Simulations using perturbation theory are very different from spectrogram simulations using matrix diagonalization

[lulunulibiye]

Hi all，

For the same system, the simulation using perturbation theory and the simulation using matrix diagonalization affect the peak position and spectrum shape. Why is there such a big difference? I read on the easyspin website that perturbation theory does not work for electron spins with S>1/2. But for the case of spin S>1/2, can simulations using two different methods make such a big difference?

Thanks
Regards,
Lu

[Stefan Stoll]

For perturbation theory to be applicable, all interaction energies (zero-field splitting, hyperfine) have to be significantly smaller than the microwave energy. In your case, D in Sys.D is 150 GHz, which is much larger than 9.8 GHz in Exp.mwFreq, so perturbation theory is inapplicable here.