Bernd G. Pfrommer

Ab Initio Theory of NMR Chemical Shifts in Solids and Liquids.

We present a theory for the {ital ab} {ital initio} computation of NMR chemical shifts ({sigma}) in condensed matter systems, using periodic boundary conditions. Our approach can be applied to periodic systems such as crystals, surfaces, or polymers and, with a supercell technique, to nonperiodic systems such as amorphous materials, liquids, or solids with defects. We have computed the hydrogen {sigma} for a set of free molecules, for an ionic crystal LiH, and for a H-bonded crystal HF, using density functional theory in the local density approximation. The results are in excellent agreement with experimental data. {copyright} {ital 1996 The American Physical Society.}

Unconstrained energy functionals for electronic structure calculations

The performance of conjugate gradient schemes for minimizing unconstrained energy functionals in the context of electronic structure calculations is studied. The unconstrained functionals allow a straightforward application of conjugate gradients by removing the explicit orthonormality constraints on the quantum-mechanical wave functions. However, the removal of the constraints can lead to slow convergence, in particular when preconditioning is used. The convergence properties of two previously suggested energy functionals are analyzed, and a new functional is proposed, which unifies some of the advantages of the other functionals. A numerical example confirms the analysis.