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

Abstract

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 ²B₂ and ²A₁ 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 ²A′ state of C_s symmetry and the ²B₂ state is the ground electronic state with C_2v symmetry. As for the b₂ mode, CCSD and CCSD(T) afford real frequencies for the ²B₂ state, while EOM approaches and DFT with most exchange–correlation functionals give rise to imaginary frequencies. The ²B₂ and ²A₁ 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 ²B₂ state. In addition, a potential matrix is introduced to describe the vibronic coupling between the ²B₂ and ²A₁ states and parameters in the matrix are fitted to the adiabatic potential curves from EOMIP-CCSD results.