NMR selection

Checking the NMR checkbox on the Properties tab displays options for calculating shielding tensors and electric field gradients.

NMR in CASTEP is part of the separately licensed module NMR CASTEP. NMR calculations can only be performed if you have purchased this module.

Shielding: Determines the chemical shielding tensor for each atom in the system.

The shielding tensor is ill-defined for spin-polarized calculations. NMR calculations are allowed wth a spin-polarized setting for a paramagnetic system, but you should check that the final electronic state is indeed paramagnetic. Similarly, such calculations are not recommended for metallic systems without a band gap; however, it is legitimate to use variable orbital occupancies and metallic settings while modeling an insulator.

EFG: Determines the electric field gradients for each atom in the system.

J-coupling: Determines the J-coupling between the selected atom(s) and all other atoms in the system. At least one atom must be selected in the input 3D Atomistic document.

J-coupling can be calculated only for non-magnetic systems. The Relativistic treatment of the OTFG pseudopotentials (on the Electronic tab of the CASTEP Calculation dialog) must be set to either Schroedinger for non-relativistic treatment or ZORA for scalar-relativistic effects to be taken into account. J-coupling calculations are supported only for systems with P1 symmetry. If your crystal has symmetry other than P1, you will be prompted to convert the crystal symmetry to P1 before you can continue.

G-tensor: Calculates the change in the EPR g-tensor for a molecule in a box or a defect in a crystal, as well as the tensor of hyperfine parameters.

The g-tensor and hyperfine parameters can only be calculated with spin-polarization turned on and for systems with P1 symmetry. If your crystal has symmetry other than P1, you will be prompted to convert the crystal symmetry to P1 before you continue.

System type: Specify the type of system used in the calculation. Options are:

Crystal
Molecule
Auto - CASTEP will assess the system and treat it as a crystal or molecule as appropriate

Calculations in the Molecule mode can only be performed for systems with P1 symmetry. If your crystal representing molecule in the box has symmetry other than P1, you will be prompted to convert the crystal symmetry to P1 before you can continue.

More...: Provides access to the CASTEP NMR Options dialog, which allows you to specify the nuclear quadrupolar moment for each element.

Calculation of the NMR parameters requires higher accuracy for the wavefunctions than total energy or geometry optimization calculations. This affects how you choose the Energy cutoff quality parameter on the Electronic tab, even choosing Ultra-fine may not be sufficient to obtain high quality NMR parameters. Therefore, it is advisable to study the convergence of the results as the Energy cutoff parameter is increased. Generally, setting the cutoff to between 270 and 400 eV should produce convergence of the shielding constant to about 1 ppm, but in some cases, the cutoff has to be increased up to 550 eV.

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NMR selection

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