A number of methods have been developed to account for electron correlation. With the exception of so-called density functional methods(see discussion following), these generally involve mixing the ground- state(Hartree-Fock) wavefunction with excited-state wavefunctions. Operationally, this entails implicit or explicit promotion of electrons from molecular orbtials which are occupied in the Hartree-Fock wavefunction to molecular orbitals which are unoccupied