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Re: Bloch Equation, NDSolve, and strange behaviors

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  • Subject: [mg117757] Re: Bloch Equation, NDSolve, and strange behaviors
  • From: Oliver Ruebenkoenig <ruebenko at>
  • Date: Thu, 31 Mar 2011 03:57:57 -0500 (EST)

On Wed, 30 Mar 2011, michael partensky wrote:

> Hi.
> My question is related to the demo
> I could not find the authors' emails, and decided to post it here. Actually,
> I have encountered  a similar problem first, and only then found the demo.
> *Consider the following set of parameter*s:
> frequency detuning - arbitrary (say, 0)
> Rabi frequency =0
> *relaxation*: all parameters =0.
> norm of *M *= 1, zenith angle of *M =*90, azimuth angle =0 (its value has no
> effect).
> Total duration: maximum (49.6)
> You are supposed to see  precession of the magnetization vector *M* in xy
> plane, around the constant magnetic field
> *B*0 ={0,0,1}. Hence, the trajectory should be circular.
> However, it strongly deviates from this trajectory at large *t*.
> *My question*:
> what is the reason? The solution uses NDSolve. I tried changing MaxSteps,
> WorkingPrecision  - it did not help.
> *Note*: The demo could be run at smaller *t*, where this error would not
> matter. I am trying to consider effects of small perturbations on a similar
> system, and having reliable 0-order trajectories becomes critical.
> Thanks
> Michael Partenskii

Have you tried fixed precision input and then increasing AccuracyGoal and 

You would like get a more useful respnse if you were to post some code 
that shows the issue - and * not * too * many *s


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