A diagnostic for perturbation theory calculations, S 2 , is defined and numerical results compared to the establishedT 1 diagnostic from coupled-cluster theory. S 2 is the lowest-order non-zero contribution to a M ller-Plesset perturbation expansion of T 1 . S 2 is a reasonable estimate of the importance of non-dynamical electron correlation, although not as reliable as T 1 . S 2 values 0.012 suggest that low orders of perturbation theory should yield reasonable results; S 2 values between 0.012 and 0.015 suggest that caution is required in interpreting results from low orders of perturbation theory; S 2 values 0.015 indicate that low orders of perturbation theory are not reliable for accurate results. Although not required mathematically, S 2 is always less than T 1 for the examples studied here.