Along CV1, we applied umbrella bias from 1.0 Å to 3.0 Å at an interval of 0.1 Å. We took κ h =2.2 × 10 3 kcal mol -1 Å -2 , however near transition state region, we placed umbrella windows at an interval of 0.05 Å and κ h =2.7 × 10 3 kcal mol -1 Å -2 . The initial structure of an umbrella window was generated arbitrarily by setting the distances to the equilibrium of the umbrella bias and then the structure was equilibrated at 300 K for 2-3 ps. For the subsequent windows, the initial structures were taken from the neighbouring windows. We used a mean-force based scheme for reconstructing free energy surfaces, and the standard error in free energy estimates was computed using procedure in Refs. after applying appropriate TASS reweighting.