The study of chemical reactions, as well as the calculation of thermodynamic properties are critical issues of modern chemistry. The development of experimental techniques and instruments allows measurements more accurate these quantities for systems more complex. The growing interest in the study of the interstellar medium and planetary atmospheres is also proving to be a major challenge in the coming decades. The difficulties encountered in the analysis of these experiences (or measures) often require the intervention of numerical simulations to clarify these observations. Another use of the calculation is to predict molecular and spectroscopic parameters of unstable species are difficult to produce in the laboratory.Current tools of theoretical chemistry ab initio are valuable tools for the prediction and interpretation of experimental results or astrophysical measurements and atmospheric. These simulation techniques have experienced significant developments in recent decades. The recent progress in calculations of interaction of large configurations can include a large part of the correlation energy. The computation time and memory size of computers, however, remain significant limitations that do not allow to perform configuration interaction in a total base large enough to hold the physical systems studied beyond small molecules. This fact led to interest in cheaper methods such as disruption, the truncated configuration interaction and coupled cluster, allowing to include a portion of electron correlation at a lower cost in computation time. These are methods that have been used in this work to determine theoretically the molecular parameters and spectroscopic systems MgO, MgO +, FEC2, FEC2 + and FEC2-with maximum accuracy.As a first step, we studied the MgO molecule. It is a system of choice because it allows you to learn the methods of ab initio calculations on molecular systems the simplest (diatomic), test and understand these methods (different approximations, validity, accuracy, ...) and to properly interpret the results (formation of chemical bonding and molecular states, their symmetry, their interactions, their stability, spectroscopy, ...) especially since it has been the subject of several theoretical and experimental studies. To take advantage of our expertise for diatomic molecules we have studied the system MgO + has been our second article will be presented in the appendix.In a second step, we targeted molecular systems FenCm like to understand the growth and dynamics of carbon nanotubes catalyzed by iron. Diatomic system FeC been several theoretical and experimental studies. The most recent is provided by Demeter Tzeli and Aristides Mavridi. This theoretical study has characterized its ground state and the 40 lowest electronic states at all internuclear distances up to dissociation, and secondly to provide spectroscopic data with an accuracy comparable to that given by the experiment. For higher-order systems, the problem faced by these small molecular systems composed of iron and carbon have very complicated electronic structures, our study is limited to the study of systems FEC2, FEC2 + and FEC2-
