4 Running nbody

Parameters are taken from the command line. The complete list of options (which you can find in the main() routine) is:

list of options
-i        name of snapshot input file       (no default)
-o        name of snapshot output file      (default: no output)
-d        timestep                          (default: 0.015625)
-D        time interval for snapshot output (default: 1)
-T        time to stop integration          (default: 10)
-e        softening parameter epsilon2      (default: 0.025)
-t        opening angle theta               (default: 0.75)
-n        ncrit for Barnes' vectrization    (default: 1024)
          (ONLY used with GRAPE/HARP inplementation)
-w        window size for PGPLOT snapshot plot (default: 10)
-c        flag for collision run
-x        relative position vector for collision run (no default)
-v        relative velocity vector for collision run (no default)
-s        scale factor for position scaling (default: 1)
-S        scale factor for velocity scaling (default: 1)
-h        print this help

In the following, I give somewhat more detailed explanation:

-i: Specify the name of input snapshot file. This is required. The file format is nemo stoa format.

-o: Specify the name of output snapshot file. This is optional. The file format is nemo stoa. When not specified, no output snapshot is made.

-d: The step size for time integration. The default value is 1/64.

-D:The time interval to output snapshot. The default value is 1.

-T:The time to stop integration. The default value is 10.

-e:The softening parameter. The default value is 1/40.

-t:The opening angle for the force calculation. The default value is 0.75.

-n:The critical number of particles in the cell, used in Josh Barnes' vectrization scheme. The default value is 1024. Only used with GRAPE/HARP implementation. Nbody uses the standard particle-based scheme on general-purpose computer.

-w: The size of PGPLOT window. The default value is 10. When compiles without -NOGRAPHICS, nbody creates a PGPLOT window and displays the projection of the system to the x-y plane. This option specifies the size of the region displayed.

-c: If specified, collision of two identical system is set up. Default is no collision.

-x: When -c option is given, specify the initial separation as, for example, -x 10 0 0 (x, y, and z coordinates).

-v: Similar to x option, but for initial relative velocity.

-s:Multiply the initial position of each particle by this parameter. Default is 1.

-S:Multiply the initial velocity of each particle by this parameter. Default is 1.

-h: Print a short summary of the command line options.

The file samplein is a Plummer model with 4096 particles, in Heggie units. You can try a test run with

nbody -i samplein

Some additional timing information goes to standard error.