# Calculate spin-spin coupling based on previous GIAO calculation with Gaussian

I used GAUSSIAN/GAUSSVIEW to calculate the GIAO Magnetic shielding tensor (ppm), but I forgot to calculate the Nuclear spin-spin coupling J (Hz). How can I use the .chk file or .log file to calculate only the spin-spin coupling constants?

I do not want to re-do the full nmr calculation. This is what I used:

# nmr=giao b3lyp/6-311g(d,p) pop=full geom=connectivity

• Did you tried the keyword ReadFC: Read the Fermi contact spin-spin terms from the checkpoint file and then compute the other spin-spin coupling terms? (from: gaussian.com/nmr) – Camps Jul 2 '20 at 14:22

You will have to run a new calculation to get the spin-spin constants. Gaussian computes the shielding tensor $$\mathbf{\sigma}$$ and spin-spin coupling constants $$\mathbf{K}$$ using a linear response formalism. For these properties, which are both 2nd rank tensors/2nd derivatives of the energy, this means they are calculated by forming a perturbed density with respect to one parameter and contracting it with the other, unperturbed parameter.
The shielding tensor component for an atom $$\sigma_\ce{A}$$ is the second derivative of the energy with respect to the dipole of $$\ce{A}$$, $$\mu_\ce{A}$$, and the external magnetic field $$\mathbf{B}^\text{ext}$$. Gaussian by default forms the magnetic perturbed density to calculate this (and I haven't seen a way to change it). The spin-spin constant $$K_\ce{AB}$$ is the second derivative of the energy with respect to the dipole of $$\ce{A}$$, $$\mu_\ce{A}$$, and $$\ce{B}$$, $$\mu_\ce{B}$$. Here is a Link explaining some of the theory. Since Gaussian didn't form the electric dipole perturbed density when calculating $$\mathbf{\sigma}$$, the data from the previous calculation doesn't allow you to compute $$\mathbf{K}$$.
You can at least avoid repeating the SCF by including guess=read and %oldchk=old_chk_name.chk to get the result from the previous calculation, but there is no getting around repeating the CPHF portion of the calculation. In principle, Gaussian could be made to form the electric dipole perturbed density when calculating the shielding tensor and store the result in the chk, which should give you most of what you would need to later run a spin-spin calculation. However, I'm not aware of a way to store this with Gaussian's existing keywords/user interface.