Potential energy curves for the lowest electronic states of LaB and LaB − have been calculated by ab initio calculations. The diffusion Monte Carlo method has been employed in combination with three different trial functions. Spectroscopic constants have also been numerically derived for the neutral molecule and compared with the only available theoretical work; [19] however, predictions are provided for the corresponding constants for the anionic species which have not been reported yet. Our calculations suggest the high spin quintet state of LaB as the ground state with the triplet state higher in energy irrespective of the type of the functional used. This suggestion is in good accordance with the previous theoretical results calculated at B3LYP/LANL2DZ level of theory, whereas it contradicts with the prediction based upon B3LYP/SDD calculations in the same study. Moreover, variations of the permanent dipole moments as a function of the internuclear separations for the two electronic states of the neutral molecule have been studied and analyzed.

Our study aim is to make highlights on the failure or success of both popular B3LYP functional and long-range corrected LC-BLYP functional at different values of the range separation parameter in improving the ground state and excited states energies calculations for 4f-lanthanides and 5f-actinides. The most popular B3LYP functional fails to provide a balanced description of excitation energies for 4f-lanthanides. However, it gives reasonable results for the actinides with exception of Pu and Am atoms. The ground state energies of 5f-actinides are improved significantly at the critical value of the range separation parameter equals 0.35 a.u. while the optimal value for the excited states lies in the critical range of 0.35-0.4 a.u. However, our results show that LC-BLYP functional is not recommended for 4f-lanthanides which have strongly localized nature.

The completely unexplored LaP molecule is investigated by ab initio methods. Potential energy curves for the low-lying states of LaP are constructed by means of the diffusion Monte Carlo method combined with three different trial functions. Spectroscopic constants are also numerically derived and the ground state is assigned, looking forward to experimental comparisons. Moreover, variations of the permanent dipole moments as a function of the internuclear separation for the two lowest states of the diatomic LaP are studied and analyzed.

ABSTRACT. This paper demonstrates the ability of extending the QMC method for lanthanides and obtaining highly accurate results.

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