PSEUDOSPECTRAL LOCALIZED GENERALIZED MOLLER-PLESSET METHODS WITH A GENERALIZED VALENCE-BOND REFERENCE WAVE-FUNCTION - THEORY AND CALCULATION OF CONFORMATIONAL ENERGIES

We describe a new multireference perturbation algorithm for ab initio electronic structure calculations, based on a generalized valence bond (GVB) reference system, a local version of second-order Moller-Plesse t perturbation theory (LMP2), and pseudospectral (PS) numerical method s. This PS-GVB-LMP2 algorithm is shown to have a computational scaling of approximately N-3 with basis set size N, and is readily applicable to medium to large size molecules using workstations with relatively modest memory and disk storage. Furthermore, the PS-GVB-LMP2 method is applicable to an arbitrary molecule in an automated fashion (although specific protocols for resonance interactions must be incorporated) a nd hence constitutes a well-defined model chemistry, in contrast to so me alternative multireference methodologies. A calculation on the alan ine dipeptide using the cc-pVTZ(-f) basis set (338 basis functions tot al) is presented as an example. We then apply the method to the calcul ation of 36 conformational energy differences assembled by Halgren and co-workers [J. Comput. Chem. 16, 1483 (1995)], where we obtain unifor mly good agreement (better than 0.4 kcal/mole) between theory and expe riment for all test cases but one, for which it appears as though the experimental measurement is less accurate than the theory. In contrast , quadratic configuration interaction QCISD(T) calculations are, surpr isingly, shown to fail badly on one test case, methyl vinyl ether, for which the calculated energy difference is 2.5 kcal/mole and the exper imental value is 1.15 kcal/mole. We hypothesize that single reference methods sometimes have difficulties describing multireference characte r due to low lying excited states in carbon-carbon pi bonds. (C) 1997 American Institute of Physics.