This tutorial will assume you have a reasonable understanding of what free energy calculations are and how they are performed. It is also important to have completed the Methane in Water Tutorial to familiarize yourself with the basics of these calculations in GROMACS. This tutorial moves beyond the simpler example by including the transformation of Coulombic terms. The objective of the tutorial is to calculate the hydration free energy (ΔG_hydr) of ethanol, a value that is well characterized experimentally.

The value of ΔG_hydr is useful for determining how well a force field models interactions between a solute and an aqueous environment, therefore this quantity is often used as validation when developing new parameters. Physically, ΔG_hydr corresponds to the transfer of a molecule of the solute in the (ideal) gas phase into an infinitely dilute aqueous solution. Although there are several simulation approaches that can be used to calculate this quantity, this tutorial will build upon the previous example by applying a λ-dependent transformation and use of the Bennett Acceptance Ratio (BAR), as before.