Virtual Orbitals in Hartree–Fock Theory. II

The nature of virtual orbitals in the Hartree–Fock theory is discussed. A practical proposal is made to obtain meaningful virtual orbitals in the molecular calculations at the level of Pariser–Parr–Pople-type and all-valence-electron-type approximations. It is achieved by adding appropriate operators to the original Hartree–Fock operator.