Ideally, one would be able to calculate the free energy difference by counting how
often the system samples the different values of the reaction coordinate ξ. However,
with simple MD simulations it is not possible to sample two states separated by a high
energy barrier, and it is thus an inefficient method for this type of calculations. In
these studies, I choose one of the most used and validated methods for the calculation
of PMF, the umbrella sampling method [143].