Choose a network and amax

Set the network at construction time — via the network parameter (Python API/CLI) or the --network/--amax flags:

from primat.backend import run_bbn

result = run_bbn({"network": "large", "amax": 8})

primat --network large --amax 8

Available networks

Two named networks (plus a Parthenope-rates variant of the small one) are available via network; amax (any positive integer) further restricts any of them to reactions whose nuclides all have mass number A ≤ amax:

network	Reactions	Nuclides	Notes
"small" (default)	12	8	the key reactions; fastest
"small_parthenope"	12	8	same reactions, Parthenope 3.0 rate tables (comparison runs)
"large"	~429	~59	from the AC2024 compilation; LT era only
"large", amax=8	68	12	the old “medium” network’s exact equivalent
"large", amax=2	3	3	the old “deuterium” network’s equivalent (n↔p + n_p__d_g + p_p_n__d_p)
any other name	—	—	loads data/nuclear/networks/<name>.txt

All networks share the HT (n↔p) and MT eras — the MT era always uses a fixed 18-reaction subset, too stiff to run the full network; only the LT reaction set is filtered by network/amax. The light-element abundances of the full large network match the amax=8 restriction to ≲1e-4; its heavy-nuclide tail (B, C, N, O, …) is approximate (limited by the AC2024 rate floors). See the BBN abundance evolution notebook for evolution plots across networks.

Related physics-configuration flags

Flag	Default	Effect
radiative_corrections	True	Coulomb + T=0 resummed radiative corrections to n↔p (CCR)
finite_mass_corrections	True	Fokker-Planck finite-nucleon-mass correction (FM)
thermal_corrections	True	Finite-temperature radiative corrections to n↔p (CCRTh)
spectral_distortions	True	Correct n↔p rates for non-FD neutrino distributions (SD)
tau_n_normalization	True	Normalise weak rates using τ_n (neutron lifetime)
numerical_precision	1e-7	solve_ivp relative tolerance (rtol) for all ODE integration

See The n↔p weak-rate cache workflow for how changing these flags interacts with the n↔p rate cache.