Abstract
AbstractWe have extended the range of systems to which the half‐projected Hartree–Fock (HPHF) method has been applied, including the triplet state of the wave function. In our implementation, DIIS overcomes the convergence difficulties reported in earlier studies. HPHF allows generation of a symmetry‐broken wave function in regions of the potential energy surface where the RHF wave function is triplet‐stable. The fractionally occupied natural orbitals (FONOS) of the HPHF wave function are good starting vectors for CAS–SCF calculations. A CAS–SCF in the space defined by the HPHF FONOS should be used instead of the unrestricted natural orbital CAS–SCF method in regions of triplet stability and for small active space problems. We draw extensive comparisons between the results of both the UNO–CAS and HPNO–CAS methods and those of full CAS–SCF calculations. © 1993 John Wiley & Sons, Inc.
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