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EditURL = "../../../../examples/boundary_mps/main.jl"

[Boundary MPS contractions of 2D networks](@id e_boundary_mps)

Instead of using CTMRG to contract the network encoding the norm of an infinite PEPS, one can also use so-called [boundary MPS methods](@cite haegeman_diagonalizing_2017) to contract this network. In this example, we will demonstrate how to use [the VUMPS algorithm](@cite vanderstraeten_tangentspace_2019) to do so.

Before we start, we'll fix the random seed for reproducability:

using Random
Random.seed!(29384293742893);

Besides TensorKit and PEPSKit, here we also need to load the MPSKit.jl package which implements a host of tools for working with 1D matrix product states (MPS), including the VUMPS algorithm:

using TensorKit, PEPSKit, MPSKit

Computing a PEPS norm

We start by initializing a random infinite PEPS. Let us use normally distributed complex entries using randn:

ψ = InfinitePEPS(randn, ComplexF64, ComplexSpace(2), ComplexSpace(2))
InfinitePEPS{TensorKit.TensorMap{ComplexF64, TensorKit.ComplexSpace, 1, 4, Vector{ComplexF64}}}(TensorKit.TensorMap{ComplexF64, TensorKit.ComplexSpace, 1, 4, Vector{ComplexF64}}[TensorMap{ComplexF64, TensorKit.ComplexSpace, 1, 4, Vector{ComplexF64}}(ComplexF64[-0.5524390176345264 - 0.07357188568178248im, -0.5455245317233405 + 0.8946618856309984im, 0.34014501646081047 - 0.7552574870030472im, 1.249282911658007 + 0.45352274131986825im, 0.33621043661988675 + 0.4400876608299719im, -0.0077250067072679235 + 1.7380910495900947im, -0.9866664087107284 - 0.28688827761325675im, -0.19071062901939098 - 1.1367500834118434im, -0.09149850722392933 + 0.3560942836258964im, -0.19309251474097275 - 0.32363899914302613im, 1.6255618447281441 - 0.5689426732891244im, -0.025356816648697236 + 0.5632279168368712im, 0.07675114584269166 - 0.011479824536308164im, -1.0116302866282385 - 0.9253070687198848im, -0.17779977372973318 + 1.1379201927122535im, 1.1649047337212566 + 0.9936369101208083im, 0.2510676919806213 - 0.182052326055189im, 0.04501645227038913 - 0.8140971172854408im, -0.5792402993550532 - 0.4309109406268341im, -0.5608346802110794 + 0.21262550530307248im, 1.5061767210554262 + 0.17190948125245623im, -0.8176938467062373 - 0.40919675695722396im, -0.8001234458239143 + 0.6764943808639017im, -0.6692181340575689 + 0.6923370271564298im, -0.16556382071485704 + 0.2540132491548349im, -0.29883021417599165 - 0.07229462525164528im, 0.05546115732751907 + 0.3723175507964387im, -1.200173153698329 - 0.45509299328832953im, 0.289873563752043 + 0.44718981087960125im, 0.5128282969211142 - 0.2865462937979091im, 0.018357838612906643 + 0.9634127683557584im, -0.44278618042821827 + 0.2612084385439659im], ℂ^2 ← (ℂ^2 ⊗ ℂ^2 ⊗ (ℂ^2)' ⊗ (ℂ^2)'));;])

To compute its norm, we have to contract a double-layer network which encodes the bra-ket PEPS overlap \langle ψ | ψ \rangle:

<center>
<img src="../../assets/figures/peps_norm_network.svg" alt="peps norm network" class="color-invertible" style="zoom: 180%"/>
</center>

In PEPSKit.jl, this structure is represented as an InfiniteSquareNetwork object, whose effective local rank-4 constituent tensor is given by the contraction of a pair of bra and ket PEPSKit.PEPSTensors across their physical legs. Until now, we have always contracted such a network using the CTMRG algorithm. Here however, we will use another approach.

If we take out a single row of this infinite norm network, we can interpret it as a 1D row-to-row transfer operator \mathbb{T},

<center>
<img src="../../assets/figures/peps_transfer_operator.svg" alt="peps transfer operator" class="color-invertible" style="zoom: 180%"/>
</center>

This transfer operator can be seen as an infinite chain of the effective local rank-4 tensors that make up the PEPS norm network. Since the network we want to contract can be interpreted as the infinite power of \mathbb{T}, we can contract it by finding its leading eigenvector as a 1D MPS | \psi_{\text{MPS}} \rangle, which we call the boundary MPS. This boundary MPS should satisfy the eigenvalue equation \mathbb{T} | \psi_{\text{MPS}} \rangle \approx \Lambda | \psi_{\text{MPS}} \rangle, or diagrammatically:

<center>
<img src="../../assets/figures/peps_transfer_fixedpoint_equation.svg" alt="peps transfer fixedpoint equation" class="color-invertible" style="zoom: 180%"/>
</center>

Note that if \mathbb{T} is Hermitian, we can formulate this eigenvalue equation in terms of a variational problem for the free energy,

$$\begin{align} f &= \lim_{N \to ∞} - \frac{1}{N} \log \left( \frac{\langle \psi_{\text{MPS}} | \mathbb{T} | \psi_{\text{MPS}} \rangle}{\langle \psi_{\text{MPS}} | \psi_{\text{MPS}} \rangle} \right), \\\ &= -\log(\lambda) \end{align}$$

where \lambda = \Lambda^{1/N} is the 'eigenvalue per site' of \mathbb{T}, giving f the meaning of a free energy density.

Since the contraction of a PEPS norm network is in essence exactly the same problem as the contraction of a 2D classical partition function, we can directly use boundary MPS algorithms designed for 2D statistical mechanics models in this context. In particular, we'll use the [the VUMPS algorithm](@cite vanderstraeten_tangentspace_2019) to perform the boundary MPS contraction, and we'll call it through the leading_boundary method from MPSKit.jl. This method precisely finds the MPS fixed point of a 1D transfer operator.

Boundary MPS contractions with PEPSKit.jl

To use leading_boundary, we first need to contruct the transfer operator \mathbb{T} as an MPSKit.InfiniteMPO object. In PEPSKit.jl, we can directly construct the transfer operator corresponding to a PEPS norm network from a given infinite PEPS as an InfiniteTransferPEPS object, which is a specific kind of MPSKit.InfiniteMPO.

To construct a 1D transfer operator from a 2D PEPS state, we need to specify which direction should be facing north (dir=1 corresponding to north, counting clockwise) and which row of the network is selected from the north - but since we have a trivial unit cell there is only one row here:

dir = 1 ## does not rotate the partition function
row = 1
T = InfiniteTransferPEPS(ψ, dir, row)
1-site InfiniteMPO(ComplexF64, TensorKit.ComplexSpace) with maximal dimension 4:
| ⋮
| (ℂ^2 ⊗ (ℂ^2)')
┼─[1]─ (ℂ^2 ⊗ (ℂ^2)')
│ (ℂ^2 ⊗ (ℂ^2)')
| ⋮

Since we'll find the leading eigenvector of \mathbb{T} as a boundary MPS, we first need to construct an initial guess to supply to our algorithm. We can do this using the initialize_mps function, which constructs a random MPS with a specific virtual space for a given transfer operator. Here, we'll build an initial guess for the boundary MPS with a bond dimension of 20:

mps₀ = initialize_mps(T, [ComplexSpace(20)])
1-site InfiniteMPS(ComplexF64, TensorKit.ComplexSpace) with maximal dimension 20:
| ⋮
| ℂ^20
├─[1]─ (ℂ^2 ⊗ (ℂ^2)')
│ ℂ^20
| ⋮

Note that this will just construct a MPS with random Gaussian entries based on the physical spaces of the supplied transfer operator. Of course, one might come up with a better initial guess (leading to better convergence) depending on the application. To find the leading boundary MPS fixed point, we call leading_boundary using the MPSKit.VUMPS algorithm:

mps, env, ϵ = leading_boundary(mps₀, T, VUMPS(; tol = 1.0e-6, verbosity = 2));
[ Info: VUMPS init:	obj = +1.674563752306e+00 +3.035692829590e+00im	err = 7.5576e-01
[ Info: VUMPS conv 120:	obj = +6.831610878219e+00 -9.723041888467e-09im	err = 9.5233523833e-07	time = 6.10 sec

The norm of the state per unit cell is then given by the expectation value $\langle \psi_\text{MPS} | \mathbb{T} | \psi_\text{MPS} \rangle$ per site:

norm_vumps = abs(prod(expectation_value(mps, T)))
6.83161087821908

This can be compared to the result obtained using CTMRG, where we see that the results match:

env_ctmrg, = leading_boundary(CTMRGEnv(ψ, ComplexSpace(20)), ψ; tol = 1.0e-6, verbosity = 2)
norm_ctmrg = abs(norm(ψ, env_ctmrg))
@show abs(norm_vumps - norm_ctmrg) / norm_vumps;
[ Info: CTMRG init:	obj = -1.495741317009e+01 +3.091851579630e-01im	err = 1.0000e+00
[ Info: CTMRG conv 30:	obj = +6.831603585666e+00	err = 6.2262595115e-07	time = 0.20 sec
abs(norm_vumps - norm_ctmrg) / norm_vumps = 1.0674719949350553e-6

Working with unit cells

For PEPS with non-trivial unit cells, the principle is exactly the same. The only difference is that now the transfer operator of the PEPS norm partition function has multiple rows or 'lines', each of which can be represented by an InfiniteTransferPEPS object. Such a multi-line transfer operator is represented by a PEPSKit.MultilineTransferPEPS object. In this case, the boundary MPS is an MultilineMPS object, which should be initialized by specifying a virtual space for each site in the partition function unit cell.

First, we construct a PEPS with a $2 \times 2$ unit cell using the unitcell keyword argument and then define the corresponding transfer operator, where we again specify the direction which will be facing north:

ψ_2x2 = InfinitePEPS(rand, ComplexF64, ComplexSpace(2), ComplexSpace(2); unitcell = (2, 2))
T_2x2 = PEPSKit.MultilineTransferPEPS(ψ_2x2, dir);

Now, the procedure is the same as before: We compute the norm once using VUMPS, once using CTMRG and then compare.

mps₀_2x2 = initialize_mps(T_2x2, fill(ComplexSpace(20), 2, 2))
mps_2x2, = leading_boundary(mps₀_2x2, T_2x2, VUMPS(; tol = 1.0e-6, verbosity = 2))
norm_2x2_vumps = abs(prod(expectation_value(mps_2x2, T_2x2)))

env_ctmrg_2x2, = leading_boundary(
    CTMRGEnv(ψ_2x2, ComplexSpace(20)), ψ_2x2; tol = 1.0e-6, verbosity = 2
)
norm_2x2_ctmrg = abs(norm(ψ_2x2, env_ctmrg_2x2))

@show abs(norm_2x2_vumps - norm_2x2_ctmrg) / norm_2x2_vumps;
[ Info: VUMPS init:	obj = +8.149302834396e+02 -8.860408249120e+01im	err = 8.6172e-01
┌ Warning: VUMPS cancel 200:	obj = +1.046766793317e+05 -1.493448261103e+02im	err = 3.7375841028e-02	time = 11.59 sec
└ @ MPSKit ~/.julia/packages/MPSKit/9ITGf/src/algorithms/groundstate/vumps.jl:76
[ Info: CTMRG init:	obj = -1.240261729401e+02 -1.672150510262e+01im	err = 1.0000e+00
[ Info: CTMRG conv 47:	obj = +1.046633714846e+05	err = 1.6993512829e-07	time = 0.76 sec
abs(norm_2x2_vumps - norm_2x2_ctmrg) / norm_2x2_vumps = 0.00012815051487696121

Again, the results are compatible. Note that for larger unit cells and non-Hermitian PEPS [the VUMPS algorithm may become unstable](@cite vanderstraeten_variational_2022), in which case the CTMRG algorithm is recommended.

Contracting PEPO overlaps

Using exactly the same machinery, we can contract 2D networks which encode the expectation value of a PEPO for a given PEPS state. As an example, we can consider the overlap of the PEPO correponding to the partition function of [3D classical Ising model](@ref e_3d_ising) with our random PEPS from before and evaluate the overlap $\langle \psi | T | \psi \rangle$.

The classical Ising PEPO is defined as follows:

function ising_pepo(β; unitcell = (1, 1, 1))
    t = ComplexF64[exp(β) exp(-β); exp(-β) exp(β)]
    q = sqrt(t)

    O = zeros(2, 2, 2, 2, 2, 2)
    O[1, 1, 1, 1, 1, 1] = 1
    O[2, 2, 2, 2, 2, 2] = 1
    @tensor o[-1 -2; -3 -4 -5 -6] :=
        O[1 2; 3 4 5 6] * q[-1; 1] * q[-2; 2] * q[-3; 3] * q[-4; 4] * q[-5; 5] * q[-6; 6]
    O = TensorMap(o, ℂ^2 ⊗ (ℂ^2)' ← ℂ^2 ⊗ ℂ^2 ⊗ (ℂ^2)' ⊗ (ℂ^2)')

    return InfinitePEPO(O; unitcell)
end;

To evaluate the overlap, we instantiate the PEPO and the corresponding InfiniteTransferPEPO in the right direction, on the right row of the partition function (trivial here):

T = ising_pepo(1)
transfer_pepo = InfiniteTransferPEPO(ψ, T, 1, 1)
1-site InfiniteMPO(ComplexF64, TensorKit.ComplexSpace) with maximal dimension 8:
| ⋮
| (ℂ^2 ⊗ ℂ^2 ⊗ (ℂ^2)')
┼─[1]─ (ℂ^2 ⊗ ℂ^2 ⊗ (ℂ^2)')
│ (ℂ^2 ⊗ ℂ^2 ⊗ (ℂ^2)')
| ⋮

As before, we converge the boundary MPS using VUMPS and then compute the expectation value:

mps₀_pepo = initialize_mps(transfer_pepo, [ComplexSpace(20)])
mps_pepo, = leading_boundary(mps₀_pepo, transfer_pepo, VUMPS(; tol = 1.0e-6, verbosity = 2))
norm_pepo = abs(prod(expectation_value(mps_pepo, transfer_pepo)));
@show norm_pepo;
[ Info: VUMPS init:	obj = +2.655321432467e+01 +3.760603778362e-01im	err = 8.9759e-01
┌ Warning: VUMPS cancel 200:	obj = +9.515362005489e+01 -2.503860489548e+00im	err = 2.3598739398e-01	time = 33.35 sec
└ @ MPSKit ~/.julia/packages/MPSKit/9ITGf/src/algorithms/groundstate/vumps.jl:76
norm_pepo = 95.18655749054498

These objects and routines can be used to optimize PEPS fixed points of 3D partition functions, see for example [Vanderstraeten et al.](@cite vanderstraeten_residual_2018)

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