### Geometry Optimization Skills of DFTB: Simple to Complex Molecular Systems

#### Abstract

As the Density-functional theory (DFT) is an exact theory in principle for computing ground state electronic structures of the multi-electron and many-body systems, it's approximate variants currently being used are far from fail-safe. One of the very fundamental problem that has become apparent is its inability to account the substantial effect of van der Waals (vdW) type interactions exist in large molecular assemblies. To overcome such problem, a density functional tight binding (DFTB) theory whose fundamental formulation is based on the DFT but implements Slater−Kirkwood model and Slater−Koster files with a focus on solid state systems having vdW interactions as a binding force has been widely used recently. Its* *self-consistent charge* *(*SCC)* approach is more promising theoretical model due to introducing self-consistent calculation of Mulliken charges. Present work is aimed at evaluating the geometry optimization skills of such DFTB method while applying to very simple to quite complex molecular systems of the order: water, benzene, crystalline 1,4-bis (tri-methylsilyl) benzene, and crystalline siloxaalkane. We fully optimized the isolated molecules of each of them as well as the unit cell geometries of the last two specimens and measured the dimensions of the particular sets of bond lengths, bond angles, and torsional angles in each optimized geometry. These values are found to be in an excellent agreement with the concerned experimental values. It makes the DFTB method very versatile and superb quantum mechanical model for computing ground state electronic structures.

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