Anomalous temperature dependence of nuclear quadrupole interactions in strongly hydrogen-bonded systems from first principles.

We present an analysis of the effect of finite temperatures on the deuteron nuclear quadrupole coupling constants in a strongly hydrogen-bonded molecular crystal by means of first-principles Car-Parrinello molecular-dynamics simulations. Our findings agree well with experiments and provide a microscopic explanation of the anomalous increase of the quadrupole coupling in this class of systems. We show that a simple model based on the anharmonicity of the hydrogen bond potential fails to describe the temperature dependence of the couplings even qualitatively. Instead, the inclusion of fluctuations and disorder in terms of atomic motion of the surrounding molecules turns out to be important to obtain the correct magnitude of the temperature effect.