windowhamiltonian poc

[maartenvd]

draft pr for a windowhamiltonian type, resolving some pain points from #411

It wasn't possible in the past to correctly create a windowmps environments. The window uses a finitehamiltonian, but then the boundaries require an infinite hamiltonian. This new windowhamiltonian would keep track of the "bath" hamiltonian, and the finite hamiltonian for the window.

Misses full support for the abstractmpo interface, and I think a propagator+window test would be nice to have.

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diff --git a/src/operators/windowhamiltonian.jl b/src/operators/windowhamiltonian.jl
index 7eed2c4..d39bfc8 100644
--- a/src/operators/windowhamiltonian.jl
+++ b/src/operators/windowhamiltonian.jl
@@ -8,22 +8,22 @@ Acts simalar as just a finite hamiltonian, but we 'remember' the boundary hamilt
 # todo - what is the required interface for abstractmpo?
 # support densempo windows?
 struct WindowMPOHamiltonian{O} <: AbstractMPO{O}
-    left_ham :: InfiniteMPOHamiltonian{O}
-    finite_ham :: FiniteMPOHamiltonian{O}
-    right_ham :: InfiniteMPOHamiltonian{O}
+    left_ham::InfiniteMPOHamiltonian{O}
+    finite_ham::FiniteMPOHamiltonian{O}
+    right_ham::InfiniteMPOHamiltonian{O}
 end
 
 #utility constructor
 function WindowMPOHamiltonian(ham::InfiniteMPOHamiltonian, interval::UnitRange)
-    
+
     # to make sure the interval corresponds with finite_ham, it is important that the unitcell of the left/right hamiltonians is circshifted correctly
-    left_edge = (interval.start-1) % length(ham)
-    left_ham = InfiniteMPOHamiltonian([ham[i] for i in (left_edge-length(ham)+1):left_edge])
-    right_edge = (interval.stop+1)%length(ham)
-    right_ham = InfiniteMPOHamiltonian([ham[i] for i in right_edge:(right_edge+length(ham)-1)])
+    left_edge = (interval.start - 1) % length(ham)
+    left_ham = InfiniteMPOHamiltonian([ham[i] for i in (left_edge - length(ham) + 1):left_edge])
+    right_edge = (interval.stop + 1) % length(ham)
+    right_ham = InfiniteMPOHamiltonian([ham[i] for i in right_edge:(right_edge + length(ham) - 1)])
 
-    finite_ham = FiniteMPOHamiltonian([ham[i] for i in  interval])
-    WindowMPOHamiltonian(left_ham, finite_ham, right_ham)
+    finite_ham = FiniteMPOHamiltonian([ham[i] for i in interval])
+    return WindowMPOHamiltonian(left_ham, finite_ham, right_ham)
 end
 #
 Base.parent(h::WindowMPOHamiltonian) = h.finite_ham
@@ -32,5 +32,5 @@ Base.copy(h::WindowMPOHamiltonian) = WindowMPOHamiltonian(copy(h.left_ham), copy
 
 # some basic linalg
 for fun in (:(Base.:+), :(Base.:-), :(Base.:*))
-    @eval $fun(a::WindowMPOHamiltonian,b::WindowMPOHamiltonian) = WindowMPOHamiltonian($fun(a.left_ham,b.left_ham),$fun(a.finite_ham,b.finite_ham),$fun(a.right_ham,b.right_ham))
+    @eval $fun(a::WindowMPOHamiltonian, b::WindowMPOHamiltonian) = WindowMPOHamiltonian($fun(a.left_ham, b.left_ham), $fun(a.finite_ham, b.finite_ham), $fun(a.right_ham, b.right_ham))
 end
diff --git a/test/states/windowmps.jl b/test/states/windowmps.jl
index 2106dab..ab37bba 100644
--- a/test/states/windowmps.jl
+++ b/test/states/windowmps.jl
@@ -63,7 +63,7 @@ using TensorKit: ℙ
 
     e1 = expectation_value(window, (2, 3) => O)
 
-    w_ham = WindowMPOHamiltonian(ham,1:length(window))
+    w_ham = WindowMPOHamiltonian(ham, 1:length(window))
     window, envs, _ = find_groundstate(window, w_ham, DMRG(; verbosity = 0))
 
     e2 = expectation_value(window, (2, 3) => O)