using-mpskit

name: using-mpskit description: Use when choosing or running MPSKit.jl for infinite / uniform matrix product states — VUMPS, IDMRG / IDMRG2, finite DMRG, TDVP — including unit-cell construction, the tangent-space gradient-norm convergence probe (calc_galerkin), symmetry setup via TensorKit, or MPSKit setup failures. MPSKit has no TEBD; route iTEBD to /using-tenpy.

MPSKit.jl

Use MPSKit as the harness's canonical Julia stack for infinite / uniform MPS ground states (VUMPS, IDMRG) and for finite DMRG / TDVP, paired with MPSKitModels.jl (ready Hamiltonians) and TensorKit.jl (symmetric tensors).

Sources

• Method card: skills/method-mps/SKILL.md
• Stack: the shared julia-env. Install with make install mpskit (adds MPSKit, MPSKitModels, TensorKit and runs the smoke); on a remote, instantiate julia-env via /setup-julia first.
• Smoke test (also run by the install target): julia --project=julia-env -e 'using MPSKit, MPSKitModels, TensorKit'
• Local API reference (key API + worked examples, with links to upstream docs): references/mpskit-api.md

Workflow

1. Confirm the model, geometry (infinite unit-cell length or finite chain), bond dimension, and symmetry before code.
2. Choose the algorithm with /method-mps: VUMPS for infinite ground states (default), IDMRG/IDMRG2 when DMRG infrastructure is wanted, DMRG/DMRG2 for finite chains, TDVP for time evolution. There is no TEBD in MPSKit — for iTEBD route to /using-tenpy.
3. Build H from MPSKitModels (or a custom MPO); build the initial InfiniteMPS at the chosen unit cell and D.
4. Run find_groundstate; converge on the tangent-space gradient norm ‖B‖ (calc_galerkin), not the energy.
5. Record bond dimension, unit-cell length, tolerance, max iterations, symmetry, BLAS/Julia thread count, and the final ‖B‖ in the run plan.

Parameter setup

Use this as the source for MPSKit-specific reproduction knobs unless the paper or official code fixes a value.

• Bond dimension D — set via the virtual ComplexSpace(D) (no symmetry) or a graded space per sector (symmetric). The accuracy lever; run a D-series until the energy stops moving. For gapless systems, scale D (finite-entanglement scaling).
• Unit cell L — the length of the InfiniteMPS (number of site tensors repeated). Must be ≥ the ground-state order period: 1-site for uniform/Haldane states, 2-site for Néel/AFM. A sublattice rotation can fold a 2-site ordered state to a 1-site cell (e.g. critical XXZ).
• Symmetry — chosen through the TensorKit element/space: Trivial (no symmetry) or an abelian/non-abelian sector (U1Irrep, SU2Irrep) for a smaller, sector-pinned tensor. Match the paper.
• Tolerance / stop criterion — tol in the algorithm (e.g. VUMPS(; tol=1e-12)); the run stops when ‖B‖ falls below it. Probe ‖B‖ explicitly with calc_galerkin.
• Max iterations — maxiter; the stop criterion should fire first. Hitting it = not converged.
• Eigensolver — VUMPS uses an adaptive Lanczos/Arnoldi tolerance internally; do not pin it to a single coarse step (that defeats the adaptive schedule and can stall IDMRG — see the iterator note below).
• Initial state — random InfiniteMPS([...], [...]), or seed from a product state in the target sector if convergence stalls.

Knobs

Code shape

Check exact constructors against the installed MPSKit / MPSKitModels docs before a production script; the harness-level shape:

using MPSKit, MPSKitModels, TensorKit, LinearAlgebra

# 1. Hamiltonian (MPSKitModels). XXZ takes a symmetry positional arg; XYZ does NOT.
H = heisenberg_XXZ(ComplexF64, Trivial, InfiniteChain(2); spin=1//2, Delta=2.0)
# H = heisenberg_XYZ(ComplexF64, InfiniteChain(1); Jx=-1.0, Jy=-1.0, Jz=1.0, spin=1//2)

# 2. Uniform MPS at unit cell L and bond dimension D (no symmetry → ComplexSpace).
L, D = 2, 54
psi = InfiniteMPS([physicalspace(H, i) for i in 1:L], fill(ComplexSpace(D), L))

# 3. Ground state by VUMPS; ‖B‖ is the convergence diagnostic.
psi, envs = find_groundstate(psi, H, VUMPS(; tol=1e-12, maxiter=300, verbosity=1))
e_site = real(expectation_value(psi, H)) / length(psi)

# 4. Tangent-space gradient norm ‖B‖ (per-site RMS Galerkin gradient).
bnorm = sqrt(sum(abs2(MPSKit.calc_galerkin(pos, psi, H, psi, envs)) for pos in 1:length(psi)) / length(psi))

Per-iteration trajectory (for a convergence / wall-time benchmark):

# VUMPS exposes a finalize callback: (iter, ψ, H, envs) -> (ψ, envs). Log inside it.
fin = (iter, psi, H, envs) -> (push!(recs, (e=real(expectation_value(psi,H))/length(psi),
                                            B=MPSKit.calc_galerkin(psi,H,psi,envs))); (psi, envs))
find_groundstate(psi, H, VUMPS(; tol=1e-12, maxiter=300, finalize=fin))

# IDMRG has NO finalize — use the public IterativeSolver iterator (non-invasive, perf-neutral):
it = MPSKit.IterativeSolver(IDMRG(; tol=1e-12, maxiter=300), state)
for (mps, envs, ϵ, ΔE) in it
    # log e/site and calc_galerkin here; break on ϵ ≤ tol or iter cap
end

Both hooks are public — no Pkg.develop needed for VUMPS/IDMRG trajectories. If a future probe needs internals MPSKit doesn't expose, Pkg.develop("MPSKit") and instrument the source non-invasively (reproduce-paper step 2).

Time estimate

Cost is (L_cell · d · D³ · n_iter) ÷ throughput. The work count is firm; throughput (BLAS rate, threads) is the unknown — settle it with one timed iteration at the target D.

• Per iteration ∝ unit-cell length × physical dim d × D³ (local effective eigensolves).
• Iteration count to a target ‖B‖ depends on the algorithm and proximity to criticality (the second soft factor in the estimate) — see /method-mps.
• Memory is modest (O(D²) tensors + environments); compute is the gate.
• D in the tens-to-low-hundreds is seconds-to-minutes on a laptop, single-thread. Larger D (cylinders, near-critical) scales as D³ → route to /using-slurm.
• For a wall-time benchmark: confirm the thread count with the user, run a warm-up pass to exclude JIT, and subtract per-checkpoint measurement time (reproduce-paper performance benchmark rules).

Use Another Route When

• The algorithm is iTEBD — MPSKit has no TEBD; route to /using-tenpy.
• The target is genuinely 2D / a wide system → PEPS (/using-pepskit).
• The paper used official non-MPSKit code that is runnable, or a finite ITensor workflow is preferred → /using-itensors.