3. RESULTS AND DISCUSSIONS
3.1 Structures
All first-principles calculations are carried out by the CASTEP program under the Material Studio 7.0 version.45 The energy cutoff is 400 eV. The ultrasoft form pseudopotentials are utilized in the calculations. The convergence criteria of energy, maximum force, maximum stress, and maximum displacement were set as 5.0×10-6eV/atom, 0.01 eV Å -1, 0.02 Gpa, and 5.0×10-4 Å, respectively, for optimization. In our previous work, generalized gradient approximation (GGA) with the revised Perdew­-Burke­-Ernerhorf (revPBE) and the Perdew­-Burke­-Ernerhorf (PBE) functionals have been used to calculate the equilibrium lattice parameters of methane hydrates. Comparing the results with previous ones and the experimental value, the prediction from revPBE is found closer to the experimental measurement and other theoretical works35. Therefore, revPBE functional is adopted in the following calculations in this work. Here, we calculated three type sI clathrate hydrates, i.e., NH3 hydrates and N2O hydrates and NO hydrates, all the crystalline structures are plotted in Figure 1 . The lattice parameters by the equations of state (EOS) fitting,46 and the fitting curve of Energy vs. Volume for NH3, N2O and NO hydrates are shown in Figure 2 . The predicted lattice parameters for NH3, NO and N2O hydrates are 12.095, 12.126 and 12.36 Å, respectively, higher than the value for the CH4hydrate(11.955 Å).47 The lattice parameters of the N2O hydrate can be found to be larger than those of NO and NH3 mainly because N2O has a larger diameter. It is interesting to further study the bulk modulus in terms of the attractive and repulsive interatomic interactions and the type of occupied cage.