On the importance of third- and fourth-order corrections in multi-reference Moller-Plesset theory

An implementation and applications of an approximate fourth-order multi-ref erence Moller-Plesset theory restricted to single and double excitations (M R-MP4(SD)) are presented. The results for small full CI benchmark systems i ndicate a substantial improvement compared to MR-MP2 and MR-MP3 treatments. For larger molecules we propose a configuration selection procedure to kee p the size of the expansion spaces within the limits of personal computer c apabilities. As typical applications we consider vertical excitation energi es of unsaturated organic molecules (up to 66 correlated electrons) and rea ction energies invoking biradicals. It is found that the systematic under(o ver)estimation of relative energies of open- Versus closed-shell states at MR-MP2(MR-MP3) levels is in most cases corrected at fourth-order. (C) 2001 Elsevier Science B.V. All rights reserved.