Functional Theory Based Model Calculations Research Articles

Structure–property relations and thermodynamic properties of monoclinic petalite, LiAlSi4O10

Structure–property relations of monoclinic petalite, LiAlSi4O10, were determined by experiment and atomistic modeling based on density functional theory. The elastic stiffness coefficients were measured between room temperature and 570 K using a combination of the plate-resonance technique and resonant ultrasound spectroscopy. The thermal expansion was studied between 100 and 740 K by means of dilatometry. The heat capacity between 2 and 398 K has been obtained by microcalorimetry using a quasi-adiabatic calorimeter. The experimentally determined elastic stiffness coefficients were employed to benchmark the results of density functional theory based model calculations. The values in the two data sets agreed to within a few GPa and the anisotropy was very well reproduced. The atomistic model was then employed to predict electric field gradients, the lattice dynamics and thermodynamic properties. The theoretical charge density was analyzed to investigate the bonding between atoms.

Compression behaviour of nitridocarbidosilicates [formula omitted][formula omitted] studied with X-ray diffraction and ab initio calculations

The compressibilities of the nitridocarbidosilicates Ln 2 [ Si 4 N 6 C ] , Ln = Ho , Er, were investigated by in situ powder synchrotron X-ray diffraction. Pressures up to 36 GPa were generated using the diamond-anvil cell technique. A fit of a third-order Birch–Murnaghan equation of state to the p – V data resulted in the bulk modulus B 0 = 162 ( 2 ) GPa and its pressure derivative B ′ = 5.1 ( 3 ) at V 0 = 605.1 ( 2 ) A ˚ 3 for Ho 2 [ Si 4 N 6 C ] . For Er 2 [ Si 4 N 6 C ] the values are B 0 = 163 ( 5 ) GPa and B ′ = 4.5 ( 6 ) at V 0 = 602.8 ( 2 ) A ˚ 3 . Complementary data of the pressure dependence of this family of compounds were obtained by density functional theory based model calculations for Y 2 [ Si 4 N 6 C ] . An anomalous compression behaviour is predicted to occur between 8 and 12 GPa for Y 2 [ Si 4 N 6 C ] . The bulk modulus was obtained from an Eulerian strain versus normalized stress plot to be about 165 GPa, i.e., a value very similar to the bulk moduli obtained experimentally for Er 2 [ Si 4 N 6 C ] and Ho 2 [ Si 4 N 6 C ] . The anomaly in the pressure dependence of the cell parameters seems to be due to an increase in the coordination number of the two symmetrically independent Y cations. This increase of coordination is mainly achieved by a rotation of the Si ( 3 ) N 3 C tetrahedron.

High-pressure properties of diaspore, AlO(OH)

The structural compression mechanism and compressibility of diaspore, AlO(OH), were investigated by in situ single-crystal synchrotron X-ray diffraction at pressures up to 7 GPa using the diamond-anvil cell technique. Complementary density functional theory based model calculations at pressures up to 40 GPa revealed additional information on the pressure-dependence of the hydrogen-bond geometry and the vibrational properties of diaspore. A fit of a second-order Birch–Murnaghan equation of state to the p–V data resulted in the bulk modulus B0 = 150(3) GPa and B0 = 150.9(4) GPa for the experimental and theoretical data, respectively, while a fit of a third-order Birch–Murnaghan equation of state resulted in B0 = 143.7(9) GPa with its pressure derivative B′ = 4.4(6) for the theoretical data. The compression is anisotropic, with the a-axis being most compressible. The compression of the crystal structure proceeds mainly by bond shortening, and particularly by compression of the hydrogen bond, which crosses the channels of the crystal structure in the (001) plane, in a direction nearly parallel to the a-axis, and hence is responsible for the pronounced compression of this axis. While the hydrogen bond strength increases with pressure, a symmetrisation is not reached in the investigated pressure range up to 40 GPa and does not seem likely to occur in diaspore even at higher pressures. The stretching frequencies of the O–H bond decrease approximately linearly with increasing pressure, and therefore also with increasing O–H bond length and decreasing hydrogen bond length.

Density Functional Theory Based Model Calculations for Accurate Bond Dissociation Enthalpies. Part 3. A Single Approach for X—H, X—X, and X—Y, (X, Y: C, N, O, S, Halogen) Bonds.

Density Functional Theory Based Model Calculations for Accurate Bond Dissociation Enthalpies. Part 3. A Single Approach for X—H, X—X, and X—Y, (X, Y: C, N, O, S, Halogen) Bonds.

Density Functional Theory Based Model Calculations for Accurate Bond Dissociation Enthalpies. 3. A Single Approach for X−H, X−X, and X−Y (X, Y = C, N, O, S, Halogen) Bonds

Molecule and radical enthalpies were computed using five model chemistries, which are differentiated by the method used for calculating geometries and scaled frequencies. For all the models, electronic energies were calculated using density functional theory (DFT) at the B3P86/6-311G(2d,2p) level of theory, which was selected following tests involving six hybrid functionals and three basis sets. The models were assessed for their ability to accurately predict the bond dissociation enthalpies (BDEs) of 34 X−H bonds and 28 X−X and X−Y bonds, where X, Y = C, N, O, S, and halogen. The mean absolute error (MAE) of the BDEs relative to experiment predicted using each of the five models is: AM1 = 2.1, PM3 = 1.7, HF/3-21G(d) = 1.6, B3P86/3-21G(d) = 1.4, and B3P86/6-31G(d) = 1.5 kcal/mol. The B3P86/6-311G(2d,2p)//B3P86/3-21G(d) and B3P86/6-311G(2d,2p)//B3P86/6-31G(d) models perform as well as G3(MP2) (MAE = 1.5) for the bonds in the test set and with a substantially lower computational cost. The models also perform well for Si−H bonds and for Si−X (X = C, N, O) bonds in radicals but not for Si−X bonds in closed-shell molecules. Comparisons are also made to a reparametrized version of B3LYP, which is also shown to perform well for most bonds in the test set. The models are shown to be applicable to the study of olefin line growth on silicon surfaces, an area of research in which we are currently involved. The basis set dependence of the X−H BDEs is examined. The shortcomings of the present models are discussed, with particular emphasis on the failure of various DFT methods to adequately describe molecules with extensive delocalization.

Density Functional Theory Based Model Calculations for Accurate Bond Dissociation Enthalpies. 2. Studies of X−X and X−Y (X, Y = C, N, O, S, Halogen) Bonds

The bond dissociation enthalpies for a set of 30 compounds containing X−Y (X, Y = C, N, O, S, halogen) bonds are computed using density functional theory based model approaches with the B3P86 functional. These types of bonds were chosen because of their particular importance in free radical organic and bio-organic chemistry, specifically redox chemistry and atom transfer reactions. A series of test calculations on hydrogen peroxide, propane, and methyl chloride led to the choice of the 6-311G(d,p) basis set for optimum performance in terms of speed and accuracy. Three models are defined and tested. The lowest level model, which is capable of treating systems containing more than 30 non-hydrogen atoms, predicts bond dissociation enthalpies with a mean absolute deviation of 2.38 kcal/mol relative to experiment. For a subset of 21 molecules, the two higher-level models predict results with mean absolute deviations of 1.88 and 2.19 kcal/mol relative to experiment. Test calculations on X−H bond energetics indicate that two separate approaches are required for the accurate treatment of both X−Y and X−H bonds.