First Principle Free Energy Calculations Made Simple: The Example Case of Alanine Dipeptide, from Classical Force-field to Hybrid Functional.*

Ab initio molecular dynamics (AIMD) is a powerful method to study the microscopic properties of broad classes of materials, but its predictive power is limited by the short timescales that can be investigated. Here, we present a scheme coupling SSAGES¹, a plug-in for enhanced sampling calculations, and Qbox², a plane-wave DFT MD code, that allows for the efficient use of several enhanced sampling techniques in AIMD. We demonstrate the power of the framework by computing the free energy surface (FES) of alanine-dipeptide in vacuum with Adaptive-Bias-Force using classical potentials and two density functionals: a semi-local (PBE) and a hybrid functional (PBE0). We find substantial differences between the results of DFT and those of the classical potentials: the latter accurately reproduce the energy minima of the system, however, they describe inaccurately high free energy states. The coupling scheme introduced here is general and allows for simple and straightforward calculations of FES from first principles.

¹ http://miccomcodes.org/manual/index.html
² http://qboxcode.org