POTENTIAL-ENERGY SURFACES OF NAFH

High-level ab initio calculations of the ground and several excited-st ate adiabatic potential surfaces of the NaFH system are reported. Thes e calculations were performed by multireference configuration interact ion on a large grid of geometries which allowed them to be used for co nstructing an accurate analytic representation of the NaFH potential s urfaces. For the ground and first excited states, using a genetic algo rithm, an analytic 2X2 matrix fit was obtained corresponding to a diab atic representation. The off-diagonal coupling was obtained by fitting the energy gap between the surfaces in the region of their avoided cr ossing, and the diagonal elements were then fit to reproduce the ab in itio adiabatic energy at 1530 points. The full fit was used to locate the barrier and the van der Waals well on the ground-state potential s urface, the exciplex on the first-excited-state potential surface, and the minimum energy path for the ground-state Na+HF-->NaF+H reaction. Additional calculations on the van der Waals and saddle point regions were carried out by a variety of ab initio methods as a check on accur acy. Major topological features of the potential energy surfaces repre senting higher-than-first excited states were examined. (C) 1998 Ameri can Institute of Physics. [S0021-9606(98)00813-7].