Our theoretical studies try to understand and model a number of molecular physics phenomena such as :

- Vibration-rotation interactions: the challenge here is to describe, using quantum mechanics and the development of effective Hamiltonians, resonances that occur between closely spaced energy levels. This enables to obtain/predict the positions and intensities of lines for many molecules of atmospheric and astrophysical interests.

- Large amplitude motions: in molecules qualified as non-rigid, some atoms perform motions whose amplitudes are large when compared to the interatomic distances. Such molecules cannot be treated within the harmonic approximation and the calculation of their energy levels is more difficult than for "rigid" species. Molecules subject to inversion, such as ammonia, or internal rotation of a CH3 methyl group, are examples but there are other manifestations of non-rigidity (abnormal centrifugal distortion in H2O or the motion of H in HNO2...). Molecules are often of atmospheric, astrophysical or even biological interest are non-rigid.

- Intermolecular collisions: the effects of Intermolecular collisions (i.e. of pressure) on absorption spectra are the subject of theoretical and experimental efforts at LISA. They occur in all atmospheric spectra and include different mechanisms: the (collisional) broadening and spectral shifting of absorption lines; the effects of the speed of the molecule on the profiles of isolated lines; collisional interference between optical transitions; the shape of the far wings of the bands as well as the light absorption and diffusion processes induced by collision. These issues are addressed with molecular dynamics, semi-classical or purely quantum approaches, or semi-empirical models. The most refined, using intermolecular potentials provided by quantum chemistry, are applied to simple systems in order to understand the mechanisms. Others, simpler and more phenomenological, are used for more complex molecular systems.

The studies at LISA within these fields are based on :

- the development of theoretical models and software adapted to different molecular systems, logiciels taking into account the effects mentioned above, but also, where appropriate, the abnormal centrifugal distortion, hyperfine structure, internal rotation of a deuterated methyl group or not, the spin-rotation coupling, etc.

- the analysis of spectra measured in the laboratory to predict, thanks to these theoretical models, spectroscopic parameters such as the positions, intensities and profiles of the lines.

These two axes are reinforced by the development of quantum chemical calculations in a complementary manner to spectral analyses..