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Hi all,

I have done an EPR computation with Orca to compare to an experiment, but I cant get a reproduction of the experimental spectra.
The molecule is an azobenze with two TEMPO groups added, so it has quite a lot of hydrogen atoms. While I initially planned on using the garlic method to generate the spectrum it apparently doesnt work with the nitrogen atoms of the azo-bridge, so I switched to the chili method. Since the resulting matrix was way too big I tried changing the gridsize, and I also tried reducing the space by using mt2mhz(0.14) as a cutoff to reduce the number of atoms to 8 hydrogens, 2 nitrogens and 2 oxygenatoms. I also changed the hydrogen-, oxygen- and nitrogen atoms to their most common isotope. However this still needs an enormous amount of memory, so i couldnt produce any spectra. However, if I reduce the number of atoms to consider even further, only the nitrogens and oxygen atoms remain and the resulting spectrum does not match the experimental spectrum (a singlett compared to a triplett).
Is there any other way to reduce the space further or get a result for a calculation with this many hydrogen atoms (it should be an isotropic epr)?

Thanks in advance,
Sebastian

Hi Sebastian,

garlic supports equivalent nuclei, which can be many without much computation time when the correlation time is omitted. See example for 2 nitrogens and 12 protons:
Sys.g = [2.0084 2.0063 2.0024];
Sys.Nucs = '14N,1H';
Sys.n = [2,12];
Sys.A = [110, 5];
Sys.lwpp = 0.1;

Exp.mwFreq = 9.5;
Exp.Range = [315 365];

garlic(Sys,Exp);

Would this solve your problem since you asked for an isotropic spectrum?
And just as a hint, if you need anisotropies like the g-anisotropy in the example above, it will likely be necessary to obtain those from low-temperature EPR, or also reduce the g-tensor to an isotropic value.
For more specific help, it would be useful to share more information on the spectrum that you want to model.
Best wishes,
Daniel

Hi Daniel,

thanks for your advice. I had a look at my System again and realised that removing the Sys.S variable allows me to use garlic directly on the system gained from orca2easyspin with a low cutoff point (including 2 nitrogen, 2 oxygen and 12 hydrogen atoms). The spectrum is still sadly a single peak, but that might be a problem with my calculation. I'll go back to the computation and try again, but at least I can now generate a spectrum without breaking my computer

Thanks again and best wishes,
Sebastian

HI all,

I tried a lot in the last few weeks to get my spectra to match the experimental spectra. However it's still not working
First, I realised that since my molecule has two TEMPO groups I need to describe it as two spin systems with Spin=1/2 each. Garlic cant compute that and just putting S=1/2 there gives me a quintet which is not what I expected.
So then I computed a coupling constant J and tried to do it with chili, copying the g value over and splitting the HFC matrix into two where half of the the cores only interact with one spin system (basically following the advice in this thread: https://easyspin.org/forum/viewtopic.ph ... 0d61#p1592)
Chili is obviously much more expensive than garlic so I had to also reduce the amount of nuclei included. I got it down to eight nuclei or 10 when I treated the ones with low HFC constants with PostConvNucs. But even when I changed the tcorr value and the J value around, the resulting spectra was always the same quintet I got when I misused garlic. What got me to the result I wanted, however, was deleting all nuclei belonging to one of the spin systems and just keeping one, with S=1/2 (which is the same as just a single TEMPO radical without anything else included, the experiment yielded the same for my doubly substituted molecule).

My Sys struct looks like this:

xyz: [84×3 double]
S: [0.5000 0.5000]
g: [2×3 double]
gFrame: [2×3 double]
Nucs: '1H,1H,1H,14N,16O,1H,1H,1H,14N,16O'
NucsIdx: [12 32 38 41 47 56 64 65 73 79]
A: [10×6 double]
AFrame: [10×6 double]
data: [1×1 struct]
lwpp: [0 0.1000]
J: 1.1992e+03

with Sys.A looking like

 0.9541    1.5357    4.0206         0         0         0
    0.9452    1.5242    4.0095         0         0         0
    1.8701   -2.5631   -4.0170         0         0         0
    4.9182    5.1867   40.8939         0         0         0
   20.9661   23.0396  -99.2039         0         0         0
         0         0         0    1.0028    1.5917    4.0709
         0         0         0    1.8712   -2.5581   -4.0105
         0         0         0    0.9621    1.5426    4.0239
         0         0         0    4.9351    5.2034   40.8908
         0         0         0   20.9802   23.0543  -99.2749

Is there anything else I can try or did I forget something important? Thanks in advance again

Hi Sebastian,

Of course you have quite a complex system, particularly in terms of hyperfine couplings. However, the description of the electron-electron interaction with only a scalar J coupling might be the problem yet to be solved.

This has been treated in simulations by EasySpin in detail in the paper here: https://doi.org/10.1039/C8CP05236K
for the case of DNP biradicals using multifrequncy EPR simulations. And I think this should help you - otherwise we would need to know more details of the system you are trying to describe.
Best wishes!