Two different local-density approximations, the Gáspár-Kohn-Sham and the Perdew-Zunger approximations, of the density-functional method have been used to calculate structural and electronic properties of polyethylene systems with several different dihedral angles. For each system, the CC bond lengths and the CCC and HCH bond angles are optimized simultaneously. All the parameters appear to be strongly coupled with torsional freedom and vary with the change in dihedral angle in a pattern similar to that of the total energy. The total energy has an absolute minimum for the planar zigzag conformation but a distinct local minimum for the quasistable helical conformation. Another minimum occurs in the energy curve close to this gauche minimum. The calculated valence and conduction bands are discussed and compared with other theoretical calculations and experiment. ͓S0163-1829͑96͒02239-4͔

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A first-principles density-functional calculation of the electronic and vibrational structure of the key melanin monomers J. Chem. Phys. 120, 8608 (2004) 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 ͑PPP͒. 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.

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Data from some recent tornado damage assessments are used to compute the percentage of damage path area by enhanced Fujita (EF) rating and to estimate kinetic energy. Only a small fraction of the damage area gets the highest damage rating, and this fraction is lower than a model used by the U.S. Nuclear Regulatory Commission. However, estimates of kinetic energy derived from a characteristic wind speed for each EF rating and the fraction of area with that rating match kinetic energy estimates using the model percentages. On average, the higher the EF rating, the larger the kinetic energy, but there is large variability in the relationship. The average total kinetic energy over the EF1 tornadoes examined in the study is 0.61 TJ, which compares with an average of 2.37 TJ, 40.1 TJ, 36.5 TJ, and 50.4 TJ for the EF2, EF3, EF4, and EF5 tornadoes, respectively. The most energetic tornado examined had a maximum damage rating of EF3.

MgSe has been studied using energy dispersive x-ray diffraction to 202 GPa and local density approximation and ultrasoft pseudopotentials to 500 GPa. MgSe undergoes a "continuous" phase transformation from the rocksalt to FeSi (B28) beginning at around 99 6 8 GPa and approaching sevenfold coordination at 202 GPa. Theoretical computation finds the B28 transition beginning at 58 GPa followed by a transition to an orthorhombic distortion of the B2 structure at 429 GPa.[S0031-9007(98)