Cheng Xin-Lu

Electronic structure and optical properties of rutile RuO2 from first principles

The systematic trends of electrionic structure and optical properties of rutile (P42/mnm) RuO2 have been calculated by using the plane-wave norm-conserving pseudopotential density functional theory (DFT) method within the generalised gradient approximation (GGA) for the exchange–correlation potential. The obtained equilibrium structure parameters are in excellent agreement with the experimental data. The calculated bulk modulus and elastic constants are also in good agreement with the experimental data and available theoretical calculations. Analysis based on electronic structure and pseudogap reveals that the bonding nature in RuO2 is a combination of covalent, ionic and metallic bonds. Based on a Kramers–Kronig analysis of the reflectivity, we have obtained the spectral dependence of the real and imaginary parts of the complex dielectric constant (1 and 2, respectively) and the refractive index (n); and comparisons have shown that the theoretical results agree well with the experimental data as well. Meanwhile, we have also calculated the absorption coefficient, reflectivity index, electron energy loss function of RuO2 for radiation up to 30 eV. As a result, the predicted reflectivity index is in good agreement with the experimental data at low energies.

The evaluation of bond dissociation energies for NO2 scission in nitro compounds using density functional and complete basis set methods

By using the density functional theory (B3LYP) and four highly accurate complete basis set (CBS-Q, CBS-QB3, CBS-Lq and CBS-4M) ab initio methods, the X(C, N, O)–NO2 bond dissociation energies (BDEs) for CH3NO2, C2H3NO2, C2H5NO2, HONO2, CH3ONO2, C2H5ONO2, NH2NO2 (CH3)2NNO2 are computed. By comparing the computed BDEs and experimental results, it is found that the B3LYP method is unable to predict satisfactorily the results of bond dissociation energy (BDE); however, all four CBS models are generally able to give reliable predication of the X(C, N, O)–NO2 BDEs for these nitro compounds. Moreover, the CBS-4M calculation is the least computationally demanding among the four CBS methods considered. Therefore, we recommend CBS-4M method as a reliable method of computing the BDEs for this nitro compound system.

Application of shell model in molecular dynamics simulation to MgO

The P-V-T equation of state of MgO has been simulated under high pressure and elevated temperature using the molecular dynamics (MD) method with the breathing shell model (BSM). It is found that the MD simulation with BSM is very successful in reproducing accurately the measured molar volumes of MgO over a wide range of temperature and pressure. In addition, the MD simulation reproduces accurately the measured volume compression data of MgO up to 100GPa at 300K. It is demonstrated that the MD simulated P-V-T equation of state of MgO could be applied as a useful internal pressure calibration standard at elevated temperatures and high pressures.

Simulated Melting Curve of NaCl up to 200 kbar

The melting curve of NaCl is studied up to 200 kbar by means of the shell-model molecular dynamics method, using massive shell core interaction potentials. The model for the interatomic interaction is shown to produce reasonable results at a wide range of pressures in bulk transitions. The pressure dependence of the melting curve of NaCl was calculated and the result was modified on the assumption of overheating due to the small system size and small time scale simulation. The final result is in good agreement with the corrected experimental values, accounting for melting mechanisms such as surface heating or superheating. Therefore, it is believed that bulk transition simulation at constant pressure indeed provides a useful tool for studying the melting transition.

Effects of hydrogen bonds on solid state TATB, RDX, and DATB under high pressures

To probe the behavior of hydrogen bonds in solid energetic materials, we conduct ReaxFF and SCC–DFTB molecular dynamics simulations of crystalline TATB, RDX, and DATB. By comparing the intra- and inter-molecular hydrogen bonding rates, we find that the crystal structures are stabilized by inter-molecular hydrogen bond networks. Under high-pressure, the inter- and intra-molecular hydrogen bonds in solid TATB and DATB are nearly equivalent. The hydrogen bonds in solid TATB and DATB are much shorter than in solid RDX, which suggests strong hydrogen bond interactions existing in these energetic materials. Stretching of the C–H bond is observed in solid RDX, which may lead to further decomposition and even detonation.

Structural, Electronic Properties and Chemical Bonding of Borate Li4CaB2O6 under High Pressure: an Ab Initio Investigation

We calculate structural, electronic properties and chemical bonding of borate Li4CaB2O6 under high pressure by means of the local density-functional pseudopotential approach. The equilibrium lattice constants, density of states, Mulliken population, bond lengths, bond angles as well as the pressure dependence of the band gap are presented. Analysis of the simulated high pressure band structure suggests that borate Li4CaB2O6 can be used as the semi-conductor optical material. Based on the Mulliken population analysis, it is found that the electron transfer of the Li atom is very different from that of other atoms in the studied range of high pressures. The charge populations of the Li atom decrease with the pressure up to 60 GPa, then increase with the pressure.

Molecular dynamics study for the melting curve of MgO at high pressure

Shell-model molecular dynamics method is used to study the melting temperatures of MgO at elevated temperatures and high pressures using interaction potentials. Equations of state for MgO simulated by molecular dynamics are in good agreement with available experimental data. The pressure dependence of the melting curve of MgO has been calculated. The surface melting and superheating are considered in the correction of experimental data and the calculated values, respectively. The results of corrections are compared with those of previous work. The corrected melting temperature of MgO is consistent with corrected experimental measurements. The melting temperature of MgO up to 140GPa is calculated.

Electronic, thermodynamic and elastic properties of pyrite RuO2

This paper calculates the elastic,thermodynamic and electronic properties of pyrite (P a3ˉ) RuO2 by the plane-wave pseudopotential density functional theory (DFT) method.The lattice parameters,normalized elastic constants,Cauchy pressure,brittle–ductile relations,heat capacity and Debye temperature are successfully obtained.The Murnaghan equation of state shows that pyrite RuO2 is a potential superhard material.Internal coordinate parameter increases with pressure,which disagrees with experimental data.An analysis based on electronic structure and the pseudogap reveals that the bonding nature in RuO2 is a combination of covalent,ionic and metallic bonding.A study of the elastic properties indicates that the pyrite phase is isotropic under usual conditions.The relationship between brittleness and ductility shows that pyrite RuO2 behaves in a ductile matter at zero pressure and the degree of ductility increases with pressure.This paper calculates the elastic, thermodynamic and electronic properties of pyrite (Pa) RuO2 by the plane-wave pseudopotential density functional theory (DFT) method. The lattice parameters, normalized elastic constants, Cauchy pressure, brittle–ductile relations, heat capacity and Debye temperature are successfully obtained. The Murnaghan equation of state shows that pyrite RuO2 is a potential superhard material. Internal coordinate parameter increases with pressure, which disagrees with experimental data. An analysis based on electronic structure and the pseudogap reveals that the bonding nature in RuO2 is a combination of covalent, ionic and metallic bonding. A study of the elastic properties indicates that the pyrite phase is isotropic under usual conditions. The relationship between brittleness and ductility shows that pyrite RuO2 behaves in a ductile matter at zero pressure and the degree of ductility increases with pressure.

Potential energy surfaces of ozone in the ground state

Equilibrium parameters of ozone, such as equilibrium geometry structure parameters, force constants and dissociation energy are presented by CBS-Q ab initio calculations. The calculated equilibrium geometry structure parameters and energy are in agreement with the corresponding experimental values. The potential energy function of ozone with a C2v symmetry in the ground state is described by the simplified Sorbie–Murrell many-body expansion potential function according to the ozone molecule symmetry. The contour of bond stretching vibration potential of an O3 in the ground state, with a bond angle (�) fixed, and the contour of O3 potential for O rotating around O1–O (R1), with O1–O bond length taken as the one at equilibrium, are plotted. Moreover, the potentials are analysed.

Computation of Bond Dissociation Energies for Removal of Nitrogen Dioxide Groups in Certain Aliphatic Nitro Compounds

Bond dissociation energies for removal of nitrogen dioxide groups in 10 aliphatic nitro compounds, including nitromethane, nitroethylene, nitroethane, dinitromethane, 1-nitropropane, 2-nitropropane, 1-nitrobutane, 2-methyl-2-nitropropane, nitropentane, and nitrohexane, are calculated using the highly accurate complete basis set (CBS-Q) and the three hybrid density functional theory (DFT) methods B3LYP, B3PW91 and B3P86 with 6-31G** basis set. By comparing the computed bond dissociation energies and experimental results, we find that the B3LYP/6-31G** and B3PW91/6-31G** methods are incapable of predicting the satisfactory bond dissociation energy (BDE). However, B3P86/6-31G** and CBS-Q computations are capable of giving the calculated BDEs, which are in extraordinary agreement with the experimental data. Nevertheless, since CBS-Q computational demands increase rapidly with the number of containing atoms in molecules, larger molecules soon become prohibitively expensive. Therefore, we suggest to take the B3P86/6-31G** method as a reliable method of computing the BDEs for removal of the NO2 groups in the aliphatic nitro compounds.