fix: change_index now correctly zeros DoubleIndexedVariable

Changelog.md

@@ -5,6 +5,12 @@ All notable changes to [`SecondQuantizedAlgebra.jl`](https://github.com/qojulia/
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [v0.6.4]
+
+### Fixed
+
+- `change_index` now correctly zeros `DoubleIndexedVariable` nodes tagged `identical=false` even when they appear nested inside a larger product or sum. Previously `_check_not_identical` only examined the root of the substituted expression, so a `J(i,j)` factor inside `J(i,j) * x` would survive a `j → i` substitution intact instead of collapsing to zero. The fix walks the full expression tree, collects every `NotIdentical`-tagged node whose two index arguments became equal, and replaces them all via a single `Symbolics.substitute` pass.
+
 ## [v0.6.3]
 
 ### Changed
@@ -160,4 +166,5 @@ These names keep their meaning across the migration. Code that only uses them sh
 [v0.6.1]: https://github.com/qojulia/SecondQuantizedAlgebra.jl/releases/tag/v0.6.1
 [v0.6.2]: https://github.com/qojulia/SecondQuantizedAlgebra.jl/releases/tag/v0.6.2
 [v0.6.3]: https://github.com/qojulia/SecondQuantizedAlgebra.jl/releases/tag/v0.6.3
+[v0.6.4]: https://github.com/qojulia/SecondQuantizedAlgebra.jl/releases/tag/v0.6.4
 [#156]: https://github.com/qojulia/SecondQuantizedAlgebra.jl/issues/156

Project.toml

@@ -1,6 +1,6 @@
 name = "SecondQuantizedAlgebra"
 uuid = "f7aa4685-e143-4cb6-a7f3-073579757907"
-version = "0.6.3"
+version = "0.6.4"
 
 [deps]
 Combinatorics = "861a8166-3701-5b0c-9a16-15d98fcdc6aa"

src/expressions/index.jl

@@ -256,18 +256,34 @@ function create_index_arrays(indices::Vector{Index}, ranges::Vector{<:AbstractRa
     return vec(collect(Iterators.product(ranges...)))
 end
 
-"""Check if a substituted BasicSymbolic node with NotIdentical metadata has equal args → 0."""
+"""
+Zero every `NotIdentical`-tagged node anywhere in a substituted expression whose
+two indices became equal. Nested occurrences matter: a `DoubleIndexedVariable`
+with `identical=false` is almost always a factor in a larger product or sum, and
+the surrounding `*`/`+` only collapses once that factor is replaced by zero.
+"""
 function _check_not_identical(result::SymbolicUtils.BasicSymbolic)
-    if SymbolicUtils.iscall(result) &&
-            SymbolicUtils.hasmetadata(result, NotIdentical) &&
-            length(SymbolicUtils.arguments(result)) == 2
-        a1, a2 = SymbolicUtils.arguments(result)
-        isequal(a1, a2) && return Num(0)
-    end
-    return Num(result)
+    zeros = _collect_identical_zeros!(Dict{SymbolicUtils.BasicSymbolic, Int}(), result)
+    isempty(zeros) && return Num(result)
+    return Num(Symbolics.substitute(result, zeros))
 end
 _check_not_identical(result::Number) = Num(result)
 
+"""Accumulate into `acc` every `NotIdentical` node in `x` with two equal args, mapped to 0."""
+function _collect_identical_zeros!(acc, x)
+    (x isa SymbolicUtils.BasicSymbolic && SymbolicUtils.iscall(x)) || return acc
+    if SymbolicUtils.hasmetadata(x, NotIdentical) &&
+            length(SymbolicUtils.arguments(x)) == 2
+        a1, a2 = SymbolicUtils.arguments(x)
+        # The args are indices, so a matched node holds no nested NotIdentical node.
+        isequal(a1, a2) && (acc[x] = 0; return acc)
+    end
+    for a in SymbolicUtils.arguments(x)
+        _collect_identical_zeros!(acc, a)
+    end
+    return acc
+end
+
 function _depends_on_index_term(c::CNum, ops::Vector{QSym}, idx::Index)
     for op in ops
         op.index == idx && return true

test/expressions/indexing_test.jl

@@ -378,6 +378,20 @@ import SecondQuantizedAlgebra: simplify, QAdd, QSym, CNum, _to_cnum, NO_INDEX,
         @test isequal(real(prefactor(mj)), gj)
     end
 
+    @testset "change_index: nested identical=false collapses to 0" begin
+        i = Index(hf, :i, 10, hf)
+        j = Index(hf, :j, 10, hf)
+        @variables x
+        Jij = DoubleIndexedVariable(:J, i, j; identical = false)  # 0 when i==j
+
+        # Bare node already worked; the regression is nested occurrences.
+        @test isequal(change_index(Jij, j, i), Symbolics.Num(0))
+        # Under a product: J(i,j)*x with j→i ⇒ J(i,i)=0 ⇒ whole product 0
+        @test isequal(change_index(Jij * x, j, i), Symbolics.Num(0))
+        # Under a sum: (J(i,j) + x) with j→i ⇒ x
+        @test isequal(change_index(Jij + x, j, i), x)
+    end
+
     @testset "change_index — QAdd" begin
         i = Index(hf, :i, 10, hf)
         j = Index(hf, :j, 10, hf)