# velocity units

From: Melanie Jo Clarke <mjdosaj_at_MIT.EDU>
Date: Sun, 29 Oct 2006 22:25:53 -0500

I have been attempting to use Grafic-1 initial conditions to run small scale
Gadget2 simulations and have run into the following problem with velocity
units.

The manual suggests to divide by sqrt(a) to convert from physical velocity to
Gadget units. When I do this, I find that the Gadget simulation does not
evolve far enough compared to a P3M simulation run with the same initial
conditions.

As I understand it, the internal units for velocity are dx/d(ln a), so to
convert from physical velocity, dx/dt, to these units, I should divide by d(ln
a)/dt. This factor is a*sqrt(omega_m/a + omega_v*a^2 + 1 - omega_m - omega_v),
where omega_m is the matter density ratio and omega_v is the vacuum energy
density ratio. This factor reduces to sqrt(a) in the case that omega_m=1 and
omega_v=0. When I use this factor to convert physical velocity to Gadget
units, I get good agreement between P3M and Gadget simulations.

Is my interpretation correct? If so, can the Gadget2 manual be corrected so
that future users don't run into this problem?

Thank you,
Melanie Clarke Dosaj