Space chemistry and computational chemistry

The neologism "computational", which appears second in the title of this article (adapted from the current English computationnal), delimits a field of research that is very active today. Here, the various computational methods of quantum chemistry are applied in the form of specific computer codes to study the properties of various chemical species (free molecules or molecules embedded in aggregates, surfaces, solids or solutions), in relation as directly as possible to well-defined problems of structure or reactivity. The boundary between reality and modelling varies according to the degree of complexity of the phenomenon studied and the precision of the theories used to simulate it on a computer. The range of energetic and structural quantities and spectroscopic or magnetic properties that can be calculated is vast. Computational chemistry also makes it possible to tackle problems of chemical reactivity, thanks to the evaluation of collision cross-sections and hence rate constants. In the case of space chemistry, for which the relevant laboratory experiments are not always feasible because they are too costly to calculate, current theoretical chemistry methods make computational chemistry a powerful and sometimes essential tool for the study of space chemistry. It can be applied to environments as diverse as interstellar space, stellar atmospheres, planetary atmospheres, comets, meteorites, etc., both in terms of observations (identification of new molecules) and chemical reactivity (formation and destruction of space molecules). This is the subject of this article.

The computational methods used to tackle these problems are essentially those of theoretical chemistry. We won't go into them here, as they have already been the subject of an article, and the interested reader can refer to it for further information. They can also consult J.L. Rivail's highly educational book (see Pour en savoir plus). Let's just say that these methods (self-consistent field theory, configuration interaction, perturbation theory, density functional theory) are inspired by the treatment of the N-body problem in quantum mechanics. Today, they enable us to evaluate the electronic and vibrational energies of space molecules, as well as their physico-chemical characteristics, on a computer with a high degree of accuracy.