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Theoretical calculations with spin orbit effects of the diatomic molecules YS, YN, ZrS, ZrN

Authors: Ayman Farhat

Ref.: PhD Thesis, Université Claude Bernard - Lyon I (2012)

Abstract: This dissertation is dedicated to the ab initio study of the electronic structures of the polar diatomic molecules YN, YS, ZrN, and ZrS. The identification of these molecules in the spectra of stars as well as the lack in literature on the electronic structures of these molecules motivated the present study. Theoretical calculations are useful in this respect since they can provide important data for the properties of the ground and excited electronic states that are not available from experimental means. In the present work the ab initio calculations were performed at the complete active space self-consistent field method (CASSCF) followed by multireference single and double configuration interaction method (MRSDCI). The Davidson correction noted as (MRSDCI+Q) was then invoked in order to account for unlinked quadruple clusters. The calculations were performed on two stages in the first spin orbit effects were neglected while in the second type of calculations spin orbit effects were included by the method of effective core potentials. All of the calculations were done by using the computational physical chemistry program MOLPRO and by taking advantage of the graphical user interface Gabedit. In the present work potential energy curves were constructed and spectroscopic constants computed, along with permanent electric dipole moments, internal molecular electric fields, and vibrational-rotational energy structures. We detected in the ZrS molecule several degenerate vibrational energy levels which can be used to search for possible variations of the fine structure constant α and the electron to proton mass ratio μ in three S-type stars, named Rand, RCas, and χCyg. A comparison with experimental and theoretical data for most of the calculated constants demonstrated a good accuracy for our predictions giving a percentage relative difference that ranged between 0.1% and 10%. Finally, we expect that the results of the present work should invoke further experimental investigations for these molecules.