Abstract
A fully unsupervised computational protocol is proposed with the aim of obtaining reliable structural properties for molecular bricks of life in the gas phase. The results of the new composite scheme approach spectroscopic accuracy at a moderate cost without any empirical parameter in addition to those of the underlying electronic structure method. The whole workflow is fully automated and provides optimized geometries and equilibrium rotational constants. Direct comparison with experimental ground state rotational constants can be performed thanks to the effective computation of vibrational corrections in the framework of second-order vibrational perturbation theory. The results for all the nucleic acid bases and several flexible molecules of biological or medicinal interest show that the accuracy of the new tool is close to that delivered by state-of-the-art composite wave function methods for small semirigid molecules.