restore G2MomCorrelator for SingleComponentFockAddress

docs/src/hamiltonians.md

@@ -79,6 +79,7 @@ AbstractHamiltonian{T} <: AbstractOperator{T} <: AbstractObservable{T}
 ParticleNumberOperator
 G2RealCorrelator
 G2RealSpace
+G2MomCorrelator
 SuperfluidCorrelator
 StringCorrelator
 DensityMatrixDiagonal

src/Hamiltonians/G2MomCorrelator.jl

@@ -0,0 +1,77 @@
+
+import Rimu.Hamiltonians: num_offdiagonals, diagonal_element, get_offdiagonal
+
+"""
+    G2MomCorrelator(d::Int) <: AbstractOperator{ComplexF64}
+
+Two-body correlation operator representing the density-density
+correlation at distance `d`. It returns a `Complex` value.
+
+Correlation within a single component:
+```math
+\\hat{G}^{(2)}(d) = \\frac{1}{M}\\sum_{spqr=1}^M e^{-id(p-q)2π/M} a^†_{s} a^†_{p}  a_q a_r δ_{s+p,q+r}
+```
+
+The diagonal element, where `(p-q)=0`, is
+```math
+\\frac{1}{M}\\sum_{k,p=1}^M a^†_{k} b^†_{p}  b_p a_k .
+```
+
+# Arguments
+- `d::Integer`: the distance between two particles.
+
+
+# See also
+
+* [`Rimu.G2RealCorrelator`](@ref)
+* [`Rimu.G2RealSpace`](@ref)
+* [`Rimu.AbstractOperator`](@ref)
+* [`Rimu.AllOverlaps`](@ref)
+"""
+struct G2MomCorrelator{C} <: AbstractOperator{ComplexF64}
+    d::Int
+end
+# The type parameter `C` is not used here, but may be used for future extensions.
+# It is kept here for consistency with `RimuLegacyHamiltonians.jl`.
+function G2MomCorrelator(d::Int)
+    return G2MomCorrelator{3}(d)
+end
+
+function Rimu.Interfaces.allows_address_type(g2m::G2MomCorrelator, ::Type{A}) where {A}
+    return num_modes(A) > g2m.d && A <: SingleComponentFockAddress
+end
+
+function Base.show(io::IO, g::G2MomCorrelator{3})
+    # 3 is the default value for the type parameter
+    print(io, "G2MomCorrelator($(g.d))")
+end
+
+function num_offdiagonals(g::G2MomCorrelator, addr::SingleComponentFockAddress)
+    m = num_modes(addr)
+    singlies, doublies = num_singly_doubly_occupied_sites(addr)
+    return singlies * (singlies - 1) * (m - 2) + doublies * (m - 1)
+end
+
+function diagonal_element(g::G2MomCorrelator, addr::SingleComponentFockAddress)
+    M = num_modes(addr)
+    onrep = onr(addr)
+    gd = 0
+    for p in 1:M
+        iszero(onrep[p]) && continue
+        for k in 1:M
+            gd += onrep[k] * onrep[p] # a†_p a_p a†_k a_k
+        end
+    end
+    return ComplexF64(gd / M)
+end
+
+function get_offdiagonal(
+    g::G2MomCorrelator,
+    addr::A,
+    chosen,
+)::Tuple{A,ComplexF64} where {A<:SingleComponentFockAddress}
+    M = num_modes(addr)
+    new_add, gamma, Δp = momentum_transfer_excitation(addr, chosen, OccupiedModeMap(addr))
+    gd = exp(-im * g.d * Δp * 2π / M) * gamma
+    return new_add, ComplexF64(gd / M)
+end

src/Hamiltonians/Hamiltonians.jl

@@ -34,6 +34,7 @@ Other
 ## [Observables](#Observables)
 - [`ParticleNumberOperator`](@ref)
 - [`G2RealCorrelator`](@ref)
+- [`G2MomCorrelator`](@ref)
 - [`G2RealSpace`](@ref)
 - [`DensityMatrixDiagonal`](@ref)
 - [`SingleParticleExcitation`](@ref)
@@ -83,7 +84,7 @@ export ParticleNumberOperator
 
 export G2RealCorrelator, G2RealSpace, SuperfluidCorrelator, DensityMatrixDiagonal, Momentum
 export SingleParticleExcitation, TwoParticleExcitation, ReducedDensityMatrix
-export StringCorrelator
+export StringCorrelator, G2MomCorrelator
 
 export CubicGrid, PeriodicBoundaries, HardwallBoundaries, LadderBoundaries
 
@@ -124,6 +125,7 @@ include("Stoquastic.jl")
 
 include("Transcorrelated1D.jl")
 include("correlation_functions.jl")
+include("G2MomCorrelator.jl")
 include("DensityMatrixDiagonal.jl")
 include("reduced_density_matrix.jl")
 include("Momentum.jl")

test/Hamiltonians.jl

@@ -85,6 +85,7 @@ end
         (SingleParticleExcitation(2, 3), BoseFS(1, 2, 3, 4)),
         (TwoParticleExcitation(3, 2, 1, 4), BoseFS(1, 2, 3, 4)),
         (Momentum(1), BoseFS(1, 2, 3, 4)),
+        (G2MomCorrelator(3), BoseFS(1, 2, 0, 3, 0, 4, 0, 1))
     ]
         test_operator_interface(op, addr)
         # Check that the result of show can be pasted into the REPL