In this work we have investigated the electronic properties of several types of molecular systems involving a metallic element. Our motivation for such applications on metallic compounds was to obtain an accurate description of close lying electronic states, in which the relativistic effects of heavy atoms are known to be important. Thus various approaches and methods have been employed to treat these effects, including the multi-configurational method, with atomic pseudopotentials and large basis sets. In the first study, we have determined the properties of the low lying electronic states of the diatomic compounds MX, whose combinations in the solid phase produce ionic semi-conductor materials with piezoelectric properties. Based on highly correlated ab initio calculations, we have elucidated the common properties of the low lying electronic states of these diatomic compounds with eight valence electrons, which can be considered as precursors for piezoelectric effects in their solid phase. Based on our electronic structure calculations, we could identify among these diatomic compounds those who could lead to good candidates for piezoelectric effects. As the second application, we have determined the electronic structure and the spectroscopic constants for the ground state of the HZnF molecule and for the low lying electronic states of its diatomic fragments. This application was initiated and motivated by interesting and puzzling results on the close system HZnCl. Comforted by our experience with the previous studies, we used the pseudopotentials approach to obtain an accurate description of the low lying states of ZnH which could be satisfactorily compared with existing data. Next, the ZnF and ZnCl diatomic molecules have been studied with the same ansatz to reveal the properties of so far unknown electronic states. Finally, the potential energy surface of the ground state of HZnF has been determined, and several spectroscopic properties have been deduced

Dans cette étude, nous avons déterminé les propriétés électroniques de plusieurs types de composés moléculaires possédant un élément métallique. Notre motivation pour l'étude de tels systèmes était de montrer qu'il était possible d'obtenir une description précise d'états électroniques très proches en énergie, pour lesquels il est connu que les effets relativistes jouent un rôle important. Pour traiter ces effets nous avons mis en oeuvre différentes approches et méthodes, en particulier des méthodes multiconfigurationnelles, des pseudopotentiels atomiques et de grandes bases de fonctions. Dans une première étude nous avons déterminé les propriétés des états électroniques de plus basse énergie de composés diatomiques MX dont l'association en phase solide conduit à des composés semi-conducteurs présentant des propriétés piézoélectrique importantes. A l'aide de calculs électroniques incluant une large part de la corrélation électronique nous avons mis en évidence les propriétés communes à une famille de composés diatomiques possédant huit électrons de valence et qui peuvent être considérés comme précurseurs des solides piézoélectriques. Il a ainsi été possible d'identifier les couples diatomiques qui constituaient les meilleurs candidats pour une production d'effet piézoélectrique en phase solide. Dans la deuxième application, nous avons calculé la structure électronique et les constantes spectroscopiques de l'état fondamental de la molécule HZnF ainsi que les états de plus basse énergie des fragments diatomiques associés. Cette étude était motivée par une analogie avec un système voisin HZnCl dont les premiers états électroniques présentent des caractéristiques non complètement résolues. Après avoir validé notre approche sur l'étude de ZnH et comparé nos résultats à ceux d'études antérieures, nous avons entrepris la détermination de la structure électronique des molécules ZnF et ZnCl pour lesquelles nous avons pu identifier des états électroniques encore mal connus. Utilisant les résultats obtenus sur ZnH et ZnF, nous avons déterminé la surface d'énergie potentielle de l'état fondamental de HZnF et nous en avons déduit plusieurs grandeurs spectroscopiques caractérisant cet état / In this work we have investigated the electronic properties of several types of molecular systems involving a metallic element. Our motivation for such applications on metallic compounds was to obtain an accurate description of close lying electronic states, in which the relativistic effects of heavy atoms are known to be important. Thus various approaches and methods have been employed to treat these effects, including the multi-configurational method, with atomic pseudopotentials and large basis sets. In the first study, we have determined the properties of the low lying electronic states of the diatomic compounds MX, whose combinations in the solid phase produce ionic semi-conductor materials with piezoelectric properties. Based on highly correlated ab initio calculations, we have elucidated the common properties of the low lying electronic states of these diatomic compounds with eight valence electrons, which can be considered as precursors for piezoelectric effects in their solid phase. Based on our electronic structure calculations, we could identify among these diatomic compounds those who could lead to good candidates for piezoelectric effects. As the second application, we have determined the electronic structure and the spectroscopic constants for the ground state of the HZnF molecule and for the low lying electronic states of its diatomic fragments. This application was initiated and motivated by interesting and puzzling results on the close system HZnCl. Comforted by our experience with the previous studies, we used the pseudopotentials approach to obtain an accurate description of the low lying states of ZnH which could be satisfactorily compared with existing data. Next, the ZnF and ZnCl diatomic molecules have been studied with the same ansatz to reveal the properties of so far unknown electronic states. Finally, the potential energy surface of the ground state of HZnF has been determined, and several spectroscopic properties have been deduced