METHODS FOR ELECTRON CORRELATION ON LARGE MOLECULES

High-quality ab initio calculations, i.e., calculations including electron correlations with large atomic basis sets, have been restricted to relatively small systems due to their excessive demand on computer facilities. To extend these methods to large systems, one is faced with two problems: the steep (6 power increase of the computational task with the molecular size, and the large number of electron repulsion integrals that must be stored and retrieved. We have recently introduced new computational methods where both these problems have been addressed. The local correlation method reduces the steep dependency of computational effort with molecular size without significant loss of accuracy by taking advantage of the localizability of the electronic structure. For electron correlation calculations integrals in the MO basis are needed in addition to the two electron integrals in the AO basis, and the only way electron correlation can be applicable to large systems is by a direct approach in the spirit of Almlof's direct SCF. We have recently extended the direct approach to post Hartree-Fock methods. Description of both these methods including their supercomputer implementations are discussed.