ENERGY DERIVATIVE VERSUS EXPECTATION VALUE APPROACH - THE DIPOLE-MOMENT OF CO

The quality of first-order properties as energy derivatives or expectation values is investigated in calculations on the electric dipole moment of CO, employing averaged coupled pair functional (ACPF) and multireference single and double excitation configuration interaction (MRSDCI) methods. A careful optimization of the molecular orbitals in a multiconfiguration self-consistent-field (MCSCF) procedure turns out to be crucial for the reliability of the ACPF and MRSDCI energy derivatives. If the orbitals are not consistent with the electron correlation treatment the gradient corrections tend to overshoot the actual value and meaningless results can be obtained for the one-electron property. In these cases the expectation value approach may still give reasonably good results.