[1] Univ Tennessee, Dept Phys & Astron, Knoxville, TN 37996 USA
[2] USN, Res Lab, Ctr Computat Mat Sci, Washington, DC 20375 USA
[3] Vanderbilt Univ, Dept Phys & Astron, Nashville, TN 37235 USA
[4] Oak Ridge Natl Lab, Condensed Matter Sci Div, Oak Ridge, TN 37831 USA