THEORETICAL STUDYOF HYDROGEN CLATHRATE HYDRATES Marcel Sluiter, Talgat M.Inerbaev ∗ , Rodion V.Belosludov, Yoshiyuki Kawazoe Institute for Materials Research Tohoku Univesity 980-8577 Sendai Japan Oleg S. Subbotin, and Vladimir R. Belosludov Nikolaev Institute of Inorganic Chemistry Siberian Branch of Russian Academy of Science 630090 Novosibirsk Russia ABSTRACT In this work, the electronic, structural, dynamic and thermodynamic properties of H 2 clathrate hy- drate of structure II have been studied theoretically and the effect of multiple occupancies on their stability has been examined using both first-principles and lattice dynamics calculations. The most favored cage occupancy has been determined using highly accurate ab initio computer simulations within local density functional approach. It has been argued that the enclathration of hydrogen molecules is based on physisorption. The calculated physisorption energy has been found to be of the order of 20 to 25 meV per H 2 molecule. Lattice dynamics investigations show that all hydrogen hydrates with multiple species per cage are dynamically stable and phonon spectra strongly depend on the cage occupancy. As a result phonon spectra and equations of state vary significantly with cage occupancy. It is shown that quantum zero-point vibrations are of fundamental importance for the thermodynamic properties. Keywords: Keywords: clathrate hydrate, ab initio, lattice dynamics, electronic structure, phonon density of states, equation of state, hydrogen storage NOMENCLATURE D 0 [kJ mol −1 ] F [kJ mol −1 ] k B [kJ K −1 ] P [GPa] r [nm] T [K] U [kJ mol −1 ] V [nm 3 ] ΔE S [eV] ω [THz] ρ [nm] ∗ Corresponding author: Phone: +81-22-215-2057 Fax: +81-22-215-2052 E-mail: talgat@imr.edu INTRODUCTION Clathrate hydrates are a special class of inclusion compounds consisting of water and small guest molecules which form an ice-like hydrogen-bonded structure. These compounds are formed when water molecules arrange themselves in a cage-like struc- ture around guest molecules and hence many of their physical and chemical properties differ from ice [1]. In recent years clathrate hydrates have at- tracted considerable interest because large accumu- lations of natural gas hydrates have been discovered in ocean floor sediments and in permafrost regions. The fact that hydrate clathrates can contain several hundred times their own volume in gas has drawn attention. For instance, hydrogen clathrate hypothet- ically contains 500 times its volume in H 2 gas (under Proceedings of the Fifth International Conference on Gas Hydrates, June 12-16, 2005. Trondheim, Norway