DFTB1 and DFTB2 Based Real-Time Flipping Motion Studies of Central Phenylene Rotator of Crystalline Siloxaalkane Molecular Gyroscope

Abstract

Despite adopting ab initio functionalities comprising mathematical formulations and many unique complex computational features, the density-functional based tight-binding (DFTB) scheme presents itself as the standalone, versatile, and efficient quantum mechanical application. The recognizably low-cost, & extremely faster MD simulation parser codes of it run under the DFTB⁺ atomic simulation environment interface plus the noticeably methodical long ranged molecular interactions addressing 'non-self-consistent-charge' (DFTB1) & 'self-consistent-charge' (DFTB2) formulations adds a substantial value to its rational applications. Herewith, both the DFTB2/ & DFTB1/ MD simulation schemes are employed separately to simulate the siloxaalkane molecular gyroscope under crystalline condition with and without 'dispersion energy corrections' features at wide temperature regimes & the experimentally observed facile flipping motions of its central phenylene rotator are confirmed theoretically in real-time scales. The concerned rotary trajectories depict that: (a) at 800 K ³ T £ 1200 K, the rotator exhibits 1p inversions & frequent flipping motions easily with sub-picoseconds to picoseconds lifetimes; (b) at T = 600 K, the rotator flips between its stable positions at the intervals of several tens of picoseconds; but (c) at T = 300 K, the rotator flips more demonstratively with longer lifespans only after addressing the dispersion energies in DFTB2 scheme. Additionally, the flipping rates of the rotator;  = 0.018 ps^-1 & = 0.021 ps^-1 predicted at T = 800 K & 1200 K respectively are retarded to 0.009 ps^-1 & 0.013 ps^-1, and the flipping barrier estimated by using  & as E_a1 = 0.66 kcal/mol is increased to E_a2 = 1.84 kcal/mol after treating dispersion energies; the quite consistent values to those retrieved from the respectively derived potential energy surface (E_a1 =0.70 kcal/mol & E_a2 =1.2 kcal/mol). These quantitative results illuminate that the DFTB2/MD simulation scheme is more demandable than the DFTB1/MD especially for simulating crystalline molecular assembly.