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

The definitions of Hartree-Fock operators able to provide virtual orbitals more suited for correlation and excited electronic states and for configuration interaction (CI) treatments have been reviewed. From the comparisons, a simple procedure to improve these operators has emerged and discussed. In our approach, the sign of the pair and total electronic densities is changed to make the interelectron potential attractive for excited electrons. The orbitals generated from the modified operators have been compared to canonical HF orbitals by performing large scale Cl computations on ground and excited states of the NO+ molecule improving the Cl energies and the dipole moments for all states and 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.