Theoretical study on low-lying states of Ga2X (X = P, As) with coupled-cluster approaches.

Low-lying states of Ga2P and Ga2As are investigated with the equation-of-motion coupled-cluster approach for ionized states at the singles and doubles level (EOMIP-CCSD) as well as at the CCSDT-3 level together with CCSD, CCSD(T), and DFT. Except for the asymmetric stretching b2 mode of the (2)B2 and (2)A1 states, all these approaches provide structures, frequencies and adiabatic electron affinities that are in reasonable agreement with each other. According to our results, the lowest-energy state of these two molecules is the (2)A' state of C(s) symmetry and the (2)B2 state is the ground electronic state with C(2v) symmetry. As for the b2 mode, CCSD and CCSD(T) afford real frequencies for the (2)B2 state, while EOM approaches and DFT with most exchange-correlation functionals give rise to imaginary frequencies. The (2)B2 and (2)A1 states couple with each other due to distortion along b2 mode through the pseudo-Jahn-Teller effect. Analysis on results shows that EOM approaches afford reasonable b2 frequencies for the two states and DFT approaches, except for BP86 and PBE, provide qualitatively correct b2 frequencies for the (2)B2 state. In addition, a potential matrix is introduced to describe the vibronic coupling between the (2)B2 and (2)A1 states and parameters in the matrix are fitted to the adiabatic potential curves from EOMIP-CCSD results.