InfiniteMPO open_boundary_conditions (#251)

* Add OBC for InfiniteMPO

* Bump v0.12.6

Author: lkdvos

Project.toml

@@ -1,7 +1,7 @@
 name = "MPSKit"
 uuid = "bb1c41ca-d63c-52ed-829e-0820dda26502"
 authors = "Lukas Devos, Maarten Van Damme and contributors"
-version = "0.12.5"
+version = "0.12.6"
 
 [deps]
 Accessors = "7d9f7c33-5ae7-4f3b-8dc6-eff91059b697"

src/algorithms/toolbox.jl

@@ -415,6 +415,27 @@ function periodic_boundary_conditions(H::InfiniteMPOHamiltonian, L=length(H))
     return remove_orphans!(FiniteMPOHamiltonian(output))
 end
 
+"""
+    open_boundary_conditions(mpo::InfiniteMPO, L::Int) -> FiniteMPO
+
+Convert an infinite MPO into a finite MPO of length `L`, by applying open boundary conditions.
+"""
+function open_boundary_conditions(mpo::InfiniteMPO{O},
+                                  L=length(mpo)) where {O<:SparseBlockTensorMap}
+    mod(L, length(mpo)) == 0 ||
+        throw(ArgumentError("length $L is not a multiple of the infinite unitcell"))
+
+    # Make a FiniteMPO, filling it up with the tensors of H
+    # Only keep top row of the first and last column of the last MPO tensor
+
+    # allocate output
+    output = Vector(repeat(copy(parent(mpo)), L ÷ length(mpo)))
+    output[1] = output[1][1, :, :, :]
+    output[end] = output[end][:, :, :, 1]
+
+    return FiniteMPO(output)
+end
+
 """
     open_boundary_conditions(mpo::InfiniteMPOHamiltonian, L::Int) -> FiniteMPOHamiltonian