Energies and spin states of FeS(0/-), FeS2(0/-), Fe2S2(0/-), Fe3S4(0/-), and Fe4S4(0/-) clusters.

The structures and energies of the electronic ground states of the FeS(0/-), FeS2(0/-), Fe2S2(0/-), Fe3S4(0/-), and Fe4S4(0/-) neutral and anionic clusters have been computed systematically with nine computational methods in combination with seven basis sets. The computed adiabatic electronic affinities (AEA) have been compared with available experimental data. Most reasonable agreements between theory and experiment have been found for both hybrid B3LYP and B3PW91 functionals in conjugation with 6-311+G* and QZVP basis sets. Detailed comparisons between the available experimental and computed AEA data at the B3LYP/6-311+G* level identified the electronic ground state of (5)Δ for FeS, (4)Δ for FeS(-), (5)B2 for FeS2, (6)A1 for FeS2(-), (1)A1 for Fe2S2, (8)A' for Fe2S2(-), (5)A'' for Fe3S4, (6)A'' for Fe3S4(-), (1)A1 for Fe4S4, and (1)A2 for Fe4S4(-). In addition, Fe2S2, Fe3S4, Fe3S4(-), Fe4S4, and Fe4S4(-) are antiferromagnetic at the B3LYP/6-311+G* level. The magnetic properties are discussed on the basis of natural bond orbital analysis.

Molecular simulation of the potential of methane reoccupation during the replacement of methane hydrate by CO(2).

Molecular dynamics simulations and stabilization energy calculations are performed in this work in order to understand the stability of CH(4) hydrate, CO(2) hydrate, and CH(4)-CO(2) mixed hydrate. The model systems of fully occupied type SI CH(4) hydrate, CO(2) hydrate, and CH(4)-CO(2) mixed hydrate are prepared in a simulation box of 2 x 2 x 2 unit cell with periodic boundary conditions. The MD simulation results reveal that the CH(4)-CO(2) mixed hydrate is the most stable one in above three hydrates. The stabilization energy calculations of small and large cavities occupied by CH(4) and CO(2) show that the CO(2) molecule is less suitable for the small cavity because of its larger size compared with the CH(4) molecule but is more suitable for the large cavity. The results in this work can also explain the possibility of CH(4) molecule in reoccupying the small cavity during the replacement of CH(4) hydrate by CO(2), from the hydrate stability point of view.