Computations of Magnetic Resonance Parameters for Crystalline Systems: Principles

Magnetic resonance experiments of increasing sophistication probe the structure and dynamics of solid-state materials, for example, to determine the arrangement of atoms in a lattice, the packing of units in a molecular crystal, or the structure of a defect site. There is a clear need for quantitative theoretical support for these experiments, particularly to link the observed spectrum to the underlying microscopic structure. First-principles quantum-mechanical methods have the potential to provide this information. Indeed, traditional quantum-mechanical techniques have long been used to study isolated molecules and establish important links between structure and spectra (see Shielding Calculations). However, when one is interested in the magnetic response of extended systems, it is important to take into account the crystal nature of the material; attempts to extend quantum chemical approaches have been made, for example, by representing the effects of the crystal lattice through an arrangement of point charges, or by constructing an appropriately sized cluster of atoms or molecules. However, an alternative approach is to start from common material modeling techniques which exploit the translation symmetry inherent in crystals. The first of these was the use of the linear augmented plane wave (LAPW) approach to compute electric field gradients (EFGs). More recently, there has been a series of developments based on the planewave-pseudopotential technique and it is now possible to compute NMR chemical shifts and J -couplings, as well as electron paramagnetic resonance (EPR) g-tensors and hyperfine couplings, for crystalline systems. 2 OVERVIEW OF THE