An efficient method for computing excess free energy of liquid
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We present a new theoretical method for efficient calculation of free energy of liquid. This interaction entropy method allows one to compute entropy and free energy of liquid from standard single step MD (molecular dynamics) simulation directly in liquid state without the need to perform MD simulations at many intermediate states as required in thermodynamic integration or free energy perturbation methods. In this new approach, one only needs to evaluate the interaction energy of a single (fixed) liquid molecule with the rest of liquid molecules as a function of time from a standard MD simulation of liquid and the fluctuation of distribution of this interaction energy is then used to calculate the interaction entropy of the liquid. Explicit theoretical derivation of this interaction entropy approach is provided and numerical calculations for the benchmark liquid water system were carried out using three different water models. Numerical analysis of the result was performed and comparison of the computational result with experimental data and other theoretical results were provided. Excellent agreement of calculated free energies with the experimental data using TIP4P model is obtained for liquid water.
Keywordsexcess free energy liquid MD simulation interaction entropy
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This work was supported by the National Natural Science Foundation of China (21603144, 21433004), Ministry of Science and Technology of China (2016YFA0501700), NYU Global Seed Grant, Shanghai Putuo District (2014-A-02), and Shanghai Sailing Program (2016YF1408400). We thank the Supercomputer Center of East China Normal University for providing us computer time. We also thank Professor Yongle Li for help in the discussion of the excess free energy of water.