Threshold flexoelectric deformations in homeotropic nematic layers

Elastic deformations of nematic liquid crystal layers subjected to a d.c. electric field were studied numerically. The flexoelectric properties of the nematic material and the presence of ionic space charge were taken into account. Homeotropic alignment with finite surface anchoring strength was assumed. The director orientation and the electric potential distribution were calculated; the space charge density was also determined. It was found that the threshold voltage strongly depended on the parameters of the system. In particular, a threshold as low as a few tenths of a volt occurred under suitable circumstances. In the case of a negative dielectric anisotropy, Δ ε, such low values of the threshold voltage existed when the ion concentration was sufficiently high, and given sufficiently large magnitudes of the flexoelectric coefficients and a sufficiently small anchoring energy. If the ion concentration was low or if the flexoelectric coefficients were small or if the surface anchoring was strong, the threshold was equal to several volts. In the case of positive dielectric anisotropy, the threshold amounted to several tenths of a volt for a weakly anisotropic and highly conductive material. If the dielectric anisotropy was sufficiently high or if the ion concentration was sufficiently low, the threshold voltage increased with Δ ε and reached tens of volts. These results can be explained as the effect of the inhomogeneous electric field arising in the vicinity of the surfaces, due to the ionic space charge redistributed by the external voltage. They are qualitatively consistent with earlier experiments which show the effect of the ion concentration on the elastic deformations in flexoelectric nematics. They correspond also with theoretical results concerning the effect of the electric field produced by the surface polarization or by the adsorption of ions.     

Molecular structure and IR spectra of bromomethanes by DFT and post-Hartree-Fock MP2 and CCSD(T) calculations

The molecular parameters (geometries, rotational constants, dipole moments) and vibrational IR spectra (harmonic wavenumbers, absolute intensities) of bromomethanes (CH₃Br, CH₂Br₂, CHBr₃, CBr₄) are predicted by a density functional theory with the hybrid Becke3-LYP functional (DFT) and post-Hartree-Fock methods (MP2, CCSD(T)) using a 6-311G(2d,2p)-type basis set. The MP2 calculations are carried out with different numbers of frozen core orbitals to find how the number of bromine orbitals used for electron correlation influences the predicted molecular parameters and IR spectra of the species in question. Three options were used: (a) all electrons (full), with both the core and valence orbitals considered; (b) partial frozen core option (pfc), when the orbitals up to 3p of bromine were frozen; and (c) full frozen core option (ffc), when all core orbitals up to 3d were frozen. The CCSD(T) calculations for geometric parameters were carried out with both the pfc and ffc options, while for the prediction of the IR spectra only the ffc option was used. In addition, the calculations at the DFT and MP2(pfc) levels with inclusion of f functions on carbon and bromine atoms in bromomethanes (and also the CCSD(T)(pfc) calculations for CH₃Br) were carried out to predict the changes in the geometric parameters and/or vibrational IR spectra of the molecules upon inclusion of f functions The geometries of bromomethanes (particularly the CBr bond lengths) are predicted better by the DFT and CCSD(T) calculations when the f functions (in particular on bromine atom) are included, while the MP2 calculations without f functions are good enough for correct predictions of the molecular geometries. The molecular parameters and vibrational IR spectra of bromomethanes in question and their deuterated species predicted by the DFT, MP2(ffc) and CCSD(T)(ffc) with the 6-311G(2d,2p) basis set agree well with the available experimental data.