Half‐projected Hartree–Fock natural orbitals for defining CAS–SCF active spaces

We 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.