The challenge associated with the calculation of µ i is the calculation of the standard chemical potential of i, µ 0 i , the first term of equation 11. q i , the internal partition function, includes the rotation, vibrational, electronic and nuclear partition functions of a single molecule and is unknown. Here, we found a way of calculating µ 0 i without the knowledge of q i by alchemically transforming a single solute molecule into a single Einstein molecule, whose absolute free energy we know how to calculate .