Although the molecular form of carbonic acid, H2CO3, is of very limited stability in aqueous solution, some form of H2CO3 is apparently stable as a solid and may be a stable species on acid-treated carbonate mineral surfaces. Experimental vibrational spectra that have been assigned to solid H2CO3 have been obtained by several research groups, although there is no information on either the local or long-range structure of this phase and calculated vibrational frequencies for monomeric H2CO3 show significant discrepancies with the experimental IR data. Previous calculations have also indicated that H-bonded H2CO3 oligomers are more stable than the monomer and have significantly different vibrational spectra, but the accuracy of the spectral calculations was not considered sufficient to reassign the experimental data. We have now calculated the harmonic vibrational spectrum of monomeric H2CO3 at the 6-311+G(2d, p) CCSD level and have calculated anharmonic corrections at the CBSB7 B3LYP level (used in CBS-QB3 calculations), combining the two to obtain an accurate description of the fundamental vibrations of gas phase H2CO3, which disagree significantly and systematically with the experimental data for solid H2CO3. For the H-bonded dimer, (H2CO3)(2), we have calculated both the harmonic spectrum and anharmonicity corrections at the CBSB7 B3LYP level, finding much better agreement with the experimental spectrum of solid H2CO3 than for the monomer, particularly for the CdO and O-H stretching vibrations, which are strongly red-shifted by both H bonding and anharmonic effects in the dimer. The free-energy changes for the formation of the (H2CO3)(n) (n = 2 and 3) oligomers and for the formation of a 1D chain structure of H-bonded monomers are negative in the gas phase, despite an unfavorable entropic contribution. However, in aqueous solution, the free-energy change for the formation of the n = 2 and 3 oligomers becomes positive because of the loss of hydration free energy, since the -OH groups of H2CO3 are removed from H bonding with the solvent. We have also calculated C-13 NMR shieldings for the H2CO3 oligomers and some other related molecules, finding that the central C is systematically deshielded by oligomerization.