EasySpin forum

Dear Stefan,
I am trying to calculate the harmonic=0 EPR spectrum of a single crystal containing three identical UNCOUPLED COLLINEAR spins 1/2 but I don't get the expected gparBosc^2 dependence of the Intensity (where gparBosc is the component of g parallel to the oscillating field in perpendicular mode) if I use gframe. In my case, g=[gx,gy,gz], where gx is along Zlab (parallel to B static), gy is along Ylab and gz is along Xlab (parallel to Boscillating). However, it works fine if gframe is not used or if I use gframe but only with a single centre. Hereafter I attach the code that should generate a self-explanatory plot.
Am I doing something wrong?
Thank you very much in advance.

clc
clear
gz=1.5;
for i=1:10
gz=gz+0.5;
Exp.Range = [200 450];
Exp.mwFreq = 9.4;
Exp.Harmonic = 0;
Exp.Mode = 'perpendicular';
Exp.Temperature = 15;
Sys.S=[1/2 1/2 1/2];
gCu=[2.1 2.2 gz];
Sys.J=[0 0 0];
Sys.g=[gCu; gCu; gCu];
Sys.gFrame = [0 -90 0; 0 -90 0; 0 -90 0]*pi/180;
Sys.lwpp = [0 20];
Exp.CrystalOrientation = [0 0 0]*pi/180;
Opt.Threshold = 1e-4;
[B,spc] = pepper(Sys,Exp,Opt);
g(i)=gz;
res(i)=max(spc);
end

plot(g,res,'r', 'Linewidth',2)
hold on

gz=1.5;
for i=1:10
gz=gz+0.5;
Exp.Range = [200 450];
Exp.mwFreq = 9.4;
Exp.Harmonic = 0;
Exp.Mode = 'perpendicular';
Exp.Temperature = 15;
Sys.S=[1/2];
gCu=[2.1 2.2 gz];
Sys.g=[gCu];
Sys.gFrame = [0 -90 0]*pi/180;
Sys.lwpp = [0 20];
Exp.CrystalOrientation = [0 0 0]*pi/180;
Opt.Threshold = 1e-4;
[B,spc] = pepper(Sys,Exp,Opt);
max(spc)
g(i)=gz;
res(i)=max(spc);
end

plot(g,res,'b', 'Linewidth',2)
hold on
xlabel('gpartoBosc')
ylabel('Absintensity')

Your system has a lot of degeneracies. EasySpin has sometimes trouble dealing with them correctly. You can get around that problem by breaking the degeneracies ever so slightly. For example, in your case,

Starting with 6.0.0-dev.22, the handling of this is now improved. EasySpin now adds a tiny bit of numerical noise to non-zero Hamiltonian matrix elements if it suspects degeneracies.You can control the noise level with the undocumented option Opt.FuzzLevel. Its default is set to 1e-10.