The equations of state (EOSs) for neutron-star matter are fitted from mass density ρ ≈ 10⁶ g cm⁻³ to ρ ∼ 10¹⁶ g cm⁻³ (baryon number density n from ≈ 0.6×10⁻⁹ fm⁻³ to a few fm⁻³). At the low end of this density range, the EOS starts to depend on temperature T. The subroutines are downloadable by clicking on the file names below:

1. bskfit18.f (updated 13.02.23) — the main file, which contains the Fortran realization of our approximations for the BSkEOSs and related quantities.
   List of subroutines:
   • BSKfit returns pressure P and log.derivative d ln P / d lnρ at a given mass density ρ (at ρ > 10⁶ g cm⁻³);
   • BSkEofN returns energy per baryon and total mass-energy density as functions of mean baryon number density n;
   • BSkEofR returns energy per baryon and baryon number density as functions of mass density, using BSkEofN by iterations;
   • CORE17 returns fractions of electrons Y_e and muons Y_μ, respective relativity paramaters, and chemical potentials of neutrons, electrons, and protons in the core;
   • INCRST (only for BSk22,24,25,26 models and for the inner crust!) returns:
     • the number of bound nucleons in a nucleus A,
     • the total number of nucleons per one nucleus A',
     • the number of bound protons in a nucleus Z,
     • the total number of protons per one nucleus Z',
     • CMUN,CMUP,CMUe - chemical potentials of n,p,e, resp.,
     • XREL - electron relativity parameter,
     • a_p, a_n, C_p, C_n - nuclear-shape parameters,
     • x_nuc and x_nuc,n - ratios of the effective proton and neutron radii to the Wigner-Seitz cell radius;
   • EFFMBSk returns effective nucleon masses as functions of n.
   The next 4 subroutines, listed below, are not necessary for BSk22+. They are provided for the sake of backward compatibility:
   • BSkRofN returns mass density ρ as a function of baryon number density n;
   • BSkNofR returns baryon number density n as a function of ρ;
   • FRACORE returns fractions of electrons Y_e and muons Y_μ as functions of n in the stellar core;
   • INCRUST (applicable for BSk19-21 models and for the inner crust only!) returns
     • the number of bound nucleons in a nucleus A,
     • the total number of nucleons per one nucleus A',
     • Z_cell - total number of protons per a nucleus,
     • Z_clust - number of clusterized protons in a nucleus;
     • 4 nuclear-shape parameters a_p, a_n, C_p, C_n,
       as well as 4 additional parameters:
       • n_0p and n_0n – the heights of the bumps of proton and neutron densities near the center of a Wigner-Seitz cell,
       • x_nuc and x_nuc,n — the ratios of the effective proton and neutron radii of these bumps, respectively, to the cell radius.
2. bskeos.f — an auxiliary file for demonstration of the use of the EOS calculation. In the latter file, the EOS fits are extended to smaller ρ by matching to an approximation for the OPAL EOS of iron at T = 10⁷ K and ρ = (0.1 — 1000) g cm⁻³.
   Arguments of the program:
   • MODE — an integer (1, 2, or 3) that identifies which of the arguments is the input (XN, RHO, or H1);
   • KEOS — an integer (19,20,21,22,24,25,26) that identifies which EOS has to be used;
   • XN — number density n in fm⁻³;
   • RHO — mass density ρ in g cm⁻³;
     RLG=log₁₀ρ;
   • H1 — (e^H−1), where H is the dimensinoless pseudo-enthalpy;
   • P — pressure P in dyn cm⁻²;
   • Gamma — adiabatic index Γ = (d log P) / (d log n).
   The subroutine BSKEOS(MODE,KEOS,XN,RHO,H1,P,Gamma) in this file returns 4 fitted arguments for 1 input (XN, RHO, or H1). It uses subroutines from the former file that realize the separate fits: P(ρ), n(ρ), ρ(n), ρ(H1). Typical accuracy of the fits is a few percent or better. To achieve the perfect thermodynamic consistency, however, it is advised not to use all the fitted outputs, but to calculate either XN(RHO,P) or RHO(XN,P) using the thermodynamic identities (e.g., Eqs.(2) and (3) of Haensel and Potekhin 2004).

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