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Free energy calculation of a molecule by removing VDW and Coulomb interactions in a transformation and treating the molecule as non interacting systems

Free energy calculations in molecular simulations have a variety of applications including determining the strength of molecular processes such as solvation and binding. It has been recently shown that when removing the VDW and Coulomb potential terms of a group of atoms in a molecule by performing a transformation, the molecule can be treated as non interacting systems in the free energy calculation. This treatment is applicable both when the molecule is in vacuum and in liquid and enables a very simple calculation of the free energies associated with the potentials that depend on the relative spherical coordinates of these atoms. Here we demonstrate the method in the free energy calculation of a Methanethiol molecule and compare the results to these obtained by MD simulations in vacuum and in water. The comparison shows agreement between the results and faster computation when using the method by factors varying between 5000 and 10^(12) for the same computational resources.