Piet Hut: README file for N-body starter code

A Starter Code for N-body Simulations: README file

The Code `nbody_sh1.C' is a simple stand-alone version of an N-body system with shared but variable time steps, based on the fourth-order Hermite integration scheme. A code similar to this one will appear in our book `Writing an N-Body Code', which will be the first volume in a series titled `Pure Gravity, or Particles at Play.'

The file `nbody_sh1.C' contains a C++ program, and can be compiled directly as is, without any need to link to anything else. For example, on a Linux system you can use the GNU g++ compiler, and simply type: "g++ -o nbody_sh1 nbody_sh1.C" to produce the executable "nbody_sh1". The 325 lines of code are pretty standard, so we don't expect any serious problems to occur in other environments, but we have only tested the code for the GNU compiler.

The postscript file `nbody_sh1.ps' contains an early draft of a discussion of the code, to be included (eventually) in our book. Note that both code and discussion are beta versions, so we would appreciate it if you would notify us of any remaining bugs.

To run the code, you have to specify initial conditions. Here is a nice example for a three-body system where the stars will run around each other in a figure-8 configuration (for background, see http://www.ams.org/new-in-math/cover/orbits1.html):

3
0
1 0.9700436 -0.24308753 0 0.466203685 0.43236573 0
1 -0.9700436 0.24308753 -0 0.466203685 0.43236573 0
1 0 0 0 -0.932407370 -0.86473146 -0

If you enter these five lines of numbers in a file "figure8.in", you can run the nbody code by typing: "nbody_sh1 < figure8.in" . Snapshots will appear by default on the standard output stream once every time unit, while energy conservation diagnostics will appear on the standard error stream. You can use your favorite plotting package to view the orbits, with a more frequent snapshot output. To accumulate the orbit data, you can try "nbody_sh1 -o 0.01 < figure8.in > figure8.out" and plot the data in the second and third column of the file "figure8.out" . A quick way to get those data directly into a file "figure8.plot", say, would be: "nbody_sh1 -o 0.01 < figure8.in | awk '{print $2 " " $3}' > figure8.plot" .

If you're feeling adventurous, you can start experimenting further. However, if you prefer to have more guidance as to generating initial conditions, plotting, and analyzing the data, please wait a few more months until the first volume of our book series will be finished.

Happy orbiting!

Piet Hut and Jun Makino,

March 2002.