Multiconfiguration self-consistent field and multireference configuration interaction methods and applications

Recent developments on gradient theory, calculation of molecular properties, nonadiabatic coupling between electronic states, and relativistic and spin-orbit effects, are discussed. Shepard et al. proposed the subspace projected approximate matrix (SPAM) method that employs a sequence of one or more approximations to the H matrix. Duch and Diercksen compared the formulas and concluded that the DuchDiercksen and Pople corrections clearly outperform the original Davidson correction and its renormalized variants. Tanaka and co-workers developed multireference coupled pair approximation (MRCPA) method that is size-consistent for noninteracting electron pairs and can be applied to excited states, but analytic gradient calculations are not available. Chan and Head-Gordon used a reverse CuthillMcKee reordering of the orbitals to make the one-electron integral matrix close to band-diagonal.