Density functional tight binding-based free energy simulations in the DFTB+ program

The timescale problem-in which high barriers on the free energy surface trap molecular dynamics simulations in local energy wells-is a key limitation of current reactive MD simulations based on the density functional tight binding (DFTB) potential. Here, we report a new interface between the DFTB+ software package and the PLUMED library for performing DFTB-based free energy calculations. We demonstrate the performance of this interface for 3 archetypal rare-event chemical reactions, (i) intramolecular proton transfer in malonaldehyde, (ii) bowl inversion in corannulene, and (iii) oxygen diffusion on graphene. Using third-order DFTB in conjunction with metadynamics (with/without multiple walkers) and well-tempered metadynamics, we report here free energies of activation (ΔG^‡ ) of 13.1 ± 0.4, 48.2 ± 1.7, and 52.0 ± 6.2 kJ mol^-1 , respectively, for these processes. In each case, our DFTB free energy barriers and local minima compare favorably with previous literature results, demonstrating the utility of the DFTB+ - PLUMED interface. © 2018 Wiley Periodicals, Inc.