## Search Constraints

## Search Results

- Creator:
- Zhang, M.-L., Van Doren, V.E., Miao, Maosheng, Mintmire, J.W., and Ladik, J.J.
- Description:
- 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 Angstrom stacking distance and 36 degrees screw angle theta) 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 theta = 36 degrees) and of the screw angle (at d = 3.36 Angstrom) 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. (C) 1999 American Institute of Physics. [S0021-9606(99)30342-1].
- Resource Type:
- Article
- Identifier:
- 0021-9606
- Campus Tesim:
- Northridge

- Creator:
- Van de Walle, Chris G., Yan, Qimin, Moses, Poul Georg, and Miao, Maosheng
- Description:
- Band gaps and band alignments for AlN, GaN, InN, and InGaN alloys are investigated using density functional theory with the with the Heyd-Scuseria-Ernzerhof {HSE06 [ J. Heyd , G. E. Scuseria , and M. Ernzerhof , J. Chem. Phys. 134 , 8207 ( 2003 ) 10.1063/1.3548872 ; 124 , 219906 ( 2006 ) ]} XC functional. The band gap of InGaN alloys as a function of In content is calculated and a strong bowing at low In content is found, described by bowing parameters 2.29 eV at 6.25% and 1.79 eV at 12.5%, indicating the band gap cannot be described by a single composition-independent bowing parameter. Valence-band maxima (VBM) and conduction-band minima (CBM) are aligned by combining bulk calculations with surface calculations for nonpolar surfaces. The influence of surface termination [(1 $\bar 1$ 1 ¯ 00) m-plane or (11 $\bar 2$ 2 ¯ 0) a-plane] is thoroughly investigated. We find that for the relaxed surfaces of the binary nitrides the difference in electron affinities between m- and a-plane is less than 0.1 eV. The absolute electron affinities are found to strongly depend on the choice of XC functional. However, we find that relative alignments are less sensitive to the choice of XC functional. In particular, we find that relative alignments may be calculated based on Perdew-Becke-Ernzerhof [ J. P. Perdew , K. Burke , and M. Ernzerhof , Phys. Rev. Lett. 134 , 3865 ( 1996 ) ] surface calculations with the HSE06 lattice parameters. For InGaN we find that the VBM is a linear function of In content and that the majority of the band-gap bowing is located in the CBM. Based on the calculated electron affinities we predict that InGaN will be suited for water splitting up to 50% In content.
- Resource Type:
- Article
- Identifier:
- 0021-9606
- Campus Tesim:
- Northridge

- Creator:
- Asfaw, Mesfin and Shiferaw, Yohannes
- Description:
- We explore the escape rate of a dimer crossing a potential barrier using both analytical and numerical approaches. We find that for small coupling strength k, the barrier hopping can be well approximated by a two step reaction scheme where one monomer hops over the barrier and is then followed by the other. In this regime the escape rate increases with k showing that the cooperativity between monomers enhances the crossing rate. However, in the limit of large coupling strength, applying the method of adiabatic elimination, we find that the escape rate is a decreasing function of k. Thus, we find that the escape rate is a non-monotonic function of the spring constant which is peaked at an optimal coupling strength. Furthermore, in the presence of a weak periodic signal, we show that the system response to the periodic signal is pronounced at a particular spring constant showing the dimer can be transported rapidly across the reaction coordinate in a half period.
- Resource Type:
- Article
- Identifier:
- 0021-9606
- Campus Tesim:
- Northridge

- Creator:
- Kioussis, Nicholas G. and Mansoori, G.A.
- Description:
- A new and analytical thermodynamic model for simple fluids with an intermolecular pair potential consisting of a hard-sphere core and an attractive Yukawa tail is presented. The model yields simple analytic expressions for all thermodynamic properties of the fluid system. The self-consistent numerical results of the present model for the compressibility factor, internal energy, and Helmholtz free energy are in excellent agreement with the Monte Carlo "experiments" for the whole range of temperatures and densities in which experimental data are available.
- Resource Type:
- Article
- Identifier:
- 0021-9606
- Campus Tesim:
- Northridge

- Creator:
- Van Camp, P.E., Miao, Maosheng, Mintmire, J.W., Ladik, J.J., and Van Doren, V.E.
- Description:
- 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 degrees. 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. (C) 1998 American Institute of Physics. [S0021-9606(98)30743-6].
- Resource Type:
- Article
- Identifier:
- 0021-9606
- Campus Tesim:
- Northridge

- Creator:
- Peric, Miroslav, Kurban, Mark R., and Bales, Barney L.
- Description:
- Bimolecular collisions between perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-l-oxyl molecules in three alkanes have been studied by measuring the electron paramagnetic resonance (EPR) spectral changes induced by spin exchange. We define an "encounter" to be a first-time collision followed by a series of re-encounters prior to the diffusing pair's escaping each other's presence. The present work stems from a recent proposal [B. L. Bales et al., J. Phys. Chem. A 107, 9086 (2003)] that an unexpected linear dependence of the spin-exchange-induced EPR line shifts on spin-exchange frequency can be explained by re-encounters of the same probe pair during one encounter. By employing nonlinear least-squares fitting, full use of the information available from the spectral changes allows us to study encounters and re-encounters separately. The encounter rate constants appear to be dominated by hydrodynamic forces, forming a common curve for hexane, decane, and hexadecane when plotted against T/eta, where eta is the shear viscosity. Unexpectedly, encounters are not dependent on the ratio mu = a/a(s), where a and a(s) are the van der Waals radii of the nitroxide probe and the solvent, respectively. It is argued that the near coincidence of the resulting encounter rate constant with the hydrodynamic prediction is likely due to a near cancellation of terms in the general diffusion coefficient. Thus, the semblance of hydrodynamic behavior is coincidental rather than intrinsic. In contrast, the mean times between re-encounters do depend on the relative sizes of probe and solvent. For hexane at lower temperatures, the Stokes-Einstein equation apparently describes re-encounters well; however, at higher temperatures and for decane and hexadecane, departures from the hydrodynamic prediction become larger as mu becomes smaller. This is in qualitative agreement with the theory of microscopic diffusion of Hynes et al. [J. Chem. Phys. 70, 1456 (1979)]. These departures are well correlated with the free volume available in the solvent; thus, the mean times between re-encounters form a common curve when plotted versus the free volume. Because free volume is manifested macroscopically by the isothermal compressibility, it is expected and observed that the re-encounter rate also forms a common curve across all three solvents when plotted with respect to compressibility. The existence of a common curve for alkanes raises the prospect of using EPR to determine the compressibility of substances such as fossil fuels and biological membranes.
- Resource Type:
- Article
- Identifier:
- 0021-9606
- Campus Tesim:
- Northridge

- Creator:
- Seshadri, Ram, Douglas, Jason E., Ma, Yanming, Wang, Yanchao, Lv, Jian, Liu, Hanyu, Birkel, Christina S., Pollock, Tresa M., Yin, Ketao, Stucky, Galen D., Miao, Maosheng, Torbet, Chris J., and Zhu, Li
- Description:
- A structure prediction method for layered materials based on two-dimensional (2D) particle swarm optimization algorithm is developed. The relaxation of atoms in the perpendicular direction within a given range is allowed. Additional techniques including structural similarity determination, symmetry constraint enforcement, and discretization of structure constructions based on space gridding are implemented and demonstrated to significantly improve the global structural search efficiency. Our method is successful in predicting the structures of known 2D materials, including single layer and multi-layer graphene, 2D boron nitride (BN) compounds, and some quasi-2D group 6 metals(VIB) chalcogenides. Furthermore, by use of this method, we predict a new family of monolayered boron nitride structures with different chemical compositions. The first-principles electronic structure calculations reveal that the band gap of these N-rich BN systems can be tuned from 5.40 eV to 2.20 eV by adjusting the composition. (C) 2012 American Institute of Physics.
- Resource Type:
- Article
- Identifier:
- 0021-9606
- Campus Tesim:
- Northridge

- Creator:
- Zhang, M.-L., Van Doren, V.E., and Miao, Maosheng
- Description:
- 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 Angstrom (3) in comparison with the x-ray scattering value of about 92 Angstrom (3) and the neutron scattering value of 88 Angstrom (3). 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 GGA. The band structures are also calculated and their changes with the external pressure are discussed. (C) 2001 American Institute of Physics.
- Resource Type:
- Article
- Identifier:
- 0021-9606
- Campus Tesim:
- Northridge