B(Z,N) = Z Δ(1H) + N Δ(n) − Δ(Z,N) from the atomic mass excesses Δ of AME2020. This is AME's own convention (atomic masses, so the electrons' binding is not removed); B/A computed this way equals AME2020's printed B/A column to its rounding for all nuclides with A ≥ 2 (tests/check_binding.py).
The fit: unweighted least squares on B (not B/A) over every nuclide with a measured mass and A at or above the cut; masses AME estimates from systematics (#) are left out. Unweighted, because the formula misses by MeV while the mass uncertainties of the fitted nuclides are far smaller (): weighting by the mass uncertainties would let a handful of precisely measured nuclei decide the coefficients. The formula is linear in the five coefficients, so the minimum is the solution of a 5×5 linear system, solved in your browser whenever the cut or the Coulomb form changes.
Pairing: apδ/A1/2. Other conventions (ap/A3/4, or a separate odd-odd term) change ap and its meaning, so coefficients from different papers can only be compared within the same form. Coulomb: Z(Z−1) removes a proton's self-energy; Z2 is the older form.
Valley floor: the integer Z of largest Bdrop at each A, pairing left out, i.e. the drop's most bound isobar; the glowing path follows the measured most bound isobars (largest B at fixed A). Neither is the β-stability line, which runs through the smallest atomic mass at fixed A: a neutron outweighs a hydrogen atom, so the two choices differ, for the measured masses at . The β valley with its decays is on beta.html.
Drip lines of the drop: a nucleus counts as bound against neutron emission when Sn = B(Z,N) − B(Z,N−1) > 0 and S2n = B(Z,N) − B(Z,N−2) > 0, and the neutron drip line is the largest such N for each Z; likewise Sp, S2p and the smallest N on the proton side. Pairing makes Sn zigzag, so odd-N isotopes can be unbound inside the line while heavier even-N ones are still bound. The heaviest isotope with a measured mass is not the drip line: some measured nuclei are unbound (25O has Sn < 0, 26O has S2n < 0), and many bound ones have no measured mass.