{
  "model": "Heisenberg",
  "lattice": "rectangular-20x40",
  "n_sites": 800,
  "boundary": "PO",
  "params": {},
  "instance_id": "Heisenberg/rectangular-20x40_800_PO",
  "rows": [
    {
      "energy": -1049.404512,
      "sigma": 0.000624,
      "energy_variance": null,
      "dof": 800,
      "einf": 0,
      "v_score": null,
      "method": "SSE QMC (stochastic series expansion), T -> 0 converged at beta/L = 32 and 64",
      "bound_type": "exact",
      "bound_type_reason": "sign-problem-free SSE QMC on a bipartite lattice, T -> 0 converged; numerically exact (RULES.md 4)",
      "reference": "Sandvik, High-precision ground state parameters of the two-dimensional spin-1/2 Heisenberg model on the square lattice, arXiv:2601.20189 (2026)",
      "peer_reviewed": false,
      "source": "exact-2026-09-15",
      "provenance": "primary",
      "verified": {
        "checked_on": "2026-09-15",
        "method": "arXiv HTML parsed locally, one value per table cell; no LLM transcription",
        "reported_as": "E0/N_b = 0.3363476(2) (magnitude, per bond, N_b = 780), L x 2L = 20 x 40, cylinder",
        "note": "Table 3, \"SSE data for L x 2L lattices with cylindrical boundary conditions (periodic in the shorter direction and open in the longer direction). The ground state energy is normalized by the number of interaction bonds N_b = 2 L^2 - L\", row L = 20. Boundary PO (VarBench lattice.md): periodic along the 20-site direction, open along the 40-site one. Printed as a magnitude; E/N = -(E0/N_b) N_b / N = -0.32793891 in S.S units, Pauli total = 4 N E/N.",
        "secondary_of": null
      }
    }
  ],
  "url": "https://qmbl.org/i/Heisenberg/rectangular-20x40_800_PO/",
  "per_site_divisor": 3200,
  "per_site_label": "E/N (S.S)",
  "record": null,
  "no_record_reason": "solved exactly"
}