Recommendations For The Calculation Of Indirect Spin-Spin Coupling Constants
note that orbital unrelaxed calculations (DIFF_TYPE=UNRELAXED) are strongly recommended for the calculation of indirect spin-spin coupling constants.
Use of MP2 and CCSD(T)
MPn and CCSD(T) cannot be recommended for the calculation of indirect spin-spin coupling constants, as both methods require proper inclusion of orbital relaxation.
Use of frozen-core orbitals cannot be recommended for unrelaxed calculations of indirect spin-spin coupling constants, as neglect of the corresponding orbital relaxation contribution (as recovered via single excitations) might lead to large errors.
FC/SD cross terms
Note that currently no FC/SD cross contributions to the anisotropic part of the indirect spin-spin coupling tensor can be computed using CFOUR.