE constraints, Method 1 requires extensive sampling of phase space points, bur method 2 relies on configurational sampling using a microcanonical statistical weight adjusted to account for the reduced local density of momentum states arising from ZPE restrictions. Simple applications to HenI2 clusters and a linear model for H-O-D dissociation show that there are !significant deviations from standard ?MCTST., Methods 1 and 2 give similar results, but method 2 is the more efficient algorithm. (C) 1998 American Institute of Physics. [S0021-9606(98)01904-7].