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The free energy perturbation method is employed to study the free energy of solvation of a drug molecule in its pure drug melt, which is supposed to be considered the second step in predicting the solubility of drugs in water. The analysis reveals contradictory results, revealing that the Coulomb interactions are much weaker in pure drug systems, whereas the van der Waals interactions are extremely important and highly influence the interaction.

The study uses the free energy of the perturbation method and the optimized potential for the liquid simulations to take the first step for the computational prediction of drug solubility by measuring the free energy of hydration. The analysis provides an accurate demonstration of the macroscopic surface tension of water, which is not in agreement with the surface tension for water around a spherical cavity that normally is supposed to be dependent on the size of the cavity.