Re: Barnes-Hut treecode (was Re: Galaxy Interaction Simulations)

Shawn Slavin (
Thu, 10 Mar 1994 02:33:35 GMT

In article <>,
Steinn Sigurdsson <> wrote:
>In article <> (Wayne Hayes) writes:

[Stuff deleted. ;)]

> >To model globular clusters with unsoftened particles (ie including
> >2bod relaxation) is not possible,
> You mean not possible with current machines/techniques, or fundamentally
> impossible, even in principle? I was under the impression that it's
> only a matter of not enough CPU cycles to accurately model large-N
> systems without softening.
>Well, in a very real sense modeling large N collisional systems
>is intractable, there are two problems, the true trajectories
>diverge from the calculated ones, and you can show that if there
>are bound subsystems then at any resolution (in the point mass
>approximation) there will be a perturbation of a bound system by an
>unbound particle that is significant at your resolution scale but
>can't be accurately modeled at that scale - there is however good reason to believe
>neither issue matters all that much.

Well, I sat in on a colloquium today, which happened to be given by Piet Hut.
The basic answer was: Simulations of globular clusters of 10^5 stars will have
to wait for GRAPE, as you need ~10^19 floating point operations to go from
initial conditions to just past core collapse => you need a teraflop-class

Also, the focus of his talk was binaries in gc's, which I found quite
interesting. These two, three, ... body interactions are really important in
gc dynamical evolution. Adding the code to handle close interactions of
scattering + binary formation/hardening in 3-body interactions really chews up
your CPU time. (i.e. I was assuming you are using a softened potential
[pretty much the backbone of the treecode], but now you may want to include [I
would anyway] close interactions and binary formation.)

Shawn Slavin
Indiana University Astronomy