Oxalic acid and oxalic acid dihydrate were studied using terahertz spectroscopy and solid-state density functional theory (DFT) in the spectral range 10-100 cm~(-1). The size of the oxalic acid molecule and its limited internal degrees of freedom make it ideal for evaluating the performance of computational methods for the structural and dynamical simulation of strongly hydrogen-bonded solids. Calculations of the solid-state structures and terahertz spectra of oxalic acid and oxalic acid dihydrate were performed using the hybrid B3LYP and B3PW91 and the nonhybrid BLYP and PW91 density functionals employing the 6-311G(2d,2p) basis set. When these simulations were compared to the experimental spectra of the oxalic acid solids, a constant overprediction of the dihydrate frequencies was observed in contrast to the results of the anhydrous system. This change in behavior is connected to the nature of the vibrational motions being accessed. The primary molecular motion contributions to the terahertz vibrations of oxalic acid dihydrate were found to originate in the external motions of the cocrystallized H_2O molecules. The observed overestimation of the vibrational energies in the simulated terahertz spectra is attributed to increased anharmonicity of the vibrational motions in the dihydrate system versus the anhydrous, resulting from weaker hydrogen bonding through the networked water molecules.
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