GENERALIZED MOLECULAR-ORBITAL THEORY .2.

The generalized molecular orbital (GMO) concept is extended to a highe r order method, which begins with a pair-excited multiconfiguration se lf-consistent field (PEMCSCF) for the orbital optimization and is foll owed by a multireference configuration interaction calculation. Here, this method is referred to as GMO2. The method has the advantage of be ing variational, of handling large numbers of active electrons, and of only needing the user to specify the number of active electrons and o rbitals without specifying a dominant MO or VB configuration. In this paper, we briefly review the PEMCSCF theory, describe in more detail a new and more efficient optimization procedure, and propose determinin g the energy with configuration interaction (CI) at the single, double , triple, and quadruple-excitation levels (SDTQ) as a replacement for the full CI, which is needed in a complete active space (GAS) method. Several examples of the application of the method are investigated: me thane, tetrahydrogen, benzene, dinitrogen dissociation, acetylene diss ociation. For the five systems studied, the PEMCSCF orbital optimizati on produces orbitals that only differ from those of a CASSCF by an ave rage of 4 kcal/mol when both localized bond type or symmetry-adapted o rbitals are used in a full CI, GMO2(FCI). The additional error of repl acing full CI with a SDTQ CI, GMO2(SDTQ), is usually less than 1 kcal/ mol.