The results of an ab-initio molecular dynamics study of the electronic and thermophysical properties of methane hydrate with a cubic sI structure are presented. Good agreement of the simulation results for heat capacity at constant volume and density with experimental data is found. Based on the analysis of the density of electronic states, the temperature dependences of the electronic properties of methane hydrate, including the Fermi energy level, width and boundaries of the band gap are determined. For the empty framework of the hydrate (water clathrate framework), the electron energy spectra E(k) were calculated along the directions M-X, X-G, G-M, and G-R. It was found that the presence of CH4 molecules in an aqueous clathrate leads to an increase in the Fermi energy of the hydrate from 2.4 to 3.0 eV.

The processes of nucleation and growth of methane hydrate in a highly supercooled two-phase methane–water system obtained using various cooling protocols are considered. It has been shown that, at sufficiently high cooling rates, crystalline forms of methane hydrate can still form in the system. It was found that, at a cooling rate of γ = 1.0 K/ps, the process of nucleation and growth of gas hydrate was observed in all independent molecular dynamics iterations, while at a cooling rate of γ = 10.0 K/ps, no nucleation event was observed in ~26.7% of numerical experiments. It was found that with an increase in the cooling rate of the system, an increase in the average time scale of nucleation τc and a decrease in the critical size of the nucleus nc are observed. It is shown that at a sufficiently deep level of supercooling of the system, the scenario of homogeneous crystalline nucleation is realized at the initial stage of the phase transition.

В данной работе в рамках метода функционала плотности исследуются механизмы структурной стабилизации гидратов sI молекулами газов CH4, H2S, H2, N2, Ar, Kr, Xe, CO2, C2H6, C3H6. Показано, что включение молекул газа в D- и T- полости гидрата приводит к деформации полостей и изменению их радиусов (до –0.23 %). Рассчитаны энергии связи газов в D- и T- полостях. Установлено, что молекулы с диаметром d < 5 Å лучше стабилизуют D-полости, в то время как молекулы с диаметром d > 5 Å лучше стабилизируют T-полости. По характеру зависимости величины энергии связи от массы молекулы выделяются две группы газов: молекулярные, для которых dEb/dM ∈ [−0.008; −0.006] эВ·моль/г и атомарные, для которых dEb/dM ∈ [−0.002; −0.0015] эВ·моль/г. Показано, что ориентации протяженных молекул CO2, C2H6 и C3H6 вдоль длинной оси T-полости является наиболее энергетически выгодной. Рассчитаны плотности электронных состояний N(E) для незаполненного гидрата sI и гидратов sI с содержанием CH4 и CO2. Обнаружено, наличие гостевой молекулы приводит к снижению энергии электронной подсистемы и повышению стабильности гидрата.

The results of calculating the dielectric and optical characteristics of solid polymorphic phases of water—ices Ih, III and lattices of hydrates sI, sH—are presented. Static dielectric tensors εik and complex frequency-dependent tensors εik(ω) are calculated for these materials. It is shown that, in terms of optical properties, the crystal lattices Ih, III, and sH are uniaxial, the tensor components εxx(ω) and εyy(ω) coincide for them, and the hydrate lattice sI is isotropic. Based on the calculated frequency-dependent dielectric functions εik ′ (ω) and εik ′′ (ω), important optical characteristics were obtained: reflection R(ω), absorption a(ω), loss function L(ω), refractive indices n(ω) and k(ω). Comparison of the dielectric and optical spectra of the sI and sH gratings with the known spectra for methane hydrate sI revealed a broadening of the spectra in the high-energy direction. For the unfilled hydrate sI, a reflection peak was found at an energy of 17.3 eV, the appearance of which is associated with a change in the electronic structure of the crystal in the absence of a methane molecule. Qualitative agreement is obtained between the reflection spectra R(ω) and the functions εik ′ (ω), εik ′′(ω), calculated by quantum mechanical simulation, with experimental spectroscopy data for hexagonal and amorphous ices.