Metadynamics combined with auxiliary density functional and density functional tight-binding methods: alanine dipeptide as a case study

Application of ab initio molecular dynamics to study free energy surfaces (FES) is still not commonly performed because of the extensive sampling required. Indeed, it generally necessitates computationally costly simulations of more than several hundreds of picoseconds. To achieve such studies, efficient density functional theory (DFT) formalisms, based on various levels of approximate computational schemes, have been developed, and provide a good alternative to commonly used DFT implementations. We report benchmark results on the conformational change FES of alanine dipeptide obtained with auxiliary density functional theory (ADFT) and second-and third-order density functional tight-binding (DFTB) methods coupled to metadynamics simulations. The influence of an explicit water solvent is also studied with DFTB, which was made possible by its lower computational cost compared to ADFT. Simulations lengths of 2.1 and 15 ns were achieved with ADFT and DFTB, respectively, in a reasonably short computational time. ADFT leads to a free energy difference (Delta Feq-ax) of similar to -3 kcal mol(-1) between the two low energy conformers, C-7eq and C-7ax, which is lower by only 1.5 kcal mol(-1) than the Delta Feq-ax computed with DFTB. The two minima in ADFT FES are separated by an energy barrier of 9 kcal mol(-1), which is higher than the DFTB barriers by 2-4 kcal mol(-1). Despite these small quantitative differences, the DFTB method reveals FES shapes, conformation geometries and energies of the stationary points in good agreement with these found with ADFT. This validates the promising applicability of DFTB to FES of reactions occurring in larger-size systems placed in complex environments.