International journal of quantum chemistry vol:65 issue:1 pages:37-48
The traditional description of pseudo-Jahn-Teller molecular configuration instability based on Bader's formula for the curvature of the adiabatic potential is reconsidered in order to make it consistent with straightforward calculations of the ground-state energy surface within Hartree-Fock-Roothaan approximation The proposed approach employs floating molecular orbitals constructed by ''frozen LCAO'' coefficients (computed in the reference geometry) and floating atomic orbitals, which allows one to exclude the vibronic mixing with the excited states irrelevant from chemical point of view. The relaxation (vibronic) contribution to the instability, expressed in terms of one-electron quantities, involves two sets of orbital vibronic constants. These are, respectively, defined as matrix element derivatives, of correct and ''frozen LCAO'' Fock operators between occupied and unoccupied molecular orbitals. The canonical form of the relaxation contribution can be achieved when the vibronic interaction is presented as a mixing between the ground electronic state and the excited states calculated within the random-phase approximation. (C) 1997 John Wiley & Sons, Inc.