ZFS vs NQI

[faq_user]

I have a question about how to compute a spin system with zero field splitting interacting with quadrupolar nuclei. I could see from the spinach manual that both (zfs and nuclear quadrupole) can be computed using full matrices or eigenvalues and Euler angles. However if I use the same syntax for both, e.g. inter.coupling.eigs and inter.coupling.euler, how does spinach know which is zfs and which is quadrupole?

[kuprov]

At the second spherical rank, the two interactions are mathematically identical. When the spin is a nucleus, it is called “quadrupole”, and when it is an electron it is called ZFS. Physical mechanisms are different, but mathematics is identically the same. So Spinach does not actually need to know which is which.