[SciPy-User] Asymmetric peak fitting

[Paweł Kwaśniewski]

Hi Matt, Jonathan,





2015-03-20 4:15 GMT+01:00 Matt Newville <newville at cars.uchicago.edu>:

> Hi Jonathon,
>
> On Thu, Mar 19, 2015 at 8:30 PM, jkhilmer at chemistry.montana.edu <
> jkhilmer at chemistry.montana.edu> wrote:
>
>>
>>> Ah, sorry the very poor memory.  There was a fix to the expgaussian()
>>> function a month ago, after the 0.8.3 release (and thanks to Tim Spillane
>>> for the fix!).    That explains the problem you're seeing, and might
>>> explain why Pawel was having trouble too.   The fix is fairly simple
>>> (change `arg2` to have `/(s2*sigma)` instead of `/s`,  see
>>> https://github.com/lmfit/lmfit-py/blob/master/lmfit/lineshapes.py#L116),
>>> and should be orthogonal to any other changes in the current branch.
>>>
>>>
That was the problem! Thanks! I'm sorry, I should also attach the fit
result figure to my post. But now it works like a charm.

Thanks!

Pawel


>
>> Thanks for checking into this.  Do you have any thoughts on the analysis
>> by Kalambet and Tikhonov in the paper I linked previously?  Specifically,
>> in the code:
>>
>> gss = gamma*sigma*sigma
>> arg1 = gamma*(center +gss/2.0 - x)
>> arg2 = (center + gss - x)/(s2*sigma)
>> return amplitude*(gamma/2) * exp(arg1) * erfc(arg2)
>>
>> The exp(arg1) factor is much too large and erfc(arg2) is much too small,
>> when x<1/gamma ?
>>
>>
>> Jonathan
>>
>
> I haven't looked at the paper in great detail, and wasn't aware of (and/or
> never experienced myself) the potential instability -- but I haven't used
> this function much myself.   If I understand correctly,  the recommendation
> is that a version of the exponentially modified Gaussian that used Eq. 6
> from the Kalambet, et al article (and scipy.special.erfcx()) would be
> better behaved.   Is that correct, and the whole story?
>
>  A Pull Request or a patch for candidate replacement function based on
> this would be greatly appreciated.
>
> --Matt
>
>
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