Half-Projected Hartree-Fock Natural Orbitals for Defining Cas Scf Active Spaces
by Bone, R. G. A.; Pulay, P.
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.
- Journal
- International Journal of Quantum Chemistry
- Volume
- 45
- Issue
- 2
- Year
- 1993
- Start Page
- 133-166
- URL
- https://dx.doi.org/10.1002/qua.560450203
- ISBN/ISSN
- 1097-461X; 0020-7608
- DOI
- 10.1002/qua.560450203