Performance Evaluation of DFTB1 and DFTB2 Methods in reference to the Crystal Structures and Molecular Energetics of Siloxaalkane Molecular Compass

Abstract

In the lately developed computational paradigms and state-of-art theoretical analyses, the density-functional based tight-binding (DFTB) scheme and its profound theoretical features extended to address the crystalline molecular systems stand as the most versatile quantum mechanical method. Being its computational parser codes extremely efficient and even assessable directly as a low-cost scheme despite hosting ab initio functionalities comprising mathematical formulations, the “non-self-consistent-charge” (DFTB1) and “self-consistent-charge" (DFTB2) approaches are extensively applied to the wide ranged molecular systems under both crystalline (PBC) and non-crystalline conditions. The "dispersion energy corrections (DECs)" features of it further add an additional value to their rational applications. Herewith, the intensive evaluations on their performances are carried out based on the results they produced for the experimentally synthesized amphidynamic type molecular crystal with macroscopic compass like supramolecular assembly possessing central dipolar difluorophenylene rotator (compass needle), axial Si-C bond spin axis, and peripheral -Si- & -Si-O- made siloxaalkane stator. It is found that the DFTB2 performed far better than the DFTB1 in reference to its datasets: (a) lengths for the bonds associated with rotator, stator, and spin axis, (b) free-volume unit around the rotator, (c) rotational energy barriers, viz. = 5.99 kcal/mol, & 5.52 kcal/mol under PBC with and without DECs, and (d) locations of the 1p-flipped (rotator) degenerate equilibrium structures, viz. at f  = 0.505p & 1.493p (Expt. f  = 0.56p & 1.56p) radians. Besides this, the dispersion constants for the F atom; cutoff =3.8Å, polarizability  =3.4ų, and effective nuclear charge =3.5au determined by DFTB2, and the precise unit-cell geometries derived by undertaking these numeral values further exemplified its superiority over DFTB1. It is believed that all the justifications and quantitative interpretations conferred throughout this article put considerable demands on the accuracy of DFTB2 for investigating crystal structures and molecular energetics explicitly.