Multiconfiguration molecular mechanics algorithm for potential energy surfaces of chemical reactions

We present an efficient algorithm for generating semiglobal potential energ y surfaces of reactive systems. The method takes as input molecular mechani cs force fields for reactants and products and a quadratic expansion of the potential energy surface around a small number of geometries whose locatio ns are determined by an iterative process. These Hessian expansions might c ome, for example, from ab initio electronic structure calculations, density functional theory, or semiempirical molecular orbital theory. A 2 x 2 elec tronic diabatic Hamiltonian matrix is constructed from these data such that , by construction, the lowest eigenvalue of this matrix provides a semiglob al approximation to the lowest electronically adiabatic potential energy su rface. The theory is illustrated and tested by applications to rate constan t calculations for three gas-phase test reactions, namely, the isomerizatio n of 1,3-cis-pentadiene, OH + CH4--> H2O + CH3, and CH2Cl + CH3F --> CH3Cl + CH2F. (C) 2000 American Institute of Physics. [S0021-9606(00)01005-9].