Free Energy of Rigid-Molecule Crystals: Application to clathrate hydrates(PPT)

摘要

1. Classical and quantum free energies of sI, sII, and sH hydrates were computed. 2. The molecular force constants are given in analytical forms as a function of their atomic contributions; so, finite-difference approximations are avoided. 3. Although using only one proton disorder isomer introduced temperature-dependent error, the nature of that error appears to be system-size independent. 4. The free energy was computed in the thermodynamic limit by extrapolating it to for only one isomer and then add the proton-disorder correction of the smallest system. 5. Quantum effects are of the order of 5 kJ/mol, and differ between structures by about 0.1 kJ/mol. 6. The local-harmonic-approximation introduced inaccuracy by about 0.8kJ/mol.