Of the full GR codes used to evolve NS-NS binaries, almost all are grid-based and make use of some form of adaptive mesh refinement. The one exception is the SpEC code developed by the SXS collaboration, formed originally by Caltech and Cornell, which has used a hybrid spectral-method field solver with grid-based hydrodynamics. Most make use of the BSSN formalism for evolving Einstein’s equations (see Section 5.2.1 below), while the HAD code uses the alternate Generalized Harmonic Gauge (GHG) approach. This technique is also used by the SXS collaboration and the Princeton group, who have both performed simulations of merging BH-NS binaries (see Section 6.6) but have yet to report any results on NS-NS mergers. Three groups have reported results for NS-NS mergers including MHD (HAD, Whisky, and UIUC), while the KT (Kyoto/Tokyo) group has reported magnetized evolutions of HMNS remnants (see  and references therein for a discussion of their work and that of other numerical relativity groups), but have yet to use that code for a NS-NS merger calculation.
While full GR codes were being developed to study NS-NS binaries, a parallel and rather independent track developed to study detailed microphysical effects in binary mergers using approximate relativistic schemes. This includes codes like that developed by the MPG group that accurately track the production of neutrinos and antineutrinos and their annihilation during a merger, as well as post-processing routines that use extensive nuclear chains to track the production of rare high-atomic number r-process elements in merger ejecta . Meanwhile, the Bremen group’s SPH code includes variable-temperature physically motivated equations of state  and magnetohydrodynamics , and has been used with a multi-group flux-limited diffusion neutrino code to generate expected neutrino signatures from merger calculations . A summary of groups performing NS-NS merger calculations is presented in Table 3.
|KT||[134, 144, 145, 265, 264, 287, 288]||GR||*||Phys. EOS, -leakage|
|[285, 286, 282, 332]|
|HAD||[7, 6]||GR (GHG)||Y||N|
|Whisky||[17, 18, 14, 15, 116, 117, 240, 241]||GR||Y||N|
|MPG||[34, 35, 33, 32, 123, 209, 208, 296]||CF||N||Quark, Phys. EOS, -leakage|
|Bremen||[82, 233, 247]||Newt||Y||Phys. EOS , -leakage|
Living Rev. Relativity 15, (2012), 8
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