Abstract: Buckling distortions, widely thought to lower the total energy of semiconductor surfaces, are shown to actually raise the energy of Si(111)-2\ifmmode\times\else\texttimes\fi{}1. The $\ensuremath{\pi}$-bonded-chain reconstruction, in contrast, stabilizes the surface, even relative to recently proposed magnetic reconstructions. Calculations for GaAs(110) reveal that the large charge transfers associated with buckling can stabilize the surface of heteropolar semiconductors, by returning the ions of the bulk to neutral atoms at the surface. These conclusions are based on self-consistent pseudopotential calculations.

Abstract: We continue our study of the effect of light fermions on the charge degree of freedom of magnetic monopoles. Even though the gauge coupling is weak, the Fermi vacuum is strongly perturbed by its coupling to the charge degree of freedom of the monopole. To obtain a correct picture of the vacuum we concentrate on the lowest partial wave of the Fermi field about the monopole core. We find that this simplified system can be transformed to an equivalent one-dimensional scalar field theory in which the original fermions appear as sine-Gordon solitons and the monopole charge is determined by the expectation value of the scalar field at spatial infinity. The scalar theory, though not soluble, is sufficiently transparent for us to extract the qualitative physics of monopole charge in the presence of light fermions: the Witten formula for the dependence of monopole charge on vacuum angle, ${Q}_{n}=e(n\ensuremath{-}\frac{\ensuremath{\theta}}{2\ensuremath{\pi}})$, is true no matter how small the Fermi mass $m$; the fractional charge is spread through the Fermi vacuum over a region size ${m}^{\ensuremath{-}1}$ and the excitation energy of a charged state is of order $m$; the existence of vacuum structure on such a small energy scale means that certain exotic fermion-monopole scattering processes have very large cross sections. In particular it appears that in grand unification theories monopoles will catalyze baryon decay at typical strong-interaction rates.

Abstract: Alternative methods of estimating atomic charges in haloalkanes are presented, derived from quantum mechanical and classical treatments. A scheme based on a breakdown of the transmission of charge by polar atoms into one-bond, two-bond, and three-bond additive contributions is given, in which the one-bond effect is proportional to the difference in the electronegativities of the bonded atoms, and the two- and three-bond effects functions of the atomic electronegativity and polarizability. Suitable developments of the basic scheme, including an iterative self-consistent process, give calculated dipole moments for a variety of haloalkanes in good agreement with the observed values. The atomic charges obtained by this scheme are compared with other estimates of these charges. They are similar to those derived from a simple LCAO–MO scheme but differ from those obtained by population analysis of more refined quantum mechanical calculations.

Abstract: The static charge density has been determined for imidazole, based on X-ray (Mo Ka) intensity data [1892 reflections with F o > 6ty(F) and sin 0/2 4o(F) and sin 0/2 < 0.8 A -1 at 293 K]. Full-matrix least-squares refinements were carried out using Stewart's rigid pseudoatom model. Fixed neutron values were assumed for the nuclear positional parameters, and for selected thermal parameters (Utj for H atoms and third-order Cjk I values for all atoms). The model gave a satisfactory fit to the X-ray data in terms of 150 variables which included population parameters for multipole expansions of the charge density as far as octapole terms for C and N atoms and quadrupoles for H atoms. Although ring-atom mean-square thermal vibrational amplitudes are considerably different at the two temperatures (U ~ 0.06 A 2 at 293 K; U ~ 0.02 A 2 at 103 K), the derived static charge distributions are in good agreement. This is attributed to the importance of the octapole deformation density for the imidazole-ring atoms. The charge density is consistent with a simple electrostatic theory for the strong NH...N hydrogen bond. It is also found that the approximate 2mm symmetry in the molecular geometry extends to the charge distribution, and that there are similarities with

Abstract: Results are presented for the effect of the three-body force on the charge form factors of the A = 3 nuclei Mesonic exchange currents, isobaric processes, and other effects of relativistic order are included in the present calculation The evaluation of the form factors is extended to values of momentum transfer q = 100 fm/sup -1/, and their model dependence is explored

Abstract: The asymptotic theory of resonance charge exchange between a ground-state diatomic molecular ion and its neutral parent is presented The parameters of the valence electron wavefunction and asymptotically precise exchange interaction potential are calculated The role of rotational transitions is discussed The vibrational excitation transfer is taken into account and the coupled equations, describing the charge exchange process between diatomics, are solved both in limiting cases and numerically The total charge transfer cross sections are calculated for many diatomic systems and the results are compared with experimental data