  A theory for the ab initio calculation of all-electron NMR chemical shifts in
insulators using pseudopotentials is presented. It is formulated for both
finite and infinitely periodic systems and is based on an extension to the
Projector Augmented Wave approach of Bloechl [P. E. Bloechl, Phys. Rev. B 50,
17953 (1994)] and the method of Mauri et al [F. Mauri, B.G. Pfrommer, and S.G.
Louie, Phys. Rev. Lett. 77, 5300 (1996)]. The theory is successfully validated
for molecules by comparison with a selection of quantum chemical results, and
in periodic systems by comparison with plane-wave all-electron results for
diamond.
