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Rhombohedral ␣-Fe 2 O 3 has been studied by using density-functional theory ͑DFT͒ and the generalized gradient approximation ͑GGA͒. For the chosen supercell all possible magnetic configurations have been taken into account. We find an antiferromagnetic ground state at the experimental volume. This state is 388 meV/͑Fe atom͒ below the ferromagnetic solution. For the magnetic moments of the iron atoms we obtain 3.4 B , which is about 1.5 B below the experimentally observed value. The insulating nature of ␣-Fe 2 O 3 is reproduced, with a band gap of 0.32 eV, compared to an experimental value of about 2.0 eV. Analysis of the density of states confirms the strong hybridization between Fe 3d and O 2p states in ␣-Fe 2 O 3 . When we consider lower volumes, we observe a transition to a metallic, ferromagnetic low-spin phase, together with a structural transition at a pressure of 14 GPa, which is not seen in experiment. In order to take into account the strong on-site Coulomb interaction U present in Fe 2 O 3 we also performed DFTϩU calculations. We find that with increasing U the size of the band gap and the magnetic moments increase, while other quantities such as equilibrium volume and Fe-Fe distances do not show a monotonic behavior. The transition observed in the GGA calculations is shifted to higher pressures and eventually vanishes for high values of U. Best overall agreement, also with respect to experimental photoemission and inverse photoemission spectra of hematite, is achieved for Uϭ4 eV. The strength of the on-site interactions is sufficient to change the character of the gap from d-d to O-p-Fe-d.

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Density-functional studies of structural and electronic properties of transitionmetal sulfides formed by 3d transition metals, based on the local spindensity approximation and including non-local corrections to the exchangecorrelation functional (generalized gradient approximation),have demonstrated the importance of magneto-volume effects and magneto-structural effects, but could not achieve full agreement with experiment. A further improvement is to consider electronic correlation effects due to tightly bound and localized d-states on the transition metal atoms. With the DFT + U method used in this work, these correlation effects are taken in account and yield improved predictions for volume, magnetic moment, exchange splitting and bandgap. For MnS the semiconducting gap is correctly predicted, and for MnS 2 the highspin AFM type-III state can be stabilized over the low-spin state. For FeS even weak correlation effects lead to better predictions for the semiconducting gap, volume and magnetic moment.

In neuroendocrine PC-12 cells, evanescent-field fluorescence microscopy was used to track motions of green fluorescent protein (GFP)-labeled actin or GFP-labeled secretory granules in a thin layer of cytoplasm where cells adhered to glass. The layer contained abundant filamentous actin (F-actin) locally condensed into stress fibers. More than 90% of the granules imaged lay within the F-actin layer. One-third of the granules did not move detectably, while two-thirds moved randomly; the average diffusion coefficient was 23 x 10(-4) microm(2)/s. A small minority (<3%) moved rapidly and in a directed fashion over distances more than a micron. Staining of F-actin suggests that such movement occurred along actin bundles. The seemingly random movement of most other granules was not due to diffusion since it was diminished by the myosin inhibitor butanedione monoxime, and blocked by chelating intracellular Mg(2+) and replacing ATP with AMP-PNP. Mobility was blocked also when F-actin was stabilized with phalloidin, and was diminished when the actin cortex was degraded with latrunculin B. We conclude that the movement of granules requires metabolic energy, and that it is mediated as well as limited by the actin cortex. Opposing actions of the actin cortex on mobility may explain why its degradation has variable effects on secretion.