Add finite_mps for slicing Infinite MPS to finite MPS function, example of computing the correlation matrix

Author: nbaldelli

examples/vumps/vumps_hubbard_extended.jl

@@ -120,19 +120,26 @@ Sz_finite = ITensors.expect(ψfinite, "Sz")[nfinite:(nfinite + 1)]
 
 energy_exact = reference(model, Observable("energy"); U=model_params.U / model_params.t)
 
+corr_infinite = correlation_matrix(finite_mps(ψ, 1:10), "Cdagup", "Cup"; sites=2:11)
+corr_finite = correlation_matrix(
+  ψfinite, "Cdagup", "Cup"; sites=Int(Nfinite / 2):Int(Nfinite / 2 + 9)
+)
+
 println("\nResults from VUMPS")
 @show energy_infinite
 @show energy_exact
 @show Nup
 @show Ndn
 @show Nup .+ Ndn
 @show Sz
+@show corr_infinite
 
 println("\nResults from DMRG")
 @show energy_finite
 @show Nup_finite
 @show Ndn_finite
 @show Nup_finite .+ Ndn_finite
 @show Sz_finite
+@show corr_finite
 
 nothing

src/ITensorInfiniteMPS.jl

@@ -81,6 +81,7 @@ export Cell,
   translator,
   tdvp,
   vumps,
+  finite_mps,
   ⊕,
   ⊗,
   ×

src/infinitecanonicalmps.jl

@@ -148,6 +148,23 @@ function InfMPS(s::CelledVector, f::Function)
   return ψ = InfiniteCanonicalMPS(ψL, ψC, ψR)
 end
 
+function finite_mps(ψ::InfiniteCanonicalMPS, range::AbstractRange)
+  @assert isone(step(range))
+  N = length(range)
+  ψ_finite = ψ.AL[range]
+  ψ_finite[N] *= ψ.C[last(range)]
+  l0 = linkind(ψ.AL, first(range) - 1 => first(range))
+  l̃0 = sim(l0)
+  lN = linkind(ψ.AR, last(range) => last(range) + 1)
+  l̃N = sim(lN)
+  δl0 = δ(dag(l̃0), l0)
+  δlN = δ(dag(l̃N), lN)
+  ψ_finite[1] *= δl0
+  ψ_finite[N] *= dag(δlN)
+  ψ_finite = MPS([dag(δl0); [ψ_finiteᵢ for ψ_finiteᵢ in ψ_finite]; δlN])
+  set_ortho_lims!(ψ_finite, (N + 1):(N + 1))
+  return ψ_finite
+end
 function ITensors.expect(ψ::InfiniteCanonicalMPS, o::String, n::Int)
   s = siteinds(only, ψ.AL)
   O = op(o, s[n])