EasySpin forum

Hi,
I am trying to fit an EPR profile with two interacting spins (1 and 1/2), defined as Sys1 and Sys2 respectively, with command "esfit('pepper', spc, {Sys1, Sys2}, {Vary1, Vary2}, Exp)".

How to include J or ee parameters for the interaction between them?

Thanks

PS: Because of some reason, defining Sys.S = [1, 1/2]; Sys.J is slowing down the fitting so I do not want to use this syntax.

The only way to include J/ee between two spin is to set them up in the same spin system. So using the syntax Sys.S = [1, 1/2] . It is entirely expected that the simulation will slow down, since you are going from a Hilbert space of 2 and 3 to one with a size of 6(and a lot more if you are including nuclei). Depending on the magnitude of the interactions, you could use Opt.Method = 'hybrid'; to treat nuclear spin pertubatively and gain back some speed.

It should be noted that the hyperfine splitting may not be correst treated in 'hybrid' mode. See the issue viewtopic.php?f=4&t=569

While I understand this aspect of Easyspin, the question is interesting in cases of two weakly coupled (e.g. dipole-dipole) systems of widely differing intrinsic relaxation properties. E.g. two systems with very different spin-lattice relaxations, or very different linewidths.

I am not sure there is even a theory to describe such situations, but the question is intriguing.

The simulation will indeed slow down if you couple two spins, since EasySpin sets up the full spin Hamiltonian matrix, and its size increases with the number of spins.

For weak couplings, it is likely possible to use perturbation approach that is faster, but this is not implemented in EasySpin.