.trr for a selected group, variable precision


Can I write trajectories for specific groups to .trr files? And, importantly, with specified precision. My overall goal is to write a full, double precision x,y,z coordinate trajectory for a 4-atom solute molecule to .trr file (so yes, with full 16 significant figure precision - the reason is not important, basically, I need more than single precision), and hence exclude the trajectories of the solvent molecules.

I know that for a compressed .xtc files, I can do the following:

nstxout-compressed = 1 (assuming that I want every frame written)
compressed-x-precision = 100000000 (as an example of full single precision)
compressed-x-grps = NAME_OF_GROUP

And that for an un-compressed, .trr trajectory I should use:

nstxout = 1 (assuming that I want every frame written)

My questions are: is there an equivalent to compressed-x-precision for uncompressed outputs? And, is there an equivalent to compressed-x-grps for uncompressed outputs?

Note that I would also gladly write a full double precision compressed .xtc file, setting compressed-x-precision = 1e16, but this does not give me a .xtc file containing the full precision data (I’ve checked, the file size does not exceed the single-precision output).

Also, I am certain that I am running a double-precision version of GROMACS.


There is no option to write part of the system to trr.

It is not possible to write full single, or double, precision to xtc. The coordinates are multiplied by the given factor and rounded to int, so there will always be rounding errors.

Maybe tng does support more than single precision. You can try to write that instead of xtc.