A precise and complete thermodynamic, Raman spectroscopic, and ultraviolet-visible (UV-vis) optical characterization of the deltic, squaric, and croconic cyclic oxocarbon acids is obtained using theoretical solid-state methods employing very demanding calculation parameters. The computed fundamental thermodynamic properties include the isobaric specific heat, the entropy, the enthalpy, and the Gibbs free energy as a function of temperature. The calculated specific heats at 298.15 K of the deltic, squaric, and croconic acids are 89.7, 111.2, and 133.2 J mol(-1) K-1, respectively, and the corresponding entropies are 98.3, 117.3, and 136.5 J mol(-1) K-1. The only value of these properties known from experimental measurements is the specific heat of the squaric acid, which differs from the computed value at 315 K by about 4.9%. The calculated values of the thermodynamic properties are then used to determine the thermodynamic properties of formation of these materials in terms of the elements. As an application of the calculated thermodynamic properties of formation, the Gibbs free energies of reaction and associated reaction constants are evaluated for the reactions of thermal decomposition and complete combustion of the squaric and croconic acids and the reaction of interconversion between them. The only available experimental values of these properties, namely, the enthalpies of combustion of squaric and croconic acids at room temperature, are reproduced theoretically with high accuracy. The Raman spectra of these materials are also computed using density functional perturbation theory. The analysis of the theoretical Raman spectra of these materials points out to significant differences with respect to their usual empirical assignment. Therefore, the Raman spectra of these materials are fully reassigned. Finally, the ultraviolet-visible (UV-vis) optical properties of the deltic, squaric, and croconic acids are computed. The UV-vis absorption spectrum of the croconic acid in the spectral region 225-425 nm and the UV absorption spectrum of the squaric acid in the region 200-350 rim, which had previously been measured experimentally, are well reproduced. The corresponding spectrum for the deltic acid and the reflectivity, optical conductivity, dielectric, refractive index, and loss optical functions of the three materials, which had never been published as far as we know, are reported as a function of the wavelength of incident radiation in the range 200-750 nm. The origin of the peaks in the absorption spectra, which had not been analyzed so far, is unveiled here by examining the interband electronic transitions in these materials.