EPR spectrum of two independent spins with equal axial g

[garlic]

Hi everyone,
I just have a simple question for you. I would like to simulate the spectrum of two independent (non-interacting) spins with equal axial g tensor, which is
a simple case. For this case, normally the spectrum should look like the one of only one spin with axial g, but with more intensity. Unfortunately,
I cannot figure out why the spectra differ from each other.

2 independent spins:
Sys.S=[1/2, 1/2];
Sys.g = [2.0023 2.24; 2.0023 2.24];
Sys.lw=8;
Sys.ee = 0;

Exp.mwFreq = 9.35;
Exp.CenterSweep=[310 140];
Exp.Harmonic = 1;
Exp.Temperature = 6.3;
Exp.CrystalOrientation = [];

[x,y1] = pepper(Sys,Exp);
plot(x,y1);

1 spin:
Sys.S=1/2;
Sys.g = [2.0023 2.24];
Sys.lw=8;
Sys.ee = 0;

Exp.mwFreq = 9.35;
Exp.CenterSweep=[310 140];
Exp.Harmonic = 1;
Exp.Temperature = 6.3;
Exp.CrystalOrientation = [];

[x,y1] = pepper(Sys,Exp);
plot(x,y1);

[Stefan Stoll]

This is a numerical problem of EasySpin/Matlab. For two uncoupled spins with the same g values, two of the four energy levels are exactly degenerate. In this case, the finite numerical accuracy leads to problems in the identification of the correct eigenstates, which then leads to inaccurate line intensities.

It is best to avoid these situations with exact degeneracies by specifying a tiny coupling, for example Sys.ee = 1e-3. Then the problem disappears.