Natural orbitals as substitutes for optimized orbitals in complete active space wavefunctions

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.