**2.3.2 Blue moon ensemble**

The blue moon ensemble method (Sprik & Ciccotti, 1998) is a way to calculate the free energy profile along a reaction coordinate (RC) while constraining RC to a specified value. The method was implemented in FMO-MD (Komeiji, 2007) and was successfully applied to drawing a free energy profile of the Menschutkin reaction (Komeiji *et al.*, 2009a).
