Research Objectives
Determination and prediction of the structure and properties of materials systems using quantum theories.
Computational Techniques
Materials properties are computed using the density functional formalism by solving a set of self-consistent Schrodinger-like equations. Computations involve extensive determination and manipulation of the eigenvalues and eigenvectors of large matrices of dimension up to several hundred thousands on the Cray C90 and T3E.
Accomplishments
First principles calculations have been performed on a number of materials systems. A new theory was formulated and implemented, allowing the ab initio calculation of nuclear magnetic resonance (NMR) chemical shifts in solids and liquids for the very first time. Calculations on clusters explained the properties of nanocrystals and fullerene materials. Studies have been performed to predict the properties of materials under pressure.
Significance
NMR is a valuable tool in chemistry and physics. By measuring the screening of an applied magnetic field, NMR experiments help understand the structure of materials. Atoms with a different chemical environment screen an applied magnetic field differently, resulting in a "chemical shift". Previously, there was no rigorous theory for first-principles calculation of the chemical shifts in solids and liquids. Our development of a new method allows us to predict and understand the NMR chemical shifts of extended systems such as crystals, amorphous materials, liquids or defects in solids. Our calculations resolved several important issues related to the nature of the NMR spectra of CVD diamond and amorphous carbon. With this method, we are now investigating amino acid and peptide crystals, systems of importance in biology.
In a joint experimental and theoretical effort, we investigated what the geochemists referred to as the "missing xenon" problem. The amount of Xe on Earth is known to be significant lower than in meteorites and the sun. A leading explanation has been that the earth's iron core might act as a storage house of the primordial Xe, owing to its high pressure and temperature. We showed that Xe has no tendency to react with Fe even at pressures exceeding a million atmospheres. With this new result, geochemists and geophysicists now have to seek another explanation for the mystery of the missing xenon.
Publications
Caldwell, W. A., J. H. Nguyen, B. G. Pfrommer, F. Mauri, S. G. Louie, and R. Jeanloz. 1997. Structure, bonding, and geochemistry of xenon at high pressures. Science 277:930.
Pfrommer, B. G., M. Cote, S. G. Louie, and M. L. Cohen. 1997. Relaxation of crystals with the quasi-Newton method. J. Computational Physics 131:233.
Mauri, F., B. G. Prommer, and S. G. Louie. 1997. Ab initio NMR chemical shift of diamond, chemical-vapor-deposited diamond, and amorphous carbon. Phys. Rev. Lett. 79:2340.
A look into an alanine crystal: carbon (green), hydrogen (red), oxygen (purple), and
nitrogen (blue). The arrows show how the electrons flow when a magnetic field is applied
perpendicular to the cutting plane.