In addition to integrating the equations of motion,
the particles are allowed to loose some (**eta**) of their random energy (smoothed
over a certain ‘‘box’’ size (**cell**)), and turn it into mean orbital motion. (NOT
IMPLEMENTED - see *potcode(1NEMO)*
.

Orbits can also be diffused: at regular
intervals the velocity vector can be rotated over a randomly gaussian
distributed angle (**sigma**).

**in**=*in-file*- Initial conditions will be read
from
*in-file*in snapshot format [default: none - but is required]. **out**=*out-file*- If given, results are written to
*out-file*in snapshot format [default: empty, no snapshot output file produced]. **potname=***name*- name of file of
*potential(5NEMO)*descriptor [no default]. - potpars=
*par-list* - List of parameters to the potential
descriptor. The first parameter must be the pattern speed in the x-y plane
(rotating frames of reference are not supported for all integration modes).
The remaining parameters are used by the
*inipotential()*routine in the potential descriptor. [default: none - let them be defined by routine itself]. **potfile=***file-name*- name of an optional datafile to the potential descriptor. This might be an N-body snapshot or list of spline fit coefficients etc. [default: none].
**freq**=*integ-freq*- Inverse time-step, to be used with the time
integrator. [Default:
**64.0**(64 steps per unit time)]. **mode**=*mode-number*- Integration
mode number: 0=Euler, 1=PC (Predictor Corrector), 2=modified PC, 3=Runge-Kutta
(JEB modified), 4=4th order Runge-Kutta, 5=LeapFrog. [default:
**4].** =**tstop***stop-time*- Time to stop integration in N-body model units. Default is
**2.0**. **freqout**=*out-freq*- Frequency of major N-body data outputs. Default is
**4.0**(4 frames per unit time). **minor_freqout**=*out-freq*- Frequency of minor diagnostic outputs. Default
is
**32.0**(32 diagnostic measurements per unit time). **options**=*option-string*- Miscellaneous control options, specified as a comma-separated list of keywords.
Currently recognized keywords are:
**reset_time**: when reading initial data, set*tnow*to zero;**new_tout**: when restarting, set new output times;**mass**,**phi**,**acc**,**aux**: output mass, potential (density really) acceleration data and auxilliary (the deflection angle from diffusion is stored here) with major data outputs. **eta**=*loss-fraction*- *** Fraction of random energy that
is dissipated [Default:
**0.0**]. **cell**=*box-size*- *** Cell size in which dissipation
is performed after every timestep. Dissipation is current performed on a
cartesian grid, in which cells are square (2D)
or a cube (3D)
. [Default:
**0.1**]. **rmax**=*max_box-size*- *** Maximum size of the "box" (actually cube) within
which dissipation is performed. If a negative number is given, the box is
allow to grow as large as is needed, though this may consume a lot of memory.
Default:
**-1**, i.e. box can grow indefinite. **fheat=***fheat*- *** The ratio of diffusion
angle to rms velocity dispersion in a cell. If
*fheat>0*, each time dissipation is applied, the rms velocity dispersion in a cell is computed, and a diffusion angle computed. The velocity vector of each particle is then rotated with a gaussian distributed value with rms*fheat*velsig*. This in effect gives a dissipation dependant heating source. See also**sigma=**, which is position independant. [Default:**0**]. **sigma**=*angle*- Diffusion angle, gaussian distributed
with this sigma, about which each velocity vector is rotated after each
timestep. Diffusion is used only when
*sigma > 0*. [Default:**0**]. **freqdiff**=*diffusion-out-freq*- Frequency of diffusion. Currently the same diffusion angle is used between
changes, instead of during a single integration step Default is the same
as the integration frequency (
**freq=**). **seed**=*random-seed*- Random number seed,
only used when diffusion (
**sigma=**) is used. 0 must be used to get the random seed from the time of the day. [Default:**0**]. **headline**=- Identifying text for this run. Default: not used.

Since **cell** is a fixed number throughout the execution,
is doesn’t deal well with systems who’s lenght-scale changes, in particular,
expanding systems will allocate more and more space to hold the dissipation
grid.

For each cell the
relative position and velocity for each particle within that cell is computed:

R = r - <r > i i i V = v - <v > i i iafter which the dimensionless viscosity parameter ’alpha’ controls the new velocity for each particle after a timestep:

< R x V > j j u = <v> - alpha R x ------------ + (1-alpha) V i i i i < R . R > j j

10-apr-96V0.1 cloned of potcodePJT 7-feb-04V0.6 implemented diffusion for modes 0,1,4PJT