Table 1.

Space group and fractional coordinates of oxygen and carbon atoms in the MH-III, MH-IIIs, MH-IV, and MH-IVs phases of methane hydrate at 40 GPa, as obtained from the AIMD simulations

PhaseGroupAtomSitexyz
MH-IIIImcmO8i0.2500.0800.830
74C4e0.2500.6800.000
MH-IIIsPmcnO4c0.2500.4000.190
62O4c0.2500.4200.810
C4c0.2500.8000.980
MH-IVPmcnO4c0.2500.4200.045
MH-IVs62O4c0.2500.4200.455
C4c0.2500.7500.715
  • For coherence with Imcm, the space group of the higher-pressure phases is given in the nonconventional representation. The uncertainty on the fractional coordinates is ±0.005. Despite the different space group between MH-III and MH-IIIs, the full oxygen positions are very similar. In MH-IVs (H-bond symmetric phase IV), the oxygen and carbon atomic positions do not change significantly from those in MH-IV. Computed lattice parameters: MH-IIIs (40 GPa), a=4:006 Ǻ , b=6:911 Ǻ , c=6:249 Ǻ ; MH-IV (40 GPa), a=4:063 Ǻ , b=6:981 Ǻ , c=6:063 Ǻ . Lattice parameter trends with pressure are reported in SI Appendix.