Looking at the source code IllinoisGRMHD in the ET seems to support piecewise polytropic EOS, at the very least for a single piece by setting up IllinoisGRMHD’s gamma_th parameter and the first entry in the eos_params_arrays2 grid array (eg by setting ID_converter_ILGRMHD’s Initial_Gamma and Initial_K parameters). However D_converter_ILGRMHD contains an explicit check for a simple Gamma-law (ie no difference between thermal and cold Gamma) in

// P = (\Gamma - 1) rho epsilon
// -> \Gamma = P/(rho epsilon) + 1
double measured_gamma = ( P[index]/(rho_b[index] * eps[index]) + 1.0 );
if(rho_b[index]>rho_b_atm && fabs(gamma_th - measured_gamma)/gamma_th > 1e-2)
  CCTK_VError(VERR_DEF_PARAMS,
              "Expected simple gamma law with gamma_th=%.15e, but found a point with gamma l
--

Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2514/broken-support-for-piecewise-polytropes-in