M. S. Miao

Density functional calculations on the structure of crystalline polyethylene under high pressures

The geometrical structures of the crystalline polyethylene under several different external pressures up to 10 GPa are optimized by a pseudopotential plane wave density functional method. Both local density (LDA) and generalized gradient (GGA) approximations for exchange-correlation energy and potential are used. It is found that LDA heavily underestimate the geometry parameters under ambient pressure but GGA successfully correct them and get results in good agreements with the experimental geometry. The calculated GGA volume is about 94 A3 in comparison with the x-ray scattering value of about 92 A3 and the neutron scattering value of 88 A3. The bulk and Young’s modulus are calculated by means of several different methods. The Young’s modulus along the chain ranges from about 350 to about 400 GPa which is in good agreement with the experimental results. But the bulk modulus is several times larger than those of experiments, indicating a different description of the interchain interactions by both LDA and...

First-principles calculation of the conformation and electronic structure of polyparaphenylene

In this article, an all-electron first-principles total energy calculation with Gaussian-type functions for the wave functions, for the exchange correlation potential, and for the charge density has been applied for single chains of polyparaphenylene (PPP). A local-density approximation within a helical band structure approach has been used. The calculated torsional potential shows a minimum at the torsion angle of 34.8°. The internal coordinates were optimized in the equilibrium conformation and are in good agreement with experimental and other theoretical results. The calculated direct band gap is 2.54 eV compared with the experimental result from UPS spectra of 3.4 eV for the gas phase. The band structure strongly depends on the conformation which suggests that the electronic properties can be modified in a wide range through doping or addition of side groups.

Density functional calculations of the structure of crystalline urea under high pressure

Abstract The geometry of the urea crystal is optimized for several different pressures using a pseudopotential density functional (DFT) approach with a plane-wave basis set and the Ceperley–Alder local density correlation potential. All the optimized intra- and intermolecular geometrical parameters, as well as the lattice vectors, are found to be in good agreement with experiment. Under high pressures, the crystal changes anisotropically; the lattice constants a and b reduce by almost 10 times more with the pressure than the constant c. Changes of the geometrical parameters within the molecule as a function of the external pressure are found to be in good agreement with the donor–acceptor theory.

Calculation of the Total Energy Per Unit Cell and of the Band Structures of the Five Nucleotide Base Stacks Using the Local-density Approximation

All-electron first-principles total energy electronic structure calculations were carried out for single chains of four nucleotide base stacks (composed of adenine, thymine, guanine, and cytosine, respectively) in the DNA B conformation (3.36 A stacking distance and 36° screw angle θ) using the local-density approximation (LDA) within a helical band structure approach. A uracil stack was also computed in the DNA B conformation and compared with the results obtained for the four DNA base stacks. The total energies per unit cell as a function of the stacking distance (at fixed screw angle θ=36°) and of the screw angle (at d=3.36 A) show in most cases rather good agreement with the experiment. As expected with LDA calculations, the band gaps were underestimated by nearly 50% compared to experimentally suggested values. Finally, some suggestions are given for the improvement of the band structures of the nucleotide base stacks.

LDA calculations of the Young's moduli of polyethylene and six polyfluoroethylenes

Abstract The Youngs moduli of the linear chain polymers, polyethylene and six polyfluoroethylenes, constrained to the planar conformation, are calculated by means of the local density approximation (LDA) using the Perdew–Zunger (PZ) correlation potential and a linear combination of Gaussian-type orbitals (LCGTO).

An LDA calculation of the conformation and electronic structure of polytetrafluoroethylene

Two different local density approximation (X{alpha} and Kohn-Sham exchange and Perdew-Zunger correlation) of the density funcitonal method have been used to calculate structural and electronic properties of six kinds of polyfluoroethylene, including polytetrafluoroethylene (PTFE), poly(1,2-difluorethylene) (PDFE), and others, for several different dihedral angles. For PTFE and PDFE, all the geometric parameters are optimized simultaneously in the stable helical conformation. The position of the minimum and the depth of the potential well are in good agreement with the experimental results. The stable helical conformation are found for PTFE and PDFE. For PDFE a shoulder close to the stable gauche conformation is found in the energy curve. The potential curves of another four kinds of polyfluorethylene are studied in detail close to the planar conformation. The side fluorine atoms strongly affect the conformation and the electronic structure. The band structure of PTFE and PDFE in optimized geometry and the other PFEs in planar zigzag conformation are calculated in good agreement with experimental results.

LDA and tight-binding: Total energy calculations of polyparaphenylene

Abstract A simple tight-binding model is presented and used to calculate the conformation and electronic structure of a single chain polyparaphenylene (PPP). A torsional angle of 52.5 ° is obtained by considering the conjugation of p z orbitals on the carbon atoms and the Coulomb repulsion between neighboring hydrogen atoms. The characteristics of the filled and the unfilled π bands are calculated by solving the p z -submatrix only at symmetry points in the Brillouin zone. The variations in these band characteristics versus the geometry parameters are discussed in detail and the results are confirmed by a full potential LDA calculation with a Ceperley—Alder term for the correlation energy.