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% fast-motional cw EPR spectrum simulation of a nitroxide radical
%==========================================================================
% bestvalues: [-4]
clear, clf
[exp,spc] = eprload('c:\users\kk\onedrive - uniwersytet Jagielloński\data\arnab280324\AD00216_dimer-ox2-kk4-160K-p2.DTA'); % Bruker BES3T

% Parameters

% S
S.g = [2.00567];
S.Nucs = '14N';
S.A = [32.0];
S.lwpp = [0.4 0.4];

% S - Dynamic parameters
S.logtcorr = [-6];

% Experimental parameters
Exp.mwFreq = 9.20526;
Exp.Range = [323, 333];
Exp.nPoints = 1024;

Vary1.logtcorr = [3];
Vary1.A = [5];
Vary1.lwpp = [0.3 0.3];
Vary1.g = [0.005];

%SimOpt.Method = 'perturb';

% Calling the fitting function

esfit(spc,@garlic,{S, Exp},{Vary1});

What is wrong with the above script?
Running optimization algorithm with 5 active parameters...
Simulation function error:
Linewidths resulting from fast-motion lindwidth parameters must be positive! Did you supply isotropic values only?

If you want to simulate the effect of rotational tumbling (S.logtcorr), you need to provide full g and A tensors in S.g and S.A, not just their isotropic averages.

thx