LAUSR

Abstract We have calculated the spectroscopic constants and the molecular properties, for the ground-state (X  1 Σ + ) of BeLi+, such as dipole moment, quadrupole moment and dipole polarizability at different levels of correlation: many-body perturbation theory (MP2), coupled cluster method with single and double excitations (CCSD) and CCSD with perturbative triples (CCSD(T)). The correlation consistent polarized valence cc-pVXZ (X = D, T, Q) basis sets and also their augmented counterparts are used together with the non-relativistic and relativistic Hamiltonians. The results are extrapolated to the complete basis set limit (CBS) using exponential - Gaussian functions. Thus, accurate and reliable results for BeLi+ with the maximum possible error bars on them are reported. The effects of diagonal Born-Oppenheimer corrections (DBOC) and of electron correlations beyond the CCSD(T) level of approximation are also estimated. In addition, by solving the vibrational Schrodinger equation using the potential energy curve obtained at CCSD(T)/QZ level, we have calculated the vibrational parameters such as energies, rotational constants, permanent dipole moments, spontaneous and black-body radiation (BBR) induced transition rates between different vibrational levels of the electronic ground state. The lifetime of the lowest rovibrational state ( v = 0 and J = 0 ) at room temperature, is found to be 4.67 s . Further, the electronic and the vibrational parameters of a few low-lying electronic excited states: 1  3 Σ + , 1  3 Π , 2–4  1 Σ + , and 1–2  1 Π are calculated using the EOM-CCSD/QZ method. The dipole moments for the electronic transitions from the ground- to the excited singlet states are also reported.