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Question Stepsize Underflow in Stepper 

Forum: Fields: Magnetic and Otherwise
Date: 15 Jul, 2013
From: Evan Adamek <Evan Adamek>

Hi all,

I have been working on a neutron simulation using Geant4UCN. I have the code working on an older machine, but when I tried to move it to the one I am now using I run into problems. The program compiles without any issues, but when I run the executable, I get the following error at the very start of the zeroth event:

G4MagIntegratorDriver::OneGoodStep:
  Stepsize underflow in Stepper
  Step's start x=2e-08 and end x= 2e-08 are equal !!
  Due to step-size= 3.98979e-25 . Note that input step was 3.98979

This repeats about ten thousand times (with marginally different step start/end points) before aborting with a G4MultiLevelLocator::EstimateIntersectionPoint()/Too many substeps error. I found the following post: http://hypernews.slac.stanford.edu/HyperNews/geant4/get/emfields/78/1.html on this forum, but the solution proposed involves smoothing out field discontinuities, and I still get this issue even when running with only gravity enabled (there shouldn't be any non-graviational fields at the generation location anyway).

I had my advisor send me her (working) version of the code, and have encountered the same issue when running it on my own computer. I have run the first 4 novice examples as well as the first two persistency/gdml examples and all of them have worked fine. Does anyone know what I might do to fix, or at least troubleshoot, this error?

I have attached a full version of the output that I get, in case that helps.

   Attachment:
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2013/07/15/11.30-38230-output.txt

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1 None: Re: Stepsize Underflow in Stepper   (John Apostolakis - 16 Jul, 2013)
(_ None: Re: Stepsize Underflow in Stepper   (Evan Adamek - 16 Jul, 2013)
(_ Feedback: Re: Stepsize Underflow in Stepper   (Gumplinger Peter - 16 Jul, 2013)
(_ Note: Re: Stepsize Underflow in Stepper   (Evan Adamek - 17 Aug, 2013)
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