An Improved DFTB/Force Field Model for Molecule-Rare Gas Interactions: Application to 1,4-Diazabicyclo[2.2.2]octane...

It is often challenging to model the evolution of the properties of molecular compounds when they interact with an environment. We will present the combination of the density functional tight binding (DFTB) scheme, describing molecules, with a force field (FF) approach, describing rare gas atoms. We have extended a former model [Iftneret al.J. Chem. Phys.2014,140, 03430110.1063/1.4861431. ] by introducing an atomic-population-dependent correction to a repulsion term between the DFTB atoms and rare gas atoms. In this new model, the dispersion contribution also depends on atomic populations. This model allows, in particular, describing neutral and ionic systems and has been applied to simulate clusters of argon atoms and a 1,4-diazabicyclo[2.2.2]octan (DABCO) molecule. New reference calculations (MP2, CCSD(T)-F12) on small DABCO- Arn=1,40/+ clusters are also reported. We show that the new DFTB-FF model performances are significantly improved with respect to the former model for small aggregates, in particular with regard to cohesive energies. Finally, by coupling the new DFTB-FF model with a minima global search algorithm, namely a combination of Parallel Tempering Monte Carlo explorations and local quenches, we provide a set of stable structures for DABCO- Arn0/+ clusters in the range n = 1-50 atoms pointing out the size evolution of geometric features and energetic properties. Ionization potentials are shown to decrease with size, consistent with new experimental measurements. This decrease is shown to undergo a transition after the completion of the first solvation shell.