Energy switching approach to potential surfaces: An accurate single-valued function for the water molecule

A novel scheme is suggested to construct a global potential energy surface by switching between representations which are optimal for different energy regimes. The idea is illustrated for the electronic ground state of water for which we use as switched functions the many-body expansion potential of Murrell and Carter [J. Chem. Phys. 88, 4887 (1984)] and the polynomial form of Polyansky, Jensen, and Tennyson, [J. Chem. Phys. 101, 7651 (1994)]. By also modifying the former to reproduce the Coulombic behavior at the collapsed molecular limits for vanishingly small interatomic distances and approximately account for the long range forces, the new potential energy surface has been given double many-body expansion quality. The result is a global H2O potential energy surface which has spectroscopic accuracy and may be used for studies of reaction dynamics. (C) 1996 American Institute of Physics.