Local weak pairs spectral and pseudospectral singles and doubles configuration interaction

A new approximate correlation method has been developed by application of the local weak pairs approximation of Sæbo/ and Pulay to pseudospectral singles and doubles configuration interaction (SDCI) as developed by Martinez and Carter. The combination of the localization and pseudospectral approximations attacks both the problems of two-electron integral storage on disk and CI vector storage in memory that, respectively, hinder nondirect local spectral and nonlocal pseudospectral SDCI calculations individually and provides a scaling advantage over even direct local spectral SDCI calculations. The reproduction of total energies to within a kcal/mol leads to speed increases with respect to nonlocal calculations that grow larger with increasing molecular size: little or no savings for ethane and a factor of 1.1–1.6 for larger molecules studied (glyoxal, glycine, C6H2, and C8H2). The prediction of conformational energy differences with the new method appears quite promising, since energy difference predictions accurate to within a kcal/mol of the exact energy differences are obtained even when the single-point total energies are individually many kcal/mol in error. The speed increases for energy difference predictions of both local spectral and pseudospectral SDCI also grow with molecular size: from a factor of 4 in ethane and glyoxal to a factor of 6 in glycine. Additionally, when compared to the exact spectral result, the fastest local pseudospectral prediction of the conformational energy difference in glyoxal is in error by 0.2 kcal/mol and saves a factor of 10 in CPU time, indicating the prospects of combining local correlation and pseudospectral methods.