A Study of the Terahertz Spectra of Crystalline Materials (Polyethylene, Poly(Vinylidene Fluoride) form II, and α-D-Glucose) Using NDDO Semiempirical Methods

Abstract

Semi-empirical quantum chemistry methods offer a very interesting compromise between accuracy and computational load. In order to assess the performance of NDDO methods in the interpretation of terahertz spectra, the low frequency vibration modes of three crystalline materials, namely, polyethylene, poly(vinylidene fluoride) form II and α-D-glucose have been studied using the PM6 and PM7 Hamiltonians and the results have been compared with the experimental data and former calculations. The results show good qualitative or semi-quantitative agreement with the experimentally observed terahertz spectra. arXiv:1604.03919v1 [physics.chem-ph] 13 Apr 2016 to permit the identification of the transition from a 2D form to a 3D arrangement in a carbon nitride polymer [3]. The absorption in the THz has been demonstrated to display a direct correlation with the stacking order along the c axis in different types of carbon materials [4].The complexity of the underlying physical effects affecting the observed attenuation, with contributions from intra-and inter-molecular modes and a strong impact of disorder, hinders the interpretation of the data. Theoretical methods are essential for this task, and the versatility of computational quantum chemistry methods is particularly attractive for the discernment of THz spectra.The theoretical prediction of the terahertz spectra of crystalline materials has been based mostly on force-field or density functional theory (DFT) methods [1]. Ab-initio and density functional theory (DFT) methods are regarded as highly accurate in the prediction of molecular vibrations [5] when used with correction factors to compensate systematic deviations. Nevertheless, the relevance of long-range and intermolecular contributions can make the study of the THz vibrational spectra of materials very demanding in terms of the required solid-state calculations over large systems. This imposes a severe limitation in the use of computationally costly ab-initio or DFT methods. On the other hand, semi-empirical methods [6,7], specially with the recently developed parallel implementations for shared-memory multiprocessor and massively-parallel graphics processing units architectures [8], permit to study the vibrations in large molecular systems and crystals at reduced computation times.Semi-empirical NDDO (neglect of the diatomic differential overlap) computations are based on approximations and data obtained from given training sets which calls for their assessment when employed in a given task, and can be placed at an intermediate theory level between force-field and ab-initio or DFT methods. Poor performance of the AM1 [9] and the PM3 [10] methods reported in [6] disfavored their use in the analysis of vibrational spectra until the appearance of the PM6 parametrization [11], with improved accuracy in the prediction of geometries and the description of the material vibrations [7]. The recently developed PM7 [12] method yields higher accuracy in the guessed structures, particularly for large molecular crystals, even ...

Nitrogen-carbon graphite-like semiconductor synthesized from uric acid

“…The crystal parameters of the geometry optimized with the PM6 Hamiltonian were a=14.870 Å, b=7.307 Å, c=6.401 Å, α=89.76 o , β=64.67 o and γ=89.78 o , very close to their actual values. In these conditions, it can be expected that the vibrations predicted in the THz range will be close to those of the measured data [48,55]. In the polymer unit, shown in Fig.…”

“…13(a) displays the results for pure material pellets -except for CN600-2, which was diluted at a 45 wt% concentration in a PE matrix (since it was impossible to prepare a pure sample pellet)-, while Fig. 13(b) [48]. In the present case, the main signature of the THz spectrum was shifted to 1.44 THz when the measurement was performed at room temperature.…”

“…calculations of vibration modes in periodic systems is the selection of the relevant vibrations from the whole set of calculated modes [48,56]. All the optimized geometries in this study corresponded to true ground states, except for the 3D system depicted in Fig.…”

A Study of the Far Infrared Spectrum of N-Acetyl-D-Glucosamine Using THz-TDS, FTIR, and Semiempirical Quantum Chemistry Methods

“…The use of two crystal cells for the calculation of the vibrations resulted in the inclusion of modes with wave vectors sampling different points of the first Brillouin zone of the crystal. Nevertheless, only ≅ 0 solutions are relevant for the optical spectrum [24]. The in-phase oscillating modes were selected from the global set of vibrations [24] by projecting the mass-scaled eigenvectors components of the two cells in the computation domain and selecting only those vibrations for which the scalar product was larger than 0.9.…”

“…Nevertheless, only ≅ 0 solutions are relevant for the optical spectrum [24]. The in-phase oscillating modes were selected from the global set of vibrations [24] by projecting the mass-scaled eigenvectors components of the two cells in the computation domain and selecting only those vibrations for which the scalar product was larger than 0.9. Table 3, where the results are compared with those of the former study by Kovács et al [15].…”