question about cw simulation

[chemshd]

Hi experts,
Herein I encountered a problem with cw simulation.
I have two different spins which I assume not coupled at all. I simulate the spectrum in two different ways:
1. Simulate the two spins separately and average the simulation;
2. Simulate the two spins as coupled spins but set the isotropic coupling constant as 0.

I enclosed the code, while you can see that at around 9 T there is a strange transition for coupled model which does not exist in isolated model.

zoom.jpg (17.51 KiB) Viewed 5831 times

I tried to increase the nKnots from default 19 to 40, but failed to eliminate this feature.

Code: Select all

clear all;


S=1;
g1=[2 2.2];
g2=g1;
lw=100;
D1=[7 1];
D2=[7.5 1.2];

Sys1.S = S;
Sys1.g= g1;
Sys1.D = D1*clight/10000;
Sys1.lw = lw;

Sys2.S = S;
Sys2.g= g2;
Sys2.D = D2*clight/10000;
Sys2.lw = lw;

Sys.S = [1 1];
Sys.g= [2 2.2;2 2.2];
Sys.D = [7 1;7.5 1.2]*clight/10000;
Sys.ee=0;
Sys.lw = lw;

Exp.Range = [0000 16000];
Exp.mwFreq= 285;
Exp.Temperature = 15;
Exp.Orientations =[];

% Opt.nKnots=40;

[simuf1,simuspec1] = pepper(Sys1,Exp);
[simuf2,simuspec2] = pepper(Sys2,Exp);
simuspecanver=0.5*(simuspec1+simuspec2);

[simuf,simuspec] = pepper(Sys,Exp);


figure(1)
plotyy(simuf1/1000,simuspecanver,simuf/1000,simuspec)
legend('isolated','coupled')

Why is this?
does this mean the two strategies are not really physically identical?
or is there a bug in the code?

looking forward to your help.

[Stefan Stoll]

You forgot to include Opt in your call to pepper: pepper(Sys,Exp,Opt). That's why your simulation did not respond to the increase in Opt.nKnots.

What you are looking at is the coalescence point of a looping transition. Around this point, the resonance field positions are extremely sensitive to the orientation of the spin system relative to the magnet. These coalescence points are notoriously difficult to simulate accurately. If you use too small a number of orientations (Opt.nKnots), there will be artifacts. Increasind Opt.nKnots will smooth them out. However, the convergence as you increase Opt.nKnots can be slow.

[chemshd]

Hi Stefan,
Thanks very much for your reply, just now I include the Opt into the pepper simulation, with

Opt.nKnots=[91,10];
Opt.Symmetry='Ci';

while the obtained spectrum is still the same, the coalescence still exists.
I am now dealing with a very weak interaction system, the accurate simulation is very important.
Therefore it is of crucial importance to figure this problem out.

regards,

ShangDa

You forgot to include Opt in your call to pepper: pepper(Sys,Exp,Opt). That's why your simulation did not respond to the increase in Opt.nKnots.

What you are looking at is the coalescence point of a looping transition. Around this point, the resonance field positions are extremely sensitive to the orientation of the spin system relative to the magnet. These coalescence points are notoriously difficult to simulate accurately. If you use too small a number of orientations (Opt.nKnots), there will be artifacts. Increasind Opt.nKnots will smooth them out. However, the convergence as you increase Opt.nKnots can be slow.

[Stefan Stoll]

You need to increase Opt.nKnots(1) further. The second value, Opt.nKnots(2), will have no effect for looping transitions.