Ab initio CASPT2//CASSCF study of the O(1D)+H2O(X 1A1) reaction

The ground potential energy surface (PES) of the O(D-1)+H2O system was studied with the CASPT2//CASSCF ab initio method. We analyzed the degree of validity of an earlier ab initio study by us that used the Moller-Plesset (MP) method. Both the present CASPT2//CASSCF calculations and the highest level MP calculations (PUMP4//UMP2) showed that the main reaction channel (OH+OH) has no energy barrier along the minimum energy path. This result is consistent with the absence of experimental activation energy. The CASPT2//CASSCF and PUMP4//UMP2 results, however, show important differences, mainly concerning the energy, due to the dominant open-shell singlet character of the ground PES. To make an accurate general description of this system, ab initio calculations using multireference methods like the one discussed here are required. Nevertheless, the earlier PUMP4//UMP2 calculations can be taken as a reasonable starting point for characterizing the ground PES of this system. Moreover, the pseudotriatomic (O(D-1)+H-(OH)) analytical potential energy surface derived in the previous work to interpret the experimental results is a reasonable model for describing the O(D-1)+H2O -->2 OH reaction. (C) 2001 American Institute of Physics.