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