Thanks, Prof. Benyahia

You can do calculate the minimum energy path (MEP) of your reaction with Nudged Elastic Band (NEB) or string method. Based on MEP, there are two options. One is to do a transition state optimization of the peak structure on the MEP to find the transition state, and then do frequency analysis to obtain the free energies. The other way is more expensive. You can do a series of ab initio molecular dynamics simulations using umbrella sampling method to calculation the free energy profile of the whole reaction.