The pseudo-Jahn–Teller
(pJT) effect in monocyclic and bicyclic
conjugated molecules was investigated by using the state-averaged
multiconfiguration self-consistent field (MCSCF) method, together
with the 6-31G(d,p) basis
sets. Following the perturbation theory, the force constant along
a normal mode Q is given by the sum of the classical
force constant and the vibronic contribution (VC) resulting from the
interaction of the ground state with excited states. The latter is
given as the sum of individual contributions arising from vibronic
interactions between the ground state and excited states. In the present
work, each VC was calculated on the basis of nonadiabatic coupling
(NAC) integrals. Furthermore, the classical force constant was estimated
by taking advantage of the VC and the force constant obtained by vibrational
analyses. For pentalene and heptalene, the present method seems to
overestimate the VC in absolute value because of the small energy
gap between the ground state and the lowest excited state. However,
we are confident that the VC and the classical force constant for
the other molecules are reasonable in magnitude in comparison with
available literature information. Thus, it is proved that the present
method is applicable and useful for numerical estimation of pJT effect.