HARTREE-FOCK OPERATORS TO IMPROVE VIRTUAL ORBITALS AND CONFIGURATION-INTERACTION ENERGIES

The definitions of Hartree-Fock operators able to provide virtual orbi tals more suited for correlation and excited electronic states and for configuration interaction (CI) treatments have been reviewed. From th e comparisons, a simple procedure to improve these operators has emerg ed and discussed. In our approach, the sign of the pair and total elec tronic densities is changed to make the interelectron potential attrac tive for excited electrons. The orbitals generated from the modified o perators have been compared to canonical HF orbitals by performing lar ge scale Cl computations on ground and excited states of the NO+ molec ule improving the Cl energies and the dipole moments for all states an d the convergence properties of Cl. Similarly, using truncated orbital subspaces, the ground state MP2 correlation energy becomes closer to the basis limit for this property.