NAME

       gmx-spatial - Calculate the spatial distribution function

SYNOPSIS

          gmx spatial [-s [<.tpr/.gro/...>]] [-f [<.xtc/.trr/...>]] [-n [<.ndx>]]
                      [-b <time>] [-e <time>] [-dt <time>] [-[no]w] [-[no]pbc]
                      [-[no]div] [-ign <int>] [-bin <real>] [-nab <int>]

DESCRIPTION

       gmx spatial calculates the spatial distribution function and outputs it in a form that can
       be read by VMD as Gaussian98 cube format.  For a system  of  32,000  atoms  and  a  50  ns
       trajectory,  the SDF can be generated in about 30 minutes, with most of the time dedicated
       to the two runs through trjconv that are required to  center  everything  properly.   This
       also  takes a whole bunch of space (3 copies of the trajectory file).  Still, the pictures
       are pretty and very informative when the fitted selection is properly made.  3-4 atoms  in
       a  widely  mobile  group  (like  a  free amino acid in solution) works well, or select the
       protein backbone in a stable folded structure to get the SDF of solvent and  look  at  the
       time-averaged solvation shell.  It is also possible using this program to generate the SDF
       based on some arbitrary Cartesian coordinate. To do that, simply omit the preliminary  gmx
       trjconv steps.

       Usage:

          1. Use  gmx  make_ndx  to create a group containing the atoms around which you want the
             SDF

          2. gmx trjconv -s a.tpr -f a.tng -o b.tng -boxcenter tric -ur compact -pbc none

          3. gmx trjconv -s a.tpr -f b.tng -o c.tng -fit rot+trans

          4. run gmx spatial on the c.tng output of step #3.

          5. Load grid.cube into VMD and view as an isosurface.

       Note that systems such as micelles will require gmx trjconv -pbc cluster between  steps  1
       and 2.

   Warnings
       The  SDF  will  be  generated  for  a  cube that contains all bins that have some non-zero
       occupancy.  However, the preparatory -fit rot+trans option to  gmx  trjconv  implies  that
       your  system  will  be rotating and translating in space (in order that the selected group
       does not). Therefore the values that are returned will  only  be  valid  for  some  region
       around  your  central group/coordinate that has full overlap with system volume throughout
       the entire translated/rotated system over the course of the trajectory.  It is up  to  the
       user to ensure that this is the case.

   Risky options
       To  reduce  the amount of space and time required, you can output only the coords that are
       going to be used in the first and subsequent run through gmx trjconv.  However, be sure to
       set  the  -nab option to a sufficiently high value since memory is allocated for cube bins
       based on the initial coordinates and the -nab option value.

OPTIONS

       Options to specify input files:

       -s [<.tpr/.gro/...>] (topol.tpr)
              Structure+mass(db): tpr gro g96 pdb brk ent

       -f [<.xtc/.trr/...>] (traj.xtc)
              Trajectory: xtc trr cpt gro g96 pdb tng

       -n [<.ndx>] (index.ndx) (Optional)
              Index file

       Other options:

       -b <time> (0)
              Time of first frame to read from trajectory (default unit ps)

       -e <time> (0)
              Time of last frame to read from trajectory (default unit ps)

       -dt <time> (0)
              Only use frame when t MOD dt = first time (default unit ps)

       -[no]w (no)
              View output .xvg, .xpm, .eps and .pdb files

       -[no]pbc (no)
              Use periodic boundary conditions for computing distances

       -[no]div (yes)
              Calculate and apply the divisor for bin occupancies  based  on  atoms/minimal  cube
              size.  Set  as  TRUE for visualization and as FALSE (-nodiv) to get accurate counts
              per frame

       -ign <int> (-1)
              Do not display this number of outer cubes  (positive  values  may  reduce  boundary
              speckles; -1 ensures outer surface is visible)

       -bin <real> (0.05)
              Width of the bins (nm)

       -nab <int> (16)
              Number of additional bins to ensure proper memory allocation

KNOWN ISSUES

       • When  the allocated memory is not large enough, an error may occur suggesting the use of
         the -nab (Number of Additional Bins) option or increasing the -nab value.

SEE ALSO

       gmx(1)

       More information about GROMACS is available at <http://www.gromacs.org/>.

COPYRIGHT

       2022, GROMACS development team