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# Recommendations For NMR Chemical Shift Calculations

GIAOs

It is strongly recommended to use GIAOs in the calculation of magnetic properties

Large-scale calculations

In large-scale calculations, it is recommended to treat the magnetic-field perturbations in a sequential mode (keyword: TREAT_PERTURBATION=SEQUENTIAL) in order to reduce disk-space requirements (storage of one set of perturbed two-electron integrals instead of simultaneous storage of all perturbed two-electron integrals). In addition, partial AO algorithms (keyword ABCDTYPE=AOBASIS) and the module xecc (keyword: CC_PROG=ECC) should be used whenever possible.

CC_PROG=ECC is mandatory for NMR chemical shift calculations at CCSDT-n, CC3 and CCSDT level.

Frozen-core calculations

Frozen-core orbitals (i.e., frozen inner-shell orbitals) can be used within all NMR chemical shift calculations.

Files CSHIFT and CSHIFTSCF

Files CSHIFT and CSHIFTSCF contain the corresponding shielding tensors as obtained at HF-SCF and correlated levels, respectivey.

Relative chemical shifts

Relative shifts {$\delta$} are obtained via {$\delta = \sigma_{ref} - \sigma$} with {$\sigma_{ref}$} and {$\sigma$} as the isotropic shieldings of the chosen reference compound (e.g., TMS for 1H and 13C) and the nucleus of interest. Note that the calculation for the reference compound should be performed at the same computational level (method, basis set, and geometry) as for the actual compound of interest.

Nuclear spin-rotation constants

Additional calculation of nuclear spin-rotation tensors is invoked via the keyword SPIN_ROTATION=ON

Magnetizabilities and rotational g-tensors

Additional calculation of magnetizabilities and rotational g-tensors is invoked via the keyword PROP=MAGNETIC instead of PROP=NMR.

Magnetically induced current densities

Magnetically induced current densities can be computed after a NMR chemical-shift calculation using the program GIMIC (not included in the standard release of CFOUR, see interface to GIMIC).