Coupling density functional based tight binding with class 1 force fields in a hybrid QM/MM scheme

Quantum Mechanics/Molecular Mechanics (QM/MM) hybrid methods have become very popular schemes to incorporate environmental effects in the calculation of molecular properties, when it is mandatory both a quantum description electrons compute these properties and an atomistic environment. However, even Density Functional Theory/MM may timecosting large part system should be treated at QM level or plenty single point energy calculations are intended done. We report new implementation, within deMonNano code, QM/MM scheme combining density functional based tight binding with class 1 force fields. Two types additive couplings can chosen, namely mechanical coupling, consisting Lennard-Jones potential electrostatic which MM also polarizing region described level. As first test-case application, harmonic infrared spectra simple molecules gas phase water clusters computed compared those obtained DFT/MM Binding energies compared. Similar trends two levels main differences discussed.