A method of calculating the excited state MCSCF electronic wave function by constraining it to be orthogonal to the predetermined ground state wave function was applied to calculate wave functions of the Li–H2 system near the center of a conical intersection of the lowest two potential energy surfaces. Combined with an additional nonorthogonal configuration interaction process, it has proved to be an efficient method to get systems of wave functions that are continuous throughout whole geometries. Qualitative characters of wave functions near the center of potential crossing previously inferred have been reconfirmed.

MCSCF calculations were performed on the two lowest states of the Li-H2 system, using a basis set of double-zeta-plus-polarization quality. The characters of the wave functions in the region of the conical intersection of the potential energy surfaces were investigated in some detail. The depth of the minimum on the A2B2 surface was estimated to be 0.46 eV. The frequency of the a , vibration at the center of the conical intersection was 3.76 X loi3 sec-I. Under the symmetry of the nuclear geometry lower than Clv in the immediate neighborhood of the center of the intersection, the upper state was characterized by a 3a' natural orbital by which the more distant H is bonded to Li, while in the lower state the nearer H is bonded to Li. Thus, the process of the nonadiabatic transition caused by a nuclear motion of b2 symmetry is visualized as a smooth bond-preserving process, and is expected to occur with high probability. For the correct description of the states in this region, MCSCF functions comprising limited CSF'S were inadequate, and an additional process of nonorthogonal CI was required.

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