The 1 Σ + electronic ground states of MgLi + and CaLi + molecular ions are investigated for their spectroscopic constants and properties such as the dipole-and quadrupole moments, and static dipole polarizabilities. The quadrupole moments and the static dipole polarizabilities for these ions have been calculated and reported here, for the first time. The maximum possible error bars, arising due to the finite basis set and the exclusion of higher correlation effects beyond partial triples, are quoted for reliability. Further, the adiabatic effects such as diagonal Born-Oppenheimer corrections are also calculated for these molecules. The vibrational energies, the wavefunctions, and the relevant vibrational parameters are obtained by solving the vibrational Schrödinger equation using the potential energy curve and the permanent dipole moment curve of the molecular electronic ground state. Thereafter, spontaneous and black-body radiation induced transition rates are calculated to obtain the lifetimes of the vibrational states. The lifetime of rovibronic ground state for MgLi + , at room temperature, is found to be 2.81 s and for CaLi + it is 3.19 s. It has been observed that the lifetime of the highly excited vibrational state is several times larger than (comparable to) that of the vibrational ground state of MgLi + (CaLi + ). In addition, a few low-lying electronic excited states of Σ and Π symmetries have been investigated for their electronic and vibrational properties, using EOM-CCSD method together with the QZ basis sets.

We have calculated the ground state spectroscopic constants and the molecular properties, of a molecular ion 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 function. Thus, accurate and reliable results for BeLi + with the most conservative error estimates on them are reported.

The ground -state permanent dipole moments (PDMs) and molecular dipole polarizabilities (DPs) of open-shell alkaline-earth monofluorides, and atomic DPs of alkaline -earth-and fluorine atoms are reported at the Kramers -restricted configuration interaction level of theory limited to single and double excitations (KRCISD), using the finite -field approach. Sufficiently large basis sets such as quadrupole -zeta (QZ) and augmented -QZ basis sets together with the generalized active space technique is employed to carry out the field dependent energy calculations at the KRCISD level. The PDMs and the components of DPs are extracted from the linear-and quadratic fit of energies against perturbative electric field, respectively. Accuracy of the present calculations for the electronic properties is examined by comparison with the measurements and calculations where ever available.

Molecular rotational transition frequencies are useful as frequency standard in the terahertz region. The transition frequencies of QH+ molecular ions (Q: an even isotope of Group II element) in a linear trap are useful as measurement uncertainties are low. An uncertainty given by the Stark and Zeeman shifts can be below 10−15 using 202HgH+ molecular ion.

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