The Novel Method for Solvation Free Energy Computation of Liquid and Solid by 2PT model

The solvation free energy and vapor pressure are important thermodynamic properties of pure substances. We proposed a novel method, which even can calculate free energy of liquid, but also can calculate free energy of solid.... [ view full abstract ]

The solvation free energy and vapor pressure are important thermodynamic properties of pure substances. We proposed a novel method, which even can calculate free energy of liquid, but also can calculate free energy of solid. In this work, we show that these physical quantities can be obtained from short, about 20 ps, molecular dynamics simulations. The method combines the determination of free energy of a chemical in vacuum using the normal-mode analysis (energy minimization), and in the liquid or solid phase using the two-phase thermodynamic (2PT). The 2PT determines the free energy based on the vibrational density of states (DOS) from the Fourier transform of the velocity autocorrelation function. In particular, a fluidicity parameter is used to separate the DOS to a gas-like diffusive and a solid-like harmonic component. Appropriate quantum statistical weighting functions are applied to the corresponding DOS components for the free energy calculations. We have examined the calculations results for 18 common chemicals, including water, alcohol, acid, aromatics, and alkanes in liquid phase. The results from the proposed method, referred to as 2PT-NMA, is comparable (within about 3 kJ/mol) to those calculated from thermodynamic integration (TI), which normally takes about 1 ns simulation for converged free energy difference. Furthermore, the same method was used for calculating sublimation pressure of 6 solids. Our results show that 2PT-NMA could be a very useful tool for fast screening of solvent solvation free energy and vapor pressure from the trajectory of MD simulations.