Extracting reliable thermochemical parameters from molecular dynamics simulations of chemical reactions, although based on ab initio methods, is generally hampered by difficulties in reproducing the results and controlling the statistical errors. This is a serious drawback with respect to the quantum chemical description based on potential energy surfaces. This work is an attempt to fill this gap. We apply molecular dynamics, based on density functional theory (DFT) and empowered by path metadynamics (MTD), to simulate the reaction of CO2 with (one, two, and three) water molecules in the gas phase. This study relies on a strategy that ensures a precise control of the accuracy of the reaction coordinates and of the reconstructed free-energy surface on this space, namely, on (i) fully reversible MTD simulations, (ii) a committor probability analysis for the diagnosis of the collective variables, and (iii) a cluster analysis for the characterization of the reconstructed free energy surfaces. This robust procedure permits a meaningful comparison with more traditional calculations of the potential energy surfaces that we also perform within the same DFT computational scheme. This comparison shows in particular that the reactants and products of systems with only three water molecules can no longer be understood in terms of one structure but must be described as statistical configuration ensembles. Calculations carried out with different prescriptions for the exchange-correlation functionals also allow us to establish their quantitative effect on the activation barriers for the formation and the dissociation of carbonic acid. Their decrease induced by the addition of one water molecule (catalytic effect) is found to be largely independent of the specific functional.