A multiconfigurational approach of the symmetry breaking problem in the cyclic C3H radical

The ground state of the cyclic C3H radical is prone to symmetry breaking due to a peculiar structure that require to distribute three electrons on two centers which are equivalent by symmetry. A pseudo Jahn-Teller effect is also observed due to the non-adiabatic interaction with the first excited state. The multiconfigurational self-consistent-field method is used to calculate wavefunctions free from symmetry breaking. The weakening of the pseudo Jahn-Teller effect is observed when the correlation energy is increased. The equilibrium geometry and harmonic vibrational frequencies are calculated with the multireference configuration interaction method and a good agreement with experimental results is obtained. The present work confirms that the ground state of the cyclic C3H has the C2v symmetry.