  We calculate harmonic frequencies of the three most abundant carbonic acid
conformers. For this, different model chemistries are investigated with respect
to their benefits and shortcomings. Based on these results we use perturbation
theory to calculate anharmonic corrections at the {\omega}B97XD/aug-cc-pVXZ,
X=D,T,Q, level of theory and compare them with recent experimental data and
theoretical predictions. A discrete variable representation method is used to
predict the large anharmonic contributions to the frequencies of the stretching
vibrations in the hydrogen bonds in the carbonic acid dimer. Moreover, we
re-investigate the energetics of the formation of the carbonic acid dimer from
its constituents water and carbon dioxide using a high-level extrapolation
method. We find that the {\omega}B97XD functional performs well in estimating
the fundamental frequencies of the carbonic acid conformers. Concerning the
reaction energetics, the accuracy of {\omega}B97XD is even comparable to the
high-level extrapolation method. We discuss possibilities to detect carbonic
acid in various natural environments such as Earth's and Martian atmospheres.
