A diagnostic for determining the quality of single‐reference electron correlation methods

It was recently proposed that the Euclidian norm of the t 1 vector of the coupled‐cluster wave function (normalized by the number of electrons included in the correlation procedure) could be used to determine whether a single‐reference‐based electron correlation procedure is appropriate. This diagnostic T 1 is defined for use with self‐consistent‐field molecular orbitals and is invariant to the same orbital rotations as the coupled‐cluster energy. T 1 is investigated for several different chemical systems which exhibit a range of multireference behavior and is shown to be an excellent measure of the importance of nondynamical electron correlation and is far superior to C 0 from a singles and doubles configuration interaction wave function. It is further suggested that when the aim is to recover a large fraction of the dynamical electron correlation energy, a large T 1 (i.e., >0.02) probably indicates the need for a multireference electron correlation procedure.