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 S ae bo and Pulay to pseudos pectral singles and doubles configuration interaction (SDCI) as develo ped by Martinez and Carter. The combination of the localization and ps eudospectral approximations attacks both the problems of two-electron integral storage on disk and CI vector storage in memory that, respect ively, hinder nondirect local spectral and nonlocal pseudospectral SDC I calculations individually and provides a scaling advantage over even direct local spectral SDCI calculations. The reproduction of total en ergies to within a kcal/mol leads to speed increases with respect to n onlocal calculations that grow larger with increasing molecular size: little or no savings for ethane and a factor of 1.1-1.6 for larger mol ecules studied (glyoxal, glycine, C6H2, and C8H2). The prediction of c onformational energy differences with the new method appears quite pro mising, 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 spee d increases for energy difference predictions of both local spectral a nd pseudospectral SDCI also grow with molecular size: from a factor of 4 in ethane and glyoxal to a factor of 6 in glycine. Additionally, wh en compared to the exact spectral result, the fastest local pseudospec tral prediction of the conformational energy difference in glyoxal is iri error by 0.2 kcal/mol and saves a factor of 10 in CPU time, indica ting the prospects of combining local correlation and pseudospectral m ethods. (C) 1996 American Institute of Physics.