An Effective-Medium Tight-Binding Model for Silicon

A new method for calculating the total energy of Si systems is presented. The method is based on the effective-medium theory concept of a reference system. Instead of calculating the energy of an atom in the system of interest a reference system is introduced where the local surroundings are similar. The energy of the reference system can be calculated selfconsistently once and for all while the energy difference to the reference system can be obtained approximately. We propose to calculate it using the tight-binding LMTO scheme with the Atomic-Sphere Approximation(ASA) for the potential, and by using the ASA with charge-conserving spheres we are able to treat open system without introducing empty spheres. All steps in the calculational method is {\em ab initio} in the sense that all quantities entering are calculated from first principles without any fitting to experiment. A complete and detailed description of the method is given together with test calculations of the energies of phonons, elastic constants, different structures, surfaces and surface reconstructions. We compare the results to calculations using an empirical tight-binding scheme.Comment: 26 pages (11 uuencoded Postscript figures appended), LaTeX, CAMP-090594-

Early and Sustained Alterations in Cerebral Metabolism after Traumatic Brain Injury in Immature Rats

Although studies have shown alterations in cerebral metabolism after traumatic brain injury (TBI), clinical data in the developing brain is limited. We hypothesized that post-traumatic metabolic changes occur early (<24 h) and persist for up to 1 week. Immature rats underwent TBI to the left parietal cortex. Brains were removed at 4 h, 24 h, and 7 days after injury, and separated into ipsilateral (injured) and contralateral (control) hemispheres. Proton nuclear magnetic resonance (NMR) spectra were obtained, and spectra were analyzed for N-acetyl-aspartate (NAA), lactate (Lac), creatine (Cr), choline, and alanine, with metabolite ratios determined (NAA/Cr, Lac/Cr). There were no metabolic differences at any time in sham controls between cerebral hemispheres. At 4 and 24 h, there was an increase in Lac/Cr, reflecting increased glycolysis and/or decreased oxidative metabolism. At 24 h and 7 days, there was a decrease in NAA/Cr, indicating loss of neuronal integrity. The NAA/Lac ratio was decreased (∼15–20%) at all times (4 h, 24 h, 7 days) in the injured hemisphere of TBI rats. In conclusion, metabolic derangements begin early (<24 h) after TBI in the immature rat and are sustained for up to 7 days. Evaluation of early metabolic alterations after TBI could identify novel targets for neuroprotection in the developing brain