Ab Initio Intermolecular Potential Of Hydrogen Sulfide And The Properties Of Liquid And Solid: A Monte Carlo Study

Hydrogen sulfide (H₂S) is a typical small molecular hydrogenous substance showing hydrogen bonding and molecular orientation. Despite the similarities in molecular structure and hydrogen bonded interaction, the thermodynamic and structure properties of H₂S and H₂O are significantly different in liquid and solid phases. This led to an interest in understanding what kinds of intermolecular interaction mechanism make the difference. The predictions of the thermodynamic and structure properties from computer simulation method (Monte Carlo and Molecular Dynamics) are intensively dependent on the accuracy of the intermolecular potential. A variety of succeed intermolecular potential models for water have been proposed. On the contrast, the quality and quantity of pair potential models for H₂S are not well studied,especially for solid/liquid phases and transitions. Hence, the development of an effective pair potential for the condensed phase of H₂S would be of considerable important in study pure component system and multicomponent mixtures, and it would also be helpful in comparing the properties of liquid and solid H₂S and H₂O.In recent years, several attempts have been made to simulate H₂S with simple pair potential model. They have achieved success, but there are also failures. These models have common in producing some of the potential parameters, which are obtained by fitting experimental data. The first atom-atom pair potential model constructed by Jorgensen gave the good description of the thermodynamic properties and structure of liquid sulfur compounds. Forester et al. proposed two models: four-site and five-site model, which were the evolution of Jorgensen'three-site model. They found only the four-site model gave a fair description of a range of properties of condensed phases. In order for simulation of liquid and especial vapor-liquid equilibrium, Kristof et al. reparametrized the original parameters of four-site model used by Forester et al., and their vapor-liquid equilibrium result was in good agreement with experiments. In attempt to improve the performance of molecular simulation with respect to the prediction of H₂S-alkane binary mixture phase behavior, Delhommelle et al. and Nath developed respective potential model which all include Coulomb term form ab initio calculations and Lennard-Jones term from fitting of experimental date. These two models gave an excellent reproduction of coexistence properties, but exhibited a slightly poor mole fraction of H₂S under higher pressure. In order to study the effect of partial charge parameterization on fluid phase behavior, Kamath et al. determined the range of partial charges for which Lennard-Jones parameters can be tuned based on three-site model. In this work, we developed a three-site potential model which is entirely derived from ab initio total energies. Firstly, we used ab initio method to calculate the intermolecular interaction energy of 24 different orientations of the hydrogen sulfur dimers at their respective 13 different distances. Then we use exp-6-1 type potential function to fit these 312 (24×13) ab initio energies. The potential model is a three sites rigid molecular model which only has nonbonding interaction energies, including exp-6 term for intermolecular repulsion and dispersion energy and Coulomb term for electrostatic energy. Although the exp-6-1type potential function have used to build a successful methane molecules potential model and directly used to simulate methane system under ambient pressure, the exp-6-1 function has a great defect in the short-range. The attractive potential rapidly tends to infinity with the decrease of interatomic distance. This shortcoming will bring a serious impact under high pressure, we correct it by adding a"switch"function to r-6 term. Unlike some models were used to study vapor-liquid equilibrium of pure component or mixture, we are more concerning about condensed phase properties of pure H₂S system, including liquid, solid, and melting behavior. In order to evaluate the accuracy of this model, Monte Carlo simulations are carried out using this model and all the previous potential models. From the point of view of the thermodynamic conditions, MC simulations covered a temperature range from 50 K to 214 K and pressure up to 30 GPa. In summary, we have checked the model for a number of properties of condensed phase and an unusually wide range of states, and the results are clearly better than all the other models. Firstly, the equation of state agrees well with two experimental data. It reflects the good performance of pressure effect of this model. Secondly, this model can stabilize the lowest temperature solid phase III, which has orthorhombic Pbcm and display weak hydrogen bonding character. The phase transition temperature from phase III to high temperature fcc structure is close to experiments. Thirdly, the melting curve reproduces well the experimental result; in particular, the ambient pressure melting point is only about 6 K higher than experimental value. The temperature-induced solid-solid phase transition and melting properties reflect the good performance of temperature effect of this model. Finally, liquid density and structure gave a near reasonable description comparing with experiments, though there are some quantitative deficient. On the whole, this ab initio based intermolecular potential model gives an impressive performance at different thermodynamic conditions, and it is a reliable model for condensed phase of H₂S.