A low-scaling method for second order Moller-Plesset calculations

A new low-order scaling algorithm for second-order Moller-Plesset perturbat ion theory (MP2) is described. The method employs localized orbitals. Howev er, in contrast to other local MP2 schemes reported in the literature, all atomic orbitals with significant contribution to the correlation energy for a given pair or orbitals are retained. Our approach introduces no signific ant approximation if all significant pairs are taken into account. However, the method is intended for the description of correlation between spatiall y close ("strong") pairs; for distant pairs, more economical alternative me thods can be used. The algorithm, including a new implementation of the sim ultaneous two-index integral transformation scheme, is described in detail. Test calculations demonstrate asymptotic linear scaling for large systems. Strong-pair MP2 calculations with more that 1800 contracted basis function s performed on a single PC are reported. As a consequence of its lower scal ing, the calculation of the MP2 energy is less expensive than the calculati ons of the Hartree-Fock energy for large systems. This has been demonstrate d for strong pair correlation but is expected to hold also for full MP2 cal culations. (C) 2001 American Institute of Physics.