NEW DIAGNOSTICS FOR COUPLED-CLUSTER AND MOLLER-PLESSET PERTURBATION-THEORY

We here present new diagnostics for coupled-cluster and Moller-Plesset perturbation theory, readily computed from the single substitution am plitudes in the coupled-cluster singles and doubles wavefunction or in the second-order Moller-Plesset wavefunction. The new diagnostics D-1 (CCSD) and D-1(MP2) have advantages over the previously proposed J(1) and Y-2 diagnostics. A clear correlation between the size of the diagn ostics and the performance of coupled-cluster and second-order Moller- Plesset perturbation theory is demonstrated for computation of optimum geometries and harmonic vibrational frequencies for a series of 29 sm all molecules. (C) 1998 Published by Elsevier Science B.V. All rights reserved.