Potential energy and free energy surfaces of the formic acid dimer: Correlated ab initio calculations and molecular dynamics simulationsElectronic supplementary information (ESI) available: Optimised geometries of the structures in Tables 1 and 2. See http://www.rsc.org/suppdata/cp/b1/b110872g/

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Abstract
A theoretical study of the potential energy surface (PES) and free energy surface (FES) of the formic acid dimer was performed. A constant energy molecular dynamics/quenching technique was employed to scan the PES and the results were compared with ab initio calculations at the MP2 and the resolution of the identity (RI) approximation MP2 (RIMP2) levels of theory. Long runs of molecular dynamics/quenching calculation allowed us to evaluate the FES. Thermodynamic data for the six most significantly populated structures of the formic acid dimer were obtained within the rigid rotor–harmonic oscillator–ideal gas approximation. The PES and FES of the formic acid dimer differ. The first local minimum on the PES becomes the most populated structure at higher temperature.