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

High Resolution Image Download MS PowerPoint Slide 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 I ND ¯ between CCSD and CCSD(T), Δ I ND ¯ [ ( T ) ] = I ND ¯ [ CCSD ( T ) ] − I ND ¯ [ CCSD ] . The second is the ratio of the same and of the corresponding change in the total correlation diagnostic I T ¯ = I ND ¯ + I D ¯, i.e., r I [ ( T ) ] = Δ I ND ¯ [ ( T ) ] / Δ I T ¯ [ ( T ) ] . The first diagnostic can be extended to higher-order improvements in the wave function, e.g., Δ I ND ¯ [ ( Q ) ] = I ND ¯ [ CCSDT ( Q ) ] − I ND ¯ [ CCSDT ] . In general, a small Δ I ND ¯ [level 1 ] value indicates that at this level 1 of theory, the density is converged and any further changes to the energy come from dynamical correlation, while larger Δ I ND ¯ [level 2 ] indicates that the density is still not converged at level 2 and some static correlation remains. r I [( T )] is found to be a moderately good predictor for the importance of post-CCSD(T) correlation effects.