Re: [ESPResSo] bug in random.tcl in mbtools

[Tristan Bereau]

Hello,

I'm not sure whether a gaussian random number generator has already  
been implemented in the TCL shell of ESPResSo, but if not, there is a  
couple of methods described at the end of the "Old Testament" of  
molecular simulations (i.e. Allen and Tildesley), and Burkhard's is  
one of them.

Best,

Tristan


On Mar 19, 2009, at 4:52 AM, Jacob Kirkensgaard wrote:

> Hi again
>
> Thanks to Tristan and Burkhard - you are of course right. For my  
> purposes my suggestion is sufficient but I will see if I have time  
> to implement a small routine to do it properly as outlined by  
> Burkhard. Is there a routine for generating Gaussian random numbers?  
> As Tristan mentioned this would be a quick way to solve the issue.
>
> Best,
> Jacob
>
>
>
> On Mar 19, 2009, at 9:40 AM, address@hidden wrote:
>
> > Hello,
> >
> > from Tristan's remarks I understand that there
> > is a question of how to get random numbers
> > uniformly in or on a sphere? If so, let me tell
> > you one possible answer:
> >
> > 1. You first draw 3 RNs u_1, u_2, u_3 from [0:1]
> > 2. Then you transform u_1 = 2 * u_1 - 1 etc.,
> >  resulting in RNs in [-1,1]
> > 3. Calculate r2 = u_1**2 + u_2**2 + u_3**2
> > 4. If r2 > 1 --> throw everything away,
> >  go to step 1, try again
> > 5. If r2 < 1: Either you are done (supposing
> >  that you want RN IN the sphere), or you set
> >  norm = 1 / sqrt(r2)
> >  u_1 = u_1 * norm etc.
> >  By this you project the output onto the
> >  surface of the unit sphere, so you get
> >  something ON the sphere.
> >
> > If that was trivial: Please accept my apologies
> > for bothering you.
> >
> > Regards
> > Burkhard.