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

Area of Science:

• Computational Chemistry
• Materials Science
• Chemical Physics

Background:

• Reactive molecular dynamics (MD) simulations face timescale limitations due to high free energy barriers trapping simulations in local energy wells.
• Density functional tight binding (DFTB) is a common potential for these simulations, but its application in free energy calculations is restricted.

Purpose of the Study:

• To develop and validate a new interface between the DFTB+ software package and the PLUMED library.
• To enable accurate free energy calculations for reactive MD simulations using DFTB.

Main Methods:

• Developed a DFTB+ and PLUMED interface for free energy calculations.
• Employed third-order DFTB with metadynamics (including multiple walkers) and well-tempered metadynamics.
• Tested the interface on three archetypal rare-event chemical reactions: proton transfer in malonaldehyde, bowl inversion in corannulene, and oxygen diffusion on graphene.

Main Results:

• Successfully calculated free energies of activation (ΔG‡) for the studied reactions: 13.1 ± 0.4 kJ mol⁻¹ (malonaldehyde), 48.2 ± 1.7 kJ mol⁻¹ (corannulene), and 52.0 ± 6.2 kJ mol⁻¹ (graphene).
• Obtained DFTB free energy barriers and local minima that favorably compare with existing literature values.
• Demonstrated the effectiveness of the DFTB+ - PLUMED interface for rare-event simulations.

Conclusions:

• The developed DFTB+ - PLUMED interface effectively overcomes the timescale problem in reactive MD simulations.
• This interface provides a robust tool for accurate free energy calculations in chemical systems.
• The results highlight the utility of the interface for studying complex chemical reactions and material processes.