First principle calculations of dipole-dipole dispersion coefficients for the ground and first π → π* excited states of some azabenzenes

Abstract

The complex polarization propagator method has been applied to the calculation of dipole-dipole dispersion coefficients (also known as $C_6$ coefficients) of pyridine, pyrazine, and s-tetrazine. These calculations refer to the electronic ground states as well as the first excited states of π → π* character. It is argued that accurate ground state dispersion coefficients are obtained with density functional theory using the B3LYP exchange-correlation functional. The proposed values for the $C_6$ coefficients of pyridine, pyrazine, and s-tetrazine in their ground states are 1543 a.u., 1398 a.u., and 1014 a.u., respectively. Multi-configurational complete active space calculations are performed on these compounds in their respective π → π* excited state. The isotropic averages of the frequency-dependent polarizabilities are smaller in the excited states, but the effective frequencies - defined in the London - van der Waals dispersion relation - are on the other hand larger.