Car–Parrinello simulation of the vibrational spectra of strong hydrogen bonds with isotopic substitution effects: Application to oxalic acid dihydrate

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.