Theoretical study of the electronic states of CuCl2.

The electronic states of the CuCl(2) molecule are studied by several theoretical methods. We report geometries, excitation energies, vibrational frequencies, rotational constants, and transition dipole moments. With the purpose to describe the correlation energy accurately enough, a set of diffuse secondary 3d(') orbitals is introduced, thus resulting in a large active space of 21 electrons in 17 orbitals. By restricting the active space and selecting dominant configurations, the results of the general multireference second-order perturbation theory with this large active space agree very well with the experimental ones. It is found that the so-called (2)Pi(u) state is asymmetric linear and the (2)Sigma(u)(+) state is bent at the minima on their adiabatic potential energy surfaces, whereas the other five gerade states are centrosymmetric linear. After including the spin-orbit coupling, the (I)(2)Pi(g3/2)-(I)(2)Pi(g1/2) splitting is computed to be 415 cm(-1), in excellent agreement with the experimental value of about 480 cm(-1).