BENCHMARK CALCULATIONS WITH CORRELATED MOLECULAR WAVE-FUNCTIONS .4. THE CLASSICAL BARRIER HEIGHT OF THE H-2-]H-2+H REACTION(H)

Using systematic sequences of correlation consistent Gaussian basis se ts from double to sextuple zeta quality, the classical barrier height of the H+H-2 exchange reaction has been calculated by multireference c onfiguration interaction (MRCI) methods. The MRCI calculations for col linear H-3 have also been calibrated against large basis set full CI ( FCI) results, which demonstrate that the MRCI treatment leads to energ ies less than 1 mu hartree (<0.001 kcal/mol) above the FCI energies. T he dependence of both the H-2 and H-3 total energies on the basis set is found to be very regular, and this behavior has been used to extrap olate to the complete basis set (CBS) limits. The resulting estimate o f the H-H-H CBS limit yields a classical barrier height, relative to e xact H+H-2, of 9.60+/-0.02 kcal/mol; the best directly calculated valu e for the barrier is equal to 9.62 kcal/mol. These results are in exce llent agreement with recent quantum Monte Carlo calculations.