Hi Stefan,

I have been contemplating changes to the MC codes. One thing we have found is 
that MCGeneral breaks down when Q starts to get large. We see unrealistic 
results at 684 GHz when using horizontally aligned particles with high aspect 
ratio. Yuli Liu, who is working with us now, did comprehensive analysis, and we 
believe that the issue is the way the backwards algorithm is using importance 
sampling to avoid the issue of inverting the extinction matrix; however, this 
approach neglects the mixing of I and Q. I believe this is a simple fix.

The other issue is that MCGeneral is not very ARTS-like. Looking at the way it 
is structured, I think a better approach would be to have an iyMC that traces a 
single "photon," and yMC would integrate these individual results. Random 
sampling of both the antenna pattern and the bandwidth could be performed at 
this level. I also think that the MC sampled antenna pattern could be more 
widely useful across ARTS.

These papers provide an interesting curveball. The ARTS MC codes are 
particularly slow, and they are not optimized for optically thin or extremely 
optically thick atmospheres. We could look at using these libraries, or at 
least techniques, but I'm not sure how intensive such a restructuring of the 
code would be.

Of course, the tricky piece here is finding someone with the time to do this 
work. But, I think these changes would make the codes significantly more 
usable, and hopefully therefore used.

Cheers,
Ian

´╗┐On 11/29/23, 11:22 AM, "arts_dev.mi on behalf of Stefan Buehler" 
<arts_dev.mi-boun...@lists.uni-hamburg.de 
<mailto:arts_dev.mi-boun...@lists.uni-hamburg.de> on behalf of 
stefan.bueh...@uni-hamburg.de <mailto:stefan.bueh...@uni-hamburg.de>> wrote:


Dear all,


I stumbled accross this interesting paper on an open C library for particularly 
efficient MC calculations. Could this be the basis of ARTS 3D MC flux and 
heating rate calculations? Using MC sampling also for the spectral dimension, 
to be efficient, as in the second paper, which is also impressive, I think. 
They use MC sampling even for the spectral lines, if I got it right! (Basically 
treating each transition as if it were its own absorption species.)


/Stefan


https://www.dropbox.com/scl/fi/smsisfgc2it3sx4gov970/J-Adv-Model-Earth-Syst-2019-Villefranque-A-Path-E2-80-90Tracing-Monte-Carlo-Library-for-3-E2-80-90D-Radiative-Transfer-in-Highly.pdf?rlkey=v5yvrm64fnljaf739j4ssllux&dl=0
 
<https://www.dropbox.com/scl/fi/smsisfgc2it3sx4gov970/J-Adv-Model-Earth-Syst-2019-Villefranque-A-Path-E2-80-90Tracing-Monte-Carlo-Library-for-3-E2-80-90D-Radiative-Transfer-in-Highly.pdf?rlkey=v5yvrm64fnljaf739j4ssllux&amp;dl=0>


https://www.dropbox.com/scl/fi/r1tm3jdzx57kb85nowmt0/Yaniss_ea_PNAS_2023_smi.pdf?rlkey=8d4a7rb4u8pehckawbfk08c9f&dl=0
 
<https://www.dropbox.com/scl/fi/r1tm3jdzx57kb85nowmt0/Yaniss_ea_PNAS_2023_smi.pdf?rlkey=8d4a7rb4u8pehckawbfk08c9f&amp;dl=0>

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