[WIP] Boundary-MPS approximate contraction of an iPEPO window with CTMRG environment

src/PEPSKit.jl

@@ -28,10 +28,13 @@ using LoggingExtras
 import TupleTools
 
 using MPSKit
-using MPSKit: MPSTensor, MPOTensor, GenericMPSTensor, MPSBondTensor, ProductTransferMatrix
+using MPSKit:
+    MPSTensor, MPOTensor, GenericMPSTensor, MPSBondTensor,
+    ProductTransferMatrix, TransferMatrix
 using MPSKit: InfiniteEnvironments
 import MPSKit: tensorexpr, leading_boundary, loginit!, logiter!, logfinish!, logcancel!, physicalspace
 import MPSKit: infinite_temperature_density_matrix
+import MPSKit: fuser
 
 import TensorKitTensors.SpinOperators as SO
 import TensorKitTensors.FermionOperators as FO
@@ -73,6 +76,8 @@ include("operators/infinitepepo.jl")
 include("operators/transfermatrix.jl")
 include("operators/localoperator.jl")
 include("operators/localcircuit.jl")
+include("operators/mpo_observable.jl")
+
 include("operators/lattices/squarelattice.jl")
 include("operators/models.jl")
 
@@ -107,6 +112,12 @@ include("algorithms/contractions/correlator/peps.jl")
 include("algorithms/contractions/correlator/pepo_purified.jl")
 include("algorithms/contractions/correlator/pepo_1layer.jl")
 
+include("algorithms/contractions/mpo_path/pepo_1layer.jl")
+include("algorithms/contractions/window/tools.jl")
+include("algorithms/contractions/window/pepo_1layer.jl")
+include("algorithms/contractions/window/twosite/caching.jl")
+include("algorithms/contractions/window/twosite/pepo_1layer.jl")
+
 include("algorithms/ctmrg/sparse_environments.jl")
 include("algorithms/ctmrg/ctmrg.jl")
 include("algorithms/ctmrg/projectors.jl")
@@ -139,6 +150,8 @@ include("algorithms/transfermatrix.jl")
 include("algorithms/toolbox.jl")
 include("algorithms/correlator_adapters.jl")
 include("algorithms/correlators.jl")
+include("algorithms/expval_approx.jl")
+include("algorithms/correlator_approx.jl")
 
 include("algorithms/optimization/fixed_point_differentiation.jl")
 include("algorithms/optimization/peps_optimization.jl")
@@ -156,9 +169,10 @@ export FixedSpaceTruncation, SiteDependentTruncation
 export HalfInfiniteProjector, FullInfiniteProjector
 export C4vCTMRG, C4vEighProjector, C4vQRProjector
 export initialize_random_c4v_env, initialize_singlet_c4v_env
-export LocalOperator, physicalspace
+export LocalOperator, MPOObservable, physicalspace
 export product_peps
-export reduced_densitymatrix, expectation_value, network_value, cost_function
+export reduced_densitymatrix, expectation_value_approx, correlator_approx
+export expectation_value, network_value, cost_function
 export correlator, correlation_length
 export leading_boundary
 export PEPSOptimize, FixedPointGradient, GeomSum, ManualIter

src/algorithms/contractions/mpo_path/pepo_1layer.jl

@@ -0,0 +1,202 @@
+"""
+Check that the physical legs of a first OBC-MPO tensor match the PEPO site's physical space.
+"""
+function _check_pepo_first_physicalspace(A, op)
+    physicalspace(A) == space(op, 1) == space(op, 2)' ||
+        throw(SpaceMismatch("first MPO tensor physical space does not match PEPO site"))
+    return nothing
+end
+
+"""
+Check that the physical legs of a last OBC-MPO tensor match the PEPO site's physical space.
+"""
+function _check_pepo_last_physicalspace(A, op)
+    physicalspace(A) == space(op, 2) == space(op, 3)' ||
+        throw(SpaceMismatch("last MPO tensor physical space does not match PEPO site"))
+    return nothing
+end
+
+"""
+Check that the physical legs of a middle MPO tensor match the PEPO site's physical space.
+"""
+function _check_pepo_middle_physicalspace(A, op)
+    physicalspace(A) == space(op, 2) == space(op, 3)' ||
+        throw(SpaceMismatch("middle MPO tensor physical space does not match PEPO site"))
+    return nothing
+end
+
+"""
+Convert a symbolic cardinal path direction to the corresponding PEPO virtual-leg index.
+"""
+function _mpo_path_direction(direction::Symbol)
+    direction === :north && return NORTH
+    direction === :east && return EAST
+    direction === :south && return SOUTH
+    direction === :west && return WEST
+    throw(ArgumentError("invalid MPO path direction: $direction"))
+end
+
+"""
+Return the tensor-expression label for the PEPO virtual leg in a cardinal direction.
+"""
+function _mpo_path_virtual_label(direction::Symbol)
+    return (:N, :E, :S, :W)[_mpo_path_direction(direction)]
+end
+
+"""
+Build the `@tensor` labels from `(direction, suffix)` pairs
+used to fuse MPO virtual strings.
+
+- Each direction is `:north`, `:east`, `:south`, or `:west`.
+- Suffix `:l` marks an incoming MPO bond and `:r` an outgoing one.
+
+Examples:
+
+- `((:east, :r),)` produces `[W S; N Er]`.
+- `((:west, :l), (:north, :r))` produces `[Wl S; Nr E]`.
+"""
+function _mpo_path_result_expr(directions)
+    labels = [:N, :E, :S, :W]
+    for (direction, suffix) in directions
+        index = _mpo_path_direction(direction)
+        labels[index] = Symbol(labels[index], suffix)
+    end
+    return tensorexpr(:t, (labels[WEST], labels[SOUTH]), (labels[NORTH], labels[EAST]))
+end
+
+"""
+Act the first tensor `op` of an OBC-MPO on PEPO tensor `A` and fuse the
+outgoing MPO string with the virtual space of `A` along `direction`.
+"""
+@generated function mpo_path_first(A::PEPOTensor, op, ::Val{direction}) where {direction}
+    direction_index = _mpo_path_direction(direction)
+    virtual_label = _mpo_path_virtual_label(direction)
+    fused_label = Symbol(virtual_label, :r)
+
+    result_e = _mpo_path_result_expr(((direction, :r),))
+    op_e = tensorexpr(:op, :dout, (:din, :r))
+    A_e = tensorexpr(:A′, (:din, :dout), (:N, :E, :S, :W))
+    F_e = tensorexpr(:F, fused_label, (virtual_label, :r))
+    rhs = Expr(:call, :*, op_e, A_e, F_e)
+    contraction = macroexpand(
+        @__MODULE__, :(return @tensoropt $result_e := $rhs)
+    )
+
+    return quote
+        _check_pepo_first_physicalspace(A, op)
+        A′ = twistdual(A, 2)
+        F = fuser(storagetype(A), domain(A, $direction_index)', space(op, 3))
+        $contraction
+    end
+end
+
+"""
+Act the last tensor `op` of an OBC-MPO on PEPO tensor `A` and fuse the
+incoming MPO string with the virtual space of `A` along `direction`.
+"""
+@generated function mpo_path_last(A::PEPOTensor, op, ::Val{direction}) where {direction}
+    direction_index = _mpo_path_direction(direction)
+    virtual_label = _mpo_path_virtual_label(direction)
+    fused_label = Symbol(virtual_label, :l)
+
+    result_e = _mpo_path_result_expr(((direction, :l),))
+    F_e = Expr(:call, :conj, tensorexpr(:F, fused_label, (virtual_label, :l)))
+    op_e = tensorexpr(:op, (:l, :dout), :din)
+    A_e = tensorexpr(:A′, (:din, :dout), (:N, :E, :S, :W))
+    rhs = Expr(:call, :*, F_e, op_e, A_e)
+    contraction = macroexpand(
+        @__MODULE__, :(return @tensoropt $result_e := $rhs)
+    )
+
+    return quote
+        _check_pepo_last_physicalspace(A, op)
+        A′ = twistdual(A, 2)
+        F = fuser(storagetype(A), domain(A, $direction_index), space(op, 1)')
+        $contraction
+    end
+end
+
+"""
+Act the middle tensor `op` of an MPO on PEPO tensor `A` and fuse the
+incoming and the outgoing MPO string with the virtual space of `A` along
+`directions = (incoming, outgoing)`.
+"""
+@generated function mpo_path_middle(
+        A::PEPOTensor, op, ::Val{directions}
+    ) where {directions}
+    incoming, outgoing = directions
+    incoming == outgoing &&
+        throw(ArgumentError("MPO path should enter and exit in different directions"))
+
+    incoming_index = _mpo_path_direction(incoming)
+    outgoing_index = _mpo_path_direction(outgoing)
+    incoming_label = _mpo_path_virtual_label(incoming)
+    outgoing_label = _mpo_path_virtual_label(outgoing)
+    fused_incoming_label = Symbol(incoming_label, :l)
+    fused_outgoing_label = Symbol(outgoing_label, :r)
+
+    result_e = _mpo_path_result_expr(((incoming, :l), (outgoing, :r)))
+    Fin_e = Expr(
+        :call, :conj,
+        tensorexpr(:Fin, fused_incoming_label, (incoming_label, :l)),
+    )
+    op_e = tensorexpr(:op, (:l, :dout), (:din, :r))
+    A_e = tensorexpr(:A′, (:din, :dout), (:N, :E, :S, :W))
+    Fout_e = tensorexpr(
+        :Fout, fused_outgoing_label, (outgoing_label, :r)
+    )
+    rhs = Expr(:call, :*, Fin_e, op_e, A_e, Fout_e)
+    contraction = macroexpand(
+        @__MODULE__, :(return @tensoropt $result_e := $rhs)
+    )
+
+    return quote
+        _check_pepo_middle_physicalspace(A, op)
+        A′ = twistdual(A, 2)
+        Fin = fuser(storagetype(A), domain(A, $incoming_index), space(op, 1)')
+        Fout = fuser(storagetype(A), domain(A, $outgoing_index)', space(op, 4))
+        $contraction
+    end
+end
+
+"""
+Route an MPO virtual string with `stringspace` through a PEPO tensor `A`
+along `directions = (incoming, outgoing)`.
+"""
+@generated function mpo_path_string(
+        A::PEPOTensor, stringspace::ElementarySpace, ::Val{directions}
+    ) where {directions}
+    incoming, outgoing = directions
+    incoming == outgoing &&
+        throw(ArgumentError("MPO path should enter and exit in different directions"))
+
+    incoming_index = _mpo_path_direction(incoming)
+    outgoing_index = _mpo_path_direction(outgoing)
+    incoming_label = _mpo_path_virtual_label(incoming)
+    outgoing_label = _mpo_path_virtual_label(outgoing)
+    fused_incoming_label = Symbol(incoming_label, :l)
+    fused_outgoing_label = Symbol(outgoing_label, :r)
+
+    result_e = _mpo_path_result_expr(((incoming, :l), (outgoing, :r)))
+    Fin_e = Expr(
+        :call, :conj,
+        tensorexpr(:Fin, fused_incoming_label, (incoming_label, :l)),
+    )
+    O_e = tensorexpr(:O, (:W, :S), (:N, :E))
+    I_e = tensorexpr(:I, :l, :r)
+    Fout_e = tensorexpr(
+        :Fout, fused_outgoing_label, (outgoing_label, :r)
+    )
+    rhs = Expr(:call, :*, Fin_e, O_e, I_e, Fout_e)
+    contraction = macroexpand(
+        @__MODULE__, :(return @tensoropt $result_e := $rhs)
+    )
+
+    return quote
+        O = trace_physicalspaces(A)
+        I = id(storagetype(A), stringspace)
+        Fin = fuser(storagetype(A), domain(A, $incoming_index), stringspace')
+        Fout = fuser(storagetype(A), domain(A, $outgoing_index)', stringspace')
+        $contraction
+    end
+end