Hubbard square 6x6, U = 2, n = 1

Exact energy and best values (rules §6)

exact (stochastic)	-1.1516(1)	AFQMC Qin et al. (2016) · sign-problem-free QMC, exact within its error bar
best variational	-1.1506	VMC (HB) Zhou et al. (2026) · the lowest unflagged variational energy, a strict variational bound

Energies as E/site. Classes are different claims and are not ranked against each other.

The Hamiltonian

H = −t ∑_⟨ij⟩,σ (c^†_iσc_jσ + h.c.) + U ∑_i n_i↑n_i↓

t = 1, U = 2, N_↑ = N_↓ = 18 on 36 sites.

lattice: square
sites: 36
boundary: periodic
Nf: 18
U: 2
energies: 2

Stored as a total energy; quoted here as E/site, which is what the papers report (how the conversion works).

Every published energy

Grouped by what the number is. The exact energy comes first where there is one; in every other group the lowest unflagged energy is its best value, and each row's distance is to that best (rules §6).

Numerically exact (1)

Exact diagonalization, an exact solution, or sign-problem-free QMC where that is established. An exact energy is shown first, before the best of each other class. QMC is exact only within its error bar, which it must state, and reads exact (stochastic); an exact diagonalization outranks it. For a diagonalization resolved by symmetry sector only the ground state's sector is listed; the lowest energies of the other sectors are the instance's spectrum, in its JSON, and are not shown as the exact energy.

E/site	distance to class best	σ	Var(E)	V-score	method	source	code	year	comment
-1.1516exact (stochastic)	0	1.4e-4	n/a	n/a	AFQMC peer reviewed	Qin et al. (2016)		2016

Strict variational upper bounds (1)

Energies that a variational state reached (VMC, DMRG and MPS at finite bond dimension, PEPS, NQS, VQE). Each is an upper bound on the ground-state energy.

E/site	distance to class best	σ	Var(E)	V-score	method	source	code	year	comment
-1.1506best variational	0	n/a	n/a	n/a	VMC (HB) preprint	Zhou et al. (2026)		2026

How these numbers were read

Where each number below was read, and how it was checked.

-1.1516(1), AFQMC

Checked 2026-09-15 · arXiv PDF extracted locally with pypdf in layout mode, E rows of Table IV parsed by column position; no LLM transcription

Reported as. E = -41.457(5) (total), 6 x 6, U = 2, PBC

Table IV, "Total ground state energy (E) ... in the Hubbard model at half-filling, for U = 2, 4, 6, 8 ... for both PBC and PBC-APBC. Statistical errors are on the last digit." Row 6 x 6, column U = 2 PBC. Half filling: N_up = N_dn = 18. Cross-checks: the 8 x 8 PBC values at U = 4 and 8 agree with VarBench's own AFQMC exact rows to 1.3 sigma, and the 8 x 8, 10 x 10 and 12 x 12 PBC U = 8 values with the AFQMC references quoted by arXiv:2507.02644 (Table S1) to the printed digits.

-1.1506, VMC (HB)

Checked 2026-09-15 · source text of arXiv:2606.00924 read locally (arXiv HTML or pypdf layout text); value copied from the harvested cell and the text, no LLM transcription of numbers; single reading

Reported as. -1.1506

Table II, row '6 × 6', column 'U = 2, K = 1' | Same row's quoted QMC value (-1.1515(9)) matches instance exact_per_site -1.151583333 to displayed precision.

Coverage

Newest published number here: 2026.

This instance as data

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