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 CAS-SCF calculations.