EasySpin forum

I am struggling to figure out how easyspin gives rise to the "forbidden" half-field transition with the introduction of a dipolar or zero-field splitting interaction between electrons.

Terms A and B of the dipolar alphabet work with the rotating wave approximation and all time dependence is removed. However, the time dependence of terms E and F, which give rise to the half-field response, do not cancel out.

How does easyspin go about simulating half-field responses with the introduction of dipolar interactions? What do you do with Rotating wave approximation to make it work out?

For CW EPR simulations, EasySpin includes the full dipolar interaction term in the spin Hamilonian. You can see this here:

clear, clc
Sys.S = [1/2 1/2];
Sys.dip = 1;

% tensor aligned with the molecular reference frame
Sys.eeFrame = [];
eeint(Sys)

% tensor tilted relative to the molecular reference frame
Sys.eeFrame = [10 20 30]*pi/180;
eeint(Sys)

This is very helpful. But how does easyspin then produce the half-field response? I would assume easyspin must transform the Hamiltonian and put it into a rotating reference frame to make an EPR signal possible. Is that true? If so, how does it eliminate the time dependence of the four terms (C,D,E,F) the come with putting the dipolar coupling into the rotating reference frame?

I am not sure if I get your question correctly. But for CW spectra, with weak perturbation, there is no need for a rotating frame transformation afaik.