[Users] Propose thorns for inclusion in the EinsteinToolkit

[Barry Wardell]

Hi,

Recently Ian Hinder and I have created an arrangement of thorns called
EinsteinExact which provide some exact solutions for use in Cactus. The
scope of these thorns is similar to that of the Exact thorn but with some
notable improvements:

* The extrinsic curvature is computed symbolically rather than using finite
differencing so it should be accurate to within roundoff. This means that
EinsteinExact is more exact than Exact.
* The solutions used in EinsteinExact come from a database of metrics which
have been correctness-tested, e.g., by checking symbolically that they are
a solution of the Einstein equation.
* EinsteinExact is written in Mathematica and uses Kranc, so the raw source
is very small (~250 lines) and easy to understand.
* Adding a spacetime is as simple as typing the components of the metric
into Mathematica.

So far, the spacetimes included are Minkowski, Kerr-Schild, gauge wave and
shifted gauge wave, but the list can easily be extended in the future.
There is also support for arbitrary rotations, using the same conventions
as the Exact thorn uses. We have not yet included the other transformations
supported by Exact, but they shouldn't be difficult to add.

We have checked that for these spacetimes the Exact solutions converge to
the EinsteinExact solutions at the correct finite differencing order used
by Exact.

We would like to propose the EinsteinExact arrangement for inclusion in the
EinsteinToolkit. If you would like to try it out, you can get it using:

git clone --recursive git://github.com/barrywardell/EinsteinExact

Any comments or suggestions are welcome.

Regards,
Barry
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