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].