An improved approach for the direct calculation of thermal rate constants, employing the multi-configurational time-dependent Hartree (MCTDH) approach and the flux-flux correlation function, is presented. In this approach, the thermal flux operator is diagonalized and its eigenstates are propagated. The cumulative reaction probability is obtained for all energies simultaneously by a simple Fourier transform of the matrix elements between the propagated flux eigenstates. Calculations for 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 temperatures between 300 K and 600 K) and extended towards lower temperatures (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 effect is obtained and compared to experiment.