VMD-L Mailing List

From: Sara baretller (sarabiocomputation_at_gmail.com)
Date: Fri Jul 08 2011 - 10:19:50 CDT

Hi Axel

Thank you so much for you help. i appreciate your time and effort.

On Thu, Jul 7, 2011 at 5:59 PM, Axel Kohlmeyer <akohlmey_at_gmail.com> wrote:

> On Thu, Jul 7, 2011 at 10:22 AM, Mark Cunningham <cunningham_at_utpa.edu>
> wrote:
> > Sara:
> >
> > In trying to convert from radians to degrees, 180/3.14159 is only correct
> to
> > 5 decimal places, where double precision arithmetic has approximately 12
> > decimal places of precision. This may seem unimportant but can lead to
> > hard-to-trace bugs in more complex calculations. Better practice is to
> use
> > the value of pi to machine precision, which can be obtained from calling
> the
> > arc cosine function with the argument -1. Also, tcl is an interpreted
>
> actually VMD provides a Pi at machine precision. just add:
>
> global M_PI
>
> and then you can use $M_PI throughout.
>
>
> > language,
> > meaning that every time you go through the loop tcl has to parse the
> string
> > 180/3.14159 and decide what to do with it. Compiled languages like
> Fortran
>
> actually, tcl does byte-compile this, so the parsing is only done once.
> but the execution of the preparsed loop body is still very slow compared
> to C++ or Fortran.
>
> cheers,
> axel.
> --
> Dr. Axel Kohlmeyer
> akohlmey_at_gmail.com http://goo.gl/1wk0
>
> Institute for Computational Molecular Science
> Temple University, Philadelphia PA, USA.
>