Toward an Affordable Density-Based Measure for the Quality of a Coupled Cluster Calculation.

We propose two new diagnostics for the degree to which static correlation impacts the quality of a coupled cluster calculation. The first is the change in the Matito static correlation diagnostic between CCSD and CCSD(T), . The second is the ratio of the same and of the corresponding change in the total correlation diagnostic , i.e., . The first diagnostic can be extended to higher-order improvements in the wave function, e.g., . In general, a small [level] value indicates that at this level of theory, the density is converged and any further changes to the energy come from dynamical correlation, while larger [level] indicates that the density is still not converged at level and some static correlation remains. [()] is found to be a moderately good predictor for the importance of post-CCSD(T) correlation effects.