``Natural Orbitals As Substitutes for Optimized Orbitals in Complete Active Space Wavefunctions,'' M. L. Abrams and and C. D. Sherrill, Chem. Phys. Lett. 395, 227-232 (2004).

Complete-active-space self-consistent-field (CASSCF) orbitals are computationally expensive and are sometimes difficult to converge. We assess complete-active-space configuration interaction (CASCI) in a basis of natural orbitals as a less expensive alternative to CASSCF. Natural orbitals are generated from various single-reference wavefunctions. The approach is applied to bond breaking in methyl fluoride and ethylene. With natural orbitals from correlated wavefunctions, CASCI parallels CASSCF potential curves, and coupled-cluster singles and doubles natural orbitals give nonparallelity errors of only 1-3 kcal mol-1 even for a very large active space in methyl fluoride or double bond breaking in ethylene.