First-order Degenerate Symmetry-Adapted Perturbation Theory
Degenerate electronic states govern the photophysics of excimers and the molecular dynamics near conical intersections. In regions where potential energy surfaces are degenerate or nearly degenerate, intermolecular interactions mix the corresponding electronic configurations. We introduce a first-order degenerate formulation of symmetry-adapted perturbation theory (dSAPT) with weak symmetry forcing that quantifies the mixing in terms of electrostatic and exchange (Pauli repulsion) contributions. We compare two alternative ways of resolving the degeneracy, in which antisymmetry is enforced either after or before diagonalization of the first-order eigenproblem. Using CASSCF monomer wave functions, we apply dSAPT to two model spatially degenerate systems, H$_2$--F(${}^2P$) and H$_2$--NO(${}^2Π$), and show that exchange effects contribute significantly to configuration mixing already at intermediate intermolecular separations. For the H$_2$--NO(${}^2Π$) complex, we further demonstrate that second-order dispersion energy is essential for qualitatively reproducing the angular dependence of the diabatic mixing angle. Finally, using water and benzene excimers with monomers described either by CASSCF or CIS wave functions, we show that Pauli repulsion controls delocalization of the excitation at short intermolecular separations.