Following the pioneering work of Jurgens-Lutovsky and Almlof [Chem. Phys. Lett. 178, 451 (1991)], a second-order Moller-Plesset program was developed which allows the use of a large basis set for the pair correlation functions and a more modest one for the self-consistent field (SCF) orbitals. For several test systems, correlation energies closely approximate the results of a large basis set calculation, at substantial savings. The SCF energy of the large basis set calculation can also be estimated using perturbation theory. (C) 2003 American Institute of Physics.