| Density functional theory (DFT) is a quantum mechanics method which is used in researching the structure of electron for multi-electron system. Theoretically, it could be used to estimate the thermodynamics properties for ideal gases. It has been rapidly developed these days due to its simplicity in complicated measurement in labs. Due to the fact that sometimes the research has to be paused just because of the lack of several thermodynamics properties for the components involved in the simulation of chemical process, the density functional theory method was employed in this paper to predict the thermodynamic properties of ideal gas through the simulation software named Gaussian03 and MS.Dmol3. This paper made a study on the thermodynamics properties of widely used refrigeration agents under low temperature, which were hardly obtained through normal analysis, to provide the basic data for chemical process simulation.The main work and method used as well as the results obtained in this paper involve the following aspects:1. The reliability of DFT in predicting thermodynamic properties was studied.The density functional software Gaussian03 was employed to do the fully optimization and vibration analysis for the eight given halogenated aromatics. The comparison between the estimated data and the ones appeared in literature showed that the two were in good agreement with each other. The maximal error for standard entropy was 7.6 J·mol-1·K-1, standard capacity of heat-4.2 J·mol-1·K-1, and the standard heat of enthalpy17 kJ·mol-1.The results obtained showed that the Gaussian03 method was in good condition in predicting the thermodynamics properties for compounds, providing the essential guidelines for Gaussian method in the same field.The density functional software MS.Dmol3 was employed to prediction of thermodynamic properties of compounds. The optimal calculating conditions were obtained through comparison among the results obtained under different exchange relevant functions as well as different basis sets. In order to obtained the reliable thermodynamics properties, the calculating conditions were determined to be LDA-PWC as the exchange relevant functions, DNP for the compounds containing H atom while DND for the ones containing no H atom. The different calculation errors of different thermodynamic properties for different materials were calculated under the same calculating conditions. The compounds with maximum errors of properties were hydrocarbon ones, with the average error of entropy to be 11.59J·mol-1·K-1, heat capacity 0.701J·mol-1·K-1 and enthalpy change 0.148kJ·mol-1. The ones with minimum average error of entropy were aromatic hydrocarbons, 8.98J·mol-1·K-1 exactly; the minimum ones for heat capacities were the compounds containing oxide element, with the value to be 0.266 J·mol-1·K-1 while the minimum ones for enthalpy variable olefins compounds, with value of 0.044 kJ·mol-1.2. The application of density functional theory method in predicting thermodynamics properties was studied.The density functional software Gaussian03 was employed to do the optimization and vibration calculation on 130 PBDTs, and their thermodynamics properties were obtained. Through the reaction namely C12H8S+nC6H5Br= C12H8-nSBrn+nC6H6, the standard entropy of formation and standard free energy of formation were obtained. The relationship among the thermodynamics properties of PBDTs, the number of brom atoms for substitution and NPBS was studied and regression equations for Sθ, CVθ,ΔfΗθ,ΔfGθand NPBS were obtained. The regression equation could be used to estimate the thermodynamics properties of PBDTs. According to theΔfGθ, the relative stability sequence for isomers could be achieved theoretically. The study showed that the steadiest isomer was the ones whose brom atoms were often distributed into the two benzene rings with their substitute position far away with each other. This paper made a study on the thermodynamics properties and the stability for PBDTs compounds, whose results obtained would offer a significant meaning to their formation, deterioration as well as the threaten to environment.The density functional software Gaussian03 was employed to do the optimization and vibration calculation on refrigeration agents, blowing agents and organic solvents, and their thermodynamics properties were obtained. The basic thermodynamics data for these compounds have not been mentioned in literature. Through study, it could be figured out that entropy S, heat capacity Cp and enthalpy H increased while the free energy G decreased with the increase of temperature. This result showed great agreement with the classic data on thermodynamics properties. The relationship between heat capacity and temperature was determined, providing the basic data for these compounds.In all, it is reliable and feasible using DFT to estimate the thermodynamics properties.
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