Direct construction of diabatic states in the CASSCF approach. Application to the conical intersection of the ¹A₂ and ¹B₁ excited states of ozone

A generally applicable scheme for the construction of diabatic electronic wavefunctions in the CASSCF/MRCI approach is proposed. It is noted that the redundancy of variational parameters in the CASSCF wavefunction can be exploited to define daibatic molecular orbitals. A maximum-overlap criterion at neighbouring nuclear goemetries is employed. In a second step. diabatic N-electron wavefunctions which retain as much as possible their configurational character are constructed by blocked-diagonalization of the Hamiltonian matrix. The procedure is illustrated to the ¹A₂-¹B₁ conical intersection in ozone.