Perspectives from Quantum Chemistry for molecules and nanograins with astrophysical Interest

Quantum Chemistry methods offer powerful tools for the computation of various properties of molecules and nanoparticles or clusters such as structure, thermodynamics, infrared and electronic spectroscopy, in some cases on a routine basis. However, neither the accuracy nor the applicability of the various methods are uniform, and may depend strongly on the nature and the size of the compounds and the desired properties. The ab initio methods for approaching electronic structure can be classified into two classes. The first class includes wavefunction-based methods, namely post-Hartree Fock schemes in the framework of Configuration Interaction or Coupled Cluster schemes which can now be used for molecules containing up to a few ten atoms (for single geometry calculations) and are likely to provide accurate results whenever applicable. The second class of methods is that of density-based methods which cover systems between a few tens up to a few hundreds of atoms. It is often of broad applicability and satisfactorily accurate in many cases. For more extended systems or extensive calculations, tight-binding type approximations or evolved force-fields (based on bond-orders for instance) are also quite promising. In this overview, we will briefly remind the basics of standard Quantum Chemistry methods and provide a survey of present trends which should be of interest for astrochemistry simulations at the atomic scale.