New diagnostics for coupled-cluster and Møller–Plesset perturbation theory

We here present new diagnostics for coupled-cluster and Møller–Plesset perturbation theory, readily computed from the single substitution amplitudes in the coupled-cluster singles and doubles wavefunction or in the second-order Møller–Plesset wavefunction. The new diagnostics D1(CCSD) and D1(MP2) have advantages over the previously proposed T1 and S2 diagnostics. A clear correlation between the size of the diagnostics and the performance of coupled-cluster and second-order Møller–Plesset perturbation theory is demonstrated for computation of optimum geometries and harmonic vibrational frequencies for a series of 29 small molecules.