Potential of mean force calculations using ab initio tight-binding molecular dynamics: Application to N–NO2 bond dissociation in DMNA and HMX

We discuss implementation of the umbrella sampling technique using the weight histogram analysis method in FIREBALL, a density functional theory method based on local-orbitals and pseudopotential theory. With this implementation, we calculate free energy curves of chosen reaction pathways from quantum mechanical forces; therefore, the potential of mean force yields energetic evaluations which include the electronic rearrangements as the structure changes during molecular dynamics simulations. The free energy curve is calculated along the entire reaction pathway rather than the traditional approach of using static single-point calculations along a reaction potential surface. We present results on the N–NO2 bond dissociation in gas-phase DMNA and HMX. The results obtained in our calculations are in good agreement with other methodologies, which demonstrates that the umbrella sampling model implemented in FIREBALL is able to correctly predict the free energies changing along the reaction potential surface.