A new diagnostic for open-shell coupled-cluster theory

We present a new diagnostic for open-shell coupled-cluster theory, readily computed from the single substitution amplitudes in the CCSD wavefunction. The new diagnostic, D1(ROCCSD), is designed to be comparable to the previously proposed D1(CCSD) diagnostic. Unlike other approaches, the D1 diagnostics are independent of system size and have the same invariance properties as the energy with respect to orbital rotations. Calibration of the D1(ROCCSD) diagnostic on 34 molecular systems indicates that for values of D1(ROCCSD) of 0.025 or below the quality of the CCSD results are, in general, excellent, whereas values larger than 0.025 signal inadequacies in the CCSD approach.