# Codes used by the Numerical Relativity group

A large amount of computational infrastructure is required to support large-scale astrophysical simulation. The AEI is involved in a number of computational projects which actively support our physics research.

## Llama – Multipatch Numerical Relativity

When simulating sources of gravitational waves, there is a complicated inner region containing the objects, either neutron stars or black holes, and a simpler outer region in which gravitational waves propagate away from the source. Traditional approaches to numerical relativity have typically used numerical grids based on Cartesian coordinates both in the region around the compact objects and in the wave zone. In the wave zone, this setup is poorly adapted to the outgoing gravitational waves, leading to very low accuracy at large distances from the source. The Llama code, developed at the AEI and CCT, makes use of multiple coordinate patches, keeping the Cartesian boxes near the compact objects, but introducing grids with spherical-type coordinates in the wave zone. This allows much higher accuracy in the computation of gravitational waves. All our black hole simulations are now performed with the Llama code, and we are working on adapting Whisky to use it.

Web page: http://llamacode.org

## Whisky

Whisky is a code to evolve the equations of hydrodynamics on curved space. It was originally written by and for members of the EU Network on Sources of Gravitational Radiation and is based on the Cactus Computational Toolkit. Whisky has grown from earlier codes such as GR3D and GRAstro_Hydro, but has been rewritten to takeadvantage of some of the latest research performed here in the EU. The motivation behind Whisky is to compute gravitational radiation waveforms for systems that involve hydrodynamics. Examples would include the merger of a binary system containing a neutron star, which are expected to be reasonably common in the universe and expected to produce substantial amounts of radiation.

Web page: http://www.whiskycode.org

## Cactus Computational Toolkit

Cactus is an open source problem solving environment designed for scientists and engineers. Its modular structure easily enables parallel computation across different architectures and collaborative code development between different groups. Cactusoriginated in the academic research community, where it was developed and used over many years by a large international collaboration of physicists and computational scientists. The main development of Cactus has moved to the Center for Computation and Technology at LSU.

Cactus is free software available under the LGPL licence.

Web page: http://www.cactuscode.org

## Carpet: Adaptive mesh refinement

Carpet is a parallel adaptive mesh refinement driver for Cactus framework. Cactus is a framework for solving time-dependent partial differential equations on uniform grids, and Carpet is an extension of Cactus that make mesh refinement possible. Carpet is developed by Erik Schnetter.

Carpet is free software available under the GPL licence.

Web page: http://www.carpetcode.org

## Kranc: Computer algebra generation of numerical codes

Kranc is a Mathematica application which turns a tensorial description of a time dependent partial differential equation into a module for the Cactus Computational Toolkit. Kranc was developed at the AEI and Southampton by Sascha Husa, Ian Hinder and Christiane Lechner. Kranc has been used at AEI, Southampton, Penn State, Georgia Tech, Palma and Louisiana State University for generating codes for, among other things, simulating binary black-hole spacetimes and comparing formulations of the Einstein equations.

Kranc is free software distributed under the GNU General Public License (GPL) (version 2) for unrestricted use.

Web page: http://kranccode.org