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. © 1996 American Institute of Physics. @S0021-9606~96!02639-6#