Hyperfine and P, T odd properties in BiO: comparison between coupled-cluster method and multi-reference perturbation method based on a Dirac Hamiltonian

Polar diatomic molecules are attractive in the search for the electron electric dipole moment (eEDM) and the scalar-pseudoscalar (S-PS) interaction, both of which violate time-reversal and parity symmetries. In this study, we examined the electronic ground state of BiO and evaluated the effective electric field (<i>E</i>_eff) of eEDM and the <i>W</i>_s coefficients of the S-PS interaction. BiO forms a complex chemical bond due to the open-shell configurations of Bi (6<i>p</i>) and O (2<i>p</i>). Consequently, we performed four-component relativistic calculations using complete-active-space second-order perturbation theory (CASPT2), as well as coupled-cluster singles and doubles (CCSD) and CCSD perturbative triples (CCSD(T)) methods. Our analysis revealed that BiO exhibited a multiconfigurational character, as the Hartree-Fock configuration accounted for only 68% of the reference wave function for CASPT2. Nevertheless, for the hyperfine coupling constant (A_//), CCSD and CCSD(T) reproduced the experimental value better than CASPT2, indicating that CC methods can capture important excited configurations, rendering multireference treatment unnecessary. The deficiency of CASPT2 in reproducing A_// could be attributed to the inadequate treatment of orbital relaxation effects. Our proposed values of <i>E</i>_eff and <i>W</i>_s for BiO (17 GV/cm and −36 kHz, respectively), derived at the CCSD level, were moderately large for the parity, time-odd experiment.