INVESTIGATION OF A DIAGNOSTIC FOR PERTURBATION-THEORY - COMPARISON TOTHE T-1 DIAGNOSTIC OF COUPLED-CLUSTER THEORY

A diagnostic for perturbation theory calculations, S-2, is defined and numerical results compared to the established I-1 diagnostic from cou pled-cluster theory. S-2 is the lowest-order non-zero contribution to a Moller-Plesset perturbation expansion of I-1. S-2 is a reasonable es timate of the importance of non-dynamical electron correlation althoug h not as reliable as I-1. S-2 values less than or equal to 0.012 sugge st that low orders of perturbation theory should yield reasonable resu lts; S-2 values between 0.012 and 0.015 suggest that caution is requir ed in interpreting results from low orders of perturbation theory; S-2 values greater than or equal to 0.015 indicate that low orders of per turbation theory are not reliable for accurate results. Although not r equired mathematically, S-2 is always less than I-1 for the examples s tudied here.