ACCURATE QUANTUM CALCULATIONS OF THERMAL RATE CONSTANTS EMPLOYING MCTDH - H-2-]H+H2O AND D-2+OH-]D+DOH(OH)

An improved approach for the direct calculation of thermal rate consta nts, employing the multi-configurational time-dependent Hartree (MCTDH ) approach and the flux-flux correlation function, is presented. In th is approach, the thermal flux operator is diagonalized and its eigenst ates are propagated. The cumulative reaction probability is obtained f or all energies simultaneously by a simple Fourier transform of the ma trix elements between the propagated flux eigenstates. Calculations fo r the H-2+OH-->H+H2O reaction, which include all six internal degrees of freedom accurately, demonstrate the accuracy and efficiency of the approach. Previous results for the rate constant are reproduced (for t emperatures between 300 K and 600 K) and extended towards lower temper atures (200 K to 300 K). Also, thermal rate constants of the D-2+OH--> D+DOH reaction have been computed. All six internal degrees of freedom are included accurately in this calculation. The kinetic isotope effe ct is obtained and compared to experiment. (C) 1998 American Institute of Physics.