NMR calculations can generate the shielding tensor and/or electric field gradients for all of the atoms in the selected system. NMR properties are very sensitive to atomic positions, which makes NMR such a useful experimental tool for structure analysis. This means that it is highly recommended to perform a geometry optimization run before the NMR calculation. If the structure of a crystal or molecule under investigation is obtained from an experimental study, it is advisable to optimize positions of hydrogen atoms first of all. If the forces on heavy atoms as reported by CASTEP are high (in excess of 1 eV/Å), complete structure optimization is recommended.
NMR in CASTEP is part of the separately licensed module NMR CASTEP. NMR calculations can only be performed if you have purchased this module.
The chemical shielding tensor, σ(r), is defined as the ratio between an external applied magnetic field, B, and the non-uniform induced magnetic field, Bin(r):
The isotropic shielding, σiso, is given by one third of the trace of σ(r). From the symmetric component of the shielding tensor, the chemical shielding anisotropy, σaniso and the asymmetry parameter, η, can be defined in terms of the principal components of the shielding tensor:
For values of σaniso close to zero the asymmetry parameter will be ill-defined.
The interaction of a quadrupolar nuclei with an external magnetic field can be characterized by the quadrupolar coupling constant, CQ, and the asymmetry parameter ηQ. If the principal components of the traceless electric field gradient tensor are labeled, Vxx, Vyy and Vzz such that:
where Q is the nuclear electric quadrupole moment, and:
For very small values of Vzz the asymmetry parameter will be ill-defined.
The value of the quadrupolar moment depends on the actual isotope; CASTEP makes a suggestion based on the most typical isotope used in solid state NMR experiments. The value of Q affects only the quadrupolar coupling constant, CQ, calculated and printed out by CASTEP. If you find that the wrong value of Q was used you can rescale the generated CQ constant using a different Q value.
The list of default values is based on the compilation by Pyykkö (2008). CASTEP versions prior to the 5.0 release used an older compilation by Harris (1996), so some results for quadrupolar coupling constants may have changed in later versions.
To calculate NMR properties
NMR parameters will be calculated only for the elements for which the on-the-fly-generated pseudopotentials are chosen.
It is not recommended to use PW91 exchange-correlation functional when requesting NMR properties. PW91 functional may result in sharp real-space features of on-the-fly-generated pseudopotentials; PBE, RPBE, or PBESOL are better options when a GGA functional is required.
Calculating NMR shielding tensors using pseudopotentials
CASTEP Calculation dialog