Hello,

I am trying to use a custom perturbation with the new XGPT capabilities to generate the doppler coefficient for materials in my core. I seem to be having a hard time getting this to go, so I want to ask some questions to clarify a few points.

There is an example of using the XGPT capability here: http://serpent.vtt.fi/mediawiki/index.php/XGPT_example

I am trying to follow this example, but I find one aspect somewhat confusing. I would think that in the 'efunc' file that the user provides, the cross-section derivatives should be provided. For my case, I want the derivatives as a function of temperature. To do this, I am doppler broadening my cross sections from 600K to 605K. I would then think that I want to calculate (sigma(605) - sigma(600))/5 to approximate the derivative. In the example provided, it says to instead calculate (Sig_301 - Sig_300)/Sig_300. I suppose this is the derivative (since the temperature difference is simply 1) divided by the absolute temperature so that the resulting units are simply 1/K. Is this correct? I just would like clarification on this point.

Either way, when calculating these values, typically the values end up being quite small, on the order of 10^-4 or 10^-5. From what I understand, the typical values in these files should be on the order of 1, as that is what would be added or subtracted in the case that the 'efunc' file is not provided. This seems like a very large difference, so I want to get some validation that my thinking is correct.

Finally, I wanted to mention that I have a case running at the moment with the XGPT implemented, along with some other "standard" perturbations. Although the run is only ~25% complete at the moment, the results that have been printed so far indicate that I have done something wrong with the XGPT portion. I am receiving real values for all of the other perturbations, but all of the XGPT results so far are simply 0. This makes met think something is wrong, possibly because the values that I have in my 'efunc' file are so small.

Any help would be appreciated. Or point me to another example of how to use the XGPT. I can send my input via email if helpful, but it is too big to upload here. Thanks for your help!