Addition to the NBODY1 Code

  The NBODY1 code by Sverre Aarseth was, as mentioned in Subsection 5.2.2, made to do the calculations in physical coordinates, while the AVP code does it in comoving coordinates. To be able to compare the orbits produced by these two codes, the physical coordinates of the NBODY1 code need to be transformed into comoving coordinates. The following subroutine do just that by reading the output data from NBODY1 and calculating the comoving coordinates by using the Runge-Kutta method to determine the scale factor at a given time using equation (3.33). The code is written in the Fortran 77 programming language, so that it can be included in the NBODY1 code.