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.