We propose in the present paper a SCC-DFTB/FF (Self-Consistent-Charge Density Functional based Tight Binding/Force-Field) scheme adapted to the investigation of molecules trapped in rare gas environments. With respect to usual FF descriptions, the model involves the interaction of quantum electrons in a molecule with rare gas atoms in an anisotropic scheme. It includes polarization and dispersion contributions and can be used for both neutral and charged species. Parameters for this model are determined for hydrocarbon-argon complexes and the model is validated for small hydrocarbons. With the future aim of studying polycyclic aromatic hydrocarbons in Ar matrices, extensive benchmark calculations are performed on (C6H6)(+/0)Arn clusters against DFT and CCSD(T) calculations for the smaller sizes, and more generally against other experimental and theoretical data. Results on the structures and energetics (isomer ordering and energy separation, cohesion energy per Ar atom) are presented in detail for n = 1-8, 13, 20, 27, and 30, for both neutrals and cations. We confirm that the clustering of Ar atoms leads to a monotonous decrease of the ionization potential of benzene for n ⩽ 20, in line with previous experimental and FF data.

Application of ab initio molecular dynamics to study free energy surfaces (FES) is still not commonly performed because of the extensive sampling required. Indeed, it generally necessitates computationally costly simulations of more than several hundreds of picoseconds. To achieve such studies, efficient density functional theory (DFT) formalisms, based on various levels of approximate computational schemes, have been developed, and provide a good alternative to commonly used DFT implementations. We report benchmark results on the conformational change FES of alanine dipeptide obtained with auxiliary density functional theory (ADFT) and second- and third-order density functional tight-binding (DFTB) methods coupled to metadynamics simulations. The influence of an explicit water solvent is also studied with DFTB, which was made possible by its lower computational cost compared to ADFT. Simulations lengths of 2.1 and 15 ns were achieved with ADFT and DFTB, respectively, in a reasonably short computational time. ADFT leads to a free energy difference (ΔF ) of ∼ -3 kcal mol between the two low energy conformers, C and C, which is lower by only 1.5 kcal mol than the ΔF computed with DFTB. The two minima in ADFT FES are separated by an energy barrier of 9 kcal mol, which is higher than the DFTB barriers by 2-4 kcal mol. Despite these small quantitative differences, the DFTB method reveals FES shapes, confor-mation geometries and energies of the stationary points in good agreement with these found with ADFT. This validates the promising applicability of DFTB to FES of reactions occurring in larger-size systems placed in complex environments.