Alfredo Correa

We develop a theory for the interactions between magnetic impurities in nanoscopic systems. The case of impurities in quantum corrals built on the (111) Cu surface is analyzed in detail. For elliptical corrals with one impurity, clear magnetic mirages are obtained. This leads to an enhancement of the inter-impurity interactions when two impurities are placed at special points in the corral. We discuss the enhancement of the conduction electron response to the local perturbation in other nanoscopic systems.

Ab initio molecular dynamics and electronic structure calculation had become one of the most useful tools to investigate properties of materials. Unfortunately these atomistic detailed results are rarely reused in calculations at a higher level of description, such as fluid dynamics and finite elements calculations. In this talk we present a concrete example showing the way that first principles results can be expressed in a way that is useful for hydrodynamics calculations, in particular we show how to build a analytic equation of state for Carbon that involves solid (diamond and BC8) and liquid phases. Applications of this newly obtained equation of state will be presented. This work was performed under the auspices of the U.S. Dept. of Energy at the University of California/Lawrence Livermore National Laboratory under contract no. W-7405-Eng-48.

We have used first principles molecular dynamics and theoretical X-ray absorption spectroscopy (XAS) to investigate the aqueous solvation of cations in MgCl2, CaCl2, and NaCl solutions. We focus our discussion on the species-specific effects that Mg^2+, Ca^2+, and Na^+ have on the X-ray absorption spectrum of the respective solutions. For the divalent cations, we find that the water molecules that form a rigid first solvation shell around Mg^2+ and a more flexible solvation shell around Ca^2+ also exhibit differing hydrogen bonding characteristics. Acceptor hydrogen bonds present in the water surrounding Ca^2+ enhance a post-edge peak near 540 eV in the XAS spectrum, while the absence of such hydrogen bonding features for the first shell surrounding Mg^2+ corresponds to a diminished intensity at the post-edge peak. For Na^+, we find that a broad tilt angle distribution results in broadened post-edge features, despite donor-and-acceptor populations comparable to Ca^2+. We present re-averaged spectra of the MgCl2, CaCl2, and NaCl solutions that provide an explanation of concentration-dependent features that have been found in corresponding experimental measurements.

The investigation of the phase diagram of carbon has been the subject of experimental research for several decades. Unfortunately progress has been slow due to the extreme temperature and/or pressure required to melt diamond. From the point of view of simulations, it is rather challenging to simulate melting lines under pressure completely from first principles, and only very recently elaborate tools to perform such simulations have been developed [1,2]. We present results for the high pressure portion of the carbon phase diagram, where we have determined new melting lines and predicted metallization pressure and temperature. Our results were obtained using ab-initio molecular dynamics, together with two phase simulation techniques and free energy calculations. This work was performed under the auspices of the US Department of Energy by the University of California at the LLNL under contract no W-7405-Eng-48. [1] S. Bonev et al., Nature 431, 669 (2004) [2] T. Ogitsu et al., Phys. Rev. Lett. 91, 175502 (2003)

Ab initio electronic structure calculations in the presence of magnetic fields have been mainly performed for isolated systems, or, in the case of periodic systems, by adopting perturbative approaches. Building on a recent formulation of electronic structure calculations in the presence of magnetic fields [1,2], we will discuss calculations for periodic systems under arbitrary conditions, which include arbitrary (finite) magnetic field, arbitrary periodic cell shapes, and magnetic field spatial variations. Preliminary results based on a planewave numerical approach and local approximations to Density Functional Theory will be presented.[1] W.Cai, G.Galli, Phys. Rev. Lett. 92, 186402 (2004).[2] E. Lee, W. Cai, G. Galli, J. Comput. Phys. 226, 1310 (2007)