Capabilities of GRTeclyn

GRTeclyn is a modern C++ based code for doing numerical relativity simulations. It solves the Einstein equations of general relativity to evolve the metric components and matter fields from chosen initial conditions. It is built on AMReX, a powerful framework for massively parallel block-structured adaptive mesh refinement (AMR), and supports both CPU and GPU architectures for high-performance computing.

GRTeclyn is the next-generation version of GRChombo, with a modular and extensible design focused on clarity, maintainability, and performance portability. It is actively under development, and many of the core numerical relativity features from GRChombo are already implemented or underway.

Note that GRTeclyn is designed as a research tool, not a black box. Users are encouraged to modify and extend it for their physics cases. If you're a student or supervisor considering using GRTeclyn, please ensure your team has familiarity with numerical relativity, C++ and HPC environments.

Current key features in the public code

Formulation of the Einstein equations

GRTeclyn uses the CCZ4 formulation with constraint damping to evolve the Einstein equations.
The implementation supports full 3+1D spacetimes with dynamic metrics and gauge evolution.
The code is written modularly, with clearly separated evolution and diagnostic systems.

Gauge choice and boundaries

The moving puncture gauge is currently implemented by default, suitable for evolving black hole spacetimes.
The gauge system is extensible and designed to support alternative slicing and shift conditions.
The current version supports Sommerfeld (radiative) and reflective boundary conditions.

Black hole spacetimes

Binary black hole (BBH) simulations are supported using Bowen–York initial data.
The BinaryBH example is fully functional for boosted cases. Currently the code uses an approximate solution but we plan to integrate the TwoPunctures code shortly, which will allow fully general initial momenta and spins.
Puncture positions are tracked and recorded throughout the simulation using particle methods.

Scalar fields on dynamical backgrounds

Oscillaton example under development but minimally coupled scalar field is working.

AMReX handles dynamic block-structured AMR with tagging criteria based on physical fields or custom diagnostics.
Time sub-cycling and regridding are fully implemented and compatible with multi-level evolution.

Finite differencing scheme

The code supports 4th-order finite differencing in space and 4th-order Runge-Kutta in time.
Support for higher-order stencils (6th order) is planned in future releases.

Checkpointing and restart

Checkpointing is implemented for recovery after interruption.
Simulations can be resumed from saved checkpoints with consistent memory and time-step restoration.

Diagnostics

Standard diagnostic variables (e.g., Hamiltonian and momentum constraints, stress-energy quantities) are implemented and can be output.
A diagnostic system allows for easy addition of new variables to be monitored during the run. These are calculated only when required for output.
Output is in AMReX-native plotfiles viewable in tools like yt or AMReX-based viewers, and small data files.

Parallel performance and GPU support

Designed for MPI-parallel runs using AMReX's distributed mesh management.
GPU execution is supported using AMReX’s GPU_LAUNCH macros (e.g., CUDA/HIP/SYCL).
OpenMP is supported for CPU parallelism on shared-memory systems.

Features under development (coming soon)

These features are implemented in working branches or are actively being ported from GRChombo. They may be available on request (at your own risk pending testing).

Gravitational wave extraction — Weyl scalar calculations for waveform analysis, and output of data for CCE extraction.
Apparent horizon finder — for locating black hole horizons in dynamical simulations.
Initial condition solver — a link to the GRTelcyn solver, to solve the Hamiltonian and Momentum constraints numerically for scalar or vacuum spacetimes. Integration of TwoPunctures as discussed above.
AMR interpolation tools — to enable conservative variable interpolation and post-processing, based on particle methods.
ADM mass and momenta diagnostics — for computing global quantities.
Higher-order spatial stencils — planned support for 6th-order differencing.

Forthcoming features

The following features have been developed internally or in GRChombo and are expected to be ported or reimplemented in GRTeclyn. If you're interested in working on one of these, feel free to contact us — we may be able to share existing code for inspiration or adaptation.

Fixed background metrics — support for scalar field evolution on Schwarzschild or Kerr backgrounds using analytic metrics - by adapting the existing GRDzhaDzha code to GRTeclyn.
Massive vector (Proca) fields — a matter class for evolving massive vector fields.
Cartoon formalism — support for dimensional reduction using symmetries (e.g., axisymmetry).
Other matter models — we will include perfect fluids and vector fields in future.

Summary of development status

Feature	Status
CCZ4 evolution	✅ Ported
Adaptive Mesh Refinement (AMR)	✅ Ported
Checkpointing & Restart	✅ Ported
Moving puncture gauge	✅ Ported
BinaryBH example	✅ Ported
Diagnostic variables	✅ Ported
AMReX GPU offload	✅ Ported
MPI parallelism	✅ Ported
Scalar field matter model	✅ Ported
Puncture tracking	✅ Ported
Particle-based diagnostics	🔧 In progress
Weyl scalar / CCE extraction	🔧 In progress
Apparent horizon finder	🔧 In progress
Initial condition solver	🔧 In progress
ADM quantities	🔧 In progress
6th-order finite differencing	🔮 Planned
Proca field matter	🔮 Planned
Perfect fluid matter	🔮 Planned
Cartoon formalism	🔮 Planned