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

The ground potential energy surface (PES) of the O(D-1)+H2O system was stud ied with the CASPT2//CASSCF ab initio method. We analyzed the degree of val idity 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 consi stent with the absence of experimental activation energy. The CASPT2//CASSC F and PUMP4//UMP2 results, however, show important differences, mainly conc erning the energy, due to the dominant open-shell singlet character of the ground PES. To make an accurate general description of this system, ab init io calculations using multireference methods like the one discussed here ar e required. Nevertheless, the earlier PUMP4//UMP2 calculations can be taken as a reasonable starting point for characterizing the ground PES of this s ystem. Moreover, the pseudotriatomic (O(D-1)+H-(OH)) analytical potential e nergy surface derived in the previous work to interpret the experimental re sults is a reasonable model for describing the O(D-1)+H2O -->2 OH reaction. (C) 2001 American Institute of Physics.