INTERPRETATION OF EXCITED-STATE HARTREE-FOCK ANALYTIC DERIVATIVE ANOMALIES FOR NO2 AND HCO2 USING THE MOLECULAR-ORBITAL HESSIAN

A simple procedure utilizing the molecular orbital (MO) Hessian, the second derivative of the Hartree-Fock (HF) energy with respect to MO coefficient perturbations, has been proposed in the interpretation of anomalous analytic HF energy derivatives. The nature of the anomalous analytic self-consistent field (SCF) harmonic vibrational frequencies and infrared (ir) intensities of the B-2(2) and 2A2, excited states of NO2 and HCO2 have been studied in detail. By suitable partitioning of the HF second energy derivative expression, an association between the abnormal force constants, the dipole moment derivatives, and specific solutions to the SCF coupled perturbed HF equations is established. This connection to unstable wave functions can now be used with the MO Hessian to predict these anomalies at the HF level. In addition, appropriate small complete active space SCF (CASSCF) methods have been utilized to overcome the inherent short-comings of the HF wave functions, and determine more realistic values for the force constants and ir intensities.