[gmx-developers] Precision in trajectory file

David van der Spoel spoel at xray.bmc.uu.se
Tue Aug 3 14:47:03 CEST 2010

On 2010-08-03 12.34, Inon Sharony wrote:
> Good afternoon!
> It seems to me that although I'm running a double-precision installation
> of GROMACS, the printout to file (of the positions and velocities) is of
> much less precision. i.e. the computation is done on numbers with
> something like 16 significant digits, but the last 10 of those are
> simply truncated and lost (e.g. positions are given in single-precision
> as 0.000 nm, and in double-precision as 0.00000 nm). Since I've already
> spent computation time at getting double-precision, I'd like to make use
> of all of it -- for my own reasons. I already searched the manuals,
> mailing lists and source code for instruction but didn't find any.
> Could you please tell me how I can change the number of digits printed
> out (e.g. to the .trr file)? I'm looking for a more elegant solution
> than adding a printf line to the source code. Something along the lines
> of changing the format of numbers in the function that prints to .trr .
> Thanks in advance!
The precision in the trr file is the complete precision. All analysis 
programs use this. Some programs have a -dp flag to print with higher 

David van der Spoel, Ph.D., Professor of Biology
Dept. of Cell & Molec. Biol., Uppsala University.
Box 596, 75124 Uppsala, Sweden. Phone:	+46184714205.
spoel at xray.bmc.uu.se    http://folding.bmc.uu.se

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