Coupled ab initio potential energy surfaces for the two lowest 2A′ electronic states of the C2H molecule

Realistic two-valued potential energy surfaces for the reaction C(P-3) + CH(X(2)Pi) --> C-2 + H have been constructed from a set of high level ab initio data describing the first two (2)A' electronic states of the C2H system. These states have linear equilibrium configurations, known as the X(2)Sigma (+) and A(2)Pi states, and are coupled by a conical intersection. They lead to the formation of C-2(X(1)Sigma (+)(g)) and C-2(a(3)Pi (u)) considering an adiabatic dissociation process. The ab initio calculations are of the multireference configuration interaction variety and were carried out using a polarized triple-zeta basis set. Using the ab initio adiabatic energies and the matrix elements of the dipole moment, a 2 x 2 diabatic representation of the electronic Hamiltonian was built. Each element of this Hamiltonian matrix was expressed within the double many-body expansion (DMBE) scheme which is based, in this case, on the extended Hartree-Fock approximate correlation energy model (EHFACE). The analytical adiabatic potential energy surfaces are then obtained as the eigenvalues of this matrix, and display correctly the Sigma/Pi conical intersection. Moreover, the non-adiabatic couplings given by our analytical model are compared with the ab initio ones, and good qualitative agreement is observed.