Direct restricted‐step MCSCF calculations on the structure and spectrum of cyclobutadiene

The structure of cyclobutadiene in the ground ¹A₉ state and the lowest excited ³B_1g, ¹A_1g, and ¹B_1g, states are investigated by means of a direct, step‐restricted, second order MCSCF method. The barrier height for interconversion between the rectangular forms of the ground state minima is predicted to be 6.2kcal/mol and the singlet‐triplet state separation at the ground state square confirmation is predicted to be 9.7 kcal/mol. All investigated excited states are fond to have square equilibrium geometry the and adiabatic transition energy to the ¹A_1g, state is predicted to be lower than that to the ¹B_1g, state.