The equilibrium geometry and 24 vibrational-normal-mode frequencies of the excited state S 2 ( 1 B 2u ) of pyrazine are calculated and characterized using the complete active space self-consistent field method in the adiabatic representation. The displaced harmonic oscillator approximation is used to simulate the absorption spectrum of the S 2 ( 1 B 2u ) state along with the Franck-Condon approximation. It is found that the totally symmetric mode ν 1 plays the most important role and this exactly agrees with the experimental observations. The simulated absorption spectrum agrees well with those experimentally observed. This indicates that the present S 2 ( 1 B 2u ) state calculated in the adiabatic representation effectively includes contribution from the diabatic vibronic coupling through the conical intersection.