The nature of strong intermolecular hydrogen bonding in oxalic acid dihydrate in the crystal phase was examined by infrared spectroscopy and Car-Parrinello molecular dynamics simulation. We studied region of infrared spectra associated with the O-H modes. The spectra were calculated using harmonic approximation with crystal field and time course of the dipole moment as obtained from Car-Parrinello simulation with quantization of the O-H motion, and isotopic substitution. We obtained good agreement of the molecular dynamic simulation with experiment. To our best knowledge, this is one of the first Car-Parrinello calculations of infrared spectra including anharmonicity effects and crystal field interactions. (C) 2012 Elsevier B.V. All rights reserved.