title: Quantum free energy landscapes from ab initio path integral metadynamics: Double proton transfer in the formic acid dimer is concerted but not correlated.
abstract: With the goal of computing quantum free energy landscapes of reactive (bio)chemical systems in multi-dimensional space, we combine the metadynamics technique for sampling potential energy surfaces with the ab initio path integral approach to treating nuclear quantum motion. This unified method is applied to the double proton transfer process in the formic acid dimer (FAD), in order to study the nuclear quantum effects at finite temperatures without imposing a one-dimensional reaction coordinate or reducing the dimensionality. Importantly, the ab initio path integral metadynamics technique allows one to treat the hydrogen bonds and concomitant proton transfers in FAD strictly independently and thus provides direct access to the much discussed issue of whether the double proton transfer proceeds via a stepwise or concerted mechanism. The quantum free energy landscape we compute for this H-bonded molecular complex reveals that the two protons move in a concerted fashion from initial to product state, yet world-line analysis of the quantum correlations demonstrates that the protons are as quantum-uncorrelated at the transition state as they are when close to the equilibrium structure.
authors.affiliation: Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, 44780 Bochum, Germany.
authors.forename: Sergei D
authors.initials: SD
authors.lastname: Ivanov
authors.affiliation: Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, 44780 Bochum, Germany.
authors.forename: Ian M
authors.initials: IM
authors.lastname: Grant
authors.affiliation: Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, 44780 Bochum, Germany.
authors.forename: Dominik
authors.initials: D
authors.lastname: Marx
mesh_terms: D019281:Dimerization; D005561:Formates / Q000737:chemistry*; D006860:Hydrogen Bonding; D008956:Models, Chemical; D011522:Protons; D011789:Quantum Theory; D013696:Temperature
chemical_list: D005561:Formates; D011522:Protons; C030544:formic acid