Multi-reference Moller-Plesset theory: computational strategies for large molecules

We propose computational strategies and algorithms to perform multi-referen ce Moller-Plesset (MR-MP2) calculations efficiently for large molecules. As zeroth-order reference we employ restricted configuration interaction wave functions expressed in terms of an active space of Hartree-Fock one-partic le functions (RAS-CI). To accelerate the convergence of the perturbation ex pansion and to keep the zeroth-order spaces as small as possible (i.e. Dim < 1000) we use improved (average) virtual orbitals. The length of the first -order space (single and double excitations with respect to all reference c onfigurations) is reduced by selecting the most important configurations fr om the full space based on the magnitude of their H-0 diagonal matrix eleme nt. The two-electron integrals in the MO basis are calculated semi-directly with the resolution of the identity (RI) method which avoids computational ly demanding 4-index transformations. The errors introduced by the approxim ations can systematically be reduced and are found to be insignificant in a pplications to chemical problems. As examples we present MR-MP2 results for excitation and reaction energies of molecules for which single-reference p erturbation theory is not adequate. With our approach, investigation of sys tems as large as porphin or C-60 are possible on low-cost personal computer s.