docs: drop redundant LCAO force/stress derivation document

Remove docs/nao_lcao_force_stress_derivation.md and the code-comment
references that pointed at it. The conventions it documented (DMK =
(C f C†)^T storage, e^{-ikR} inverse Fourier sign, closed-trace
protection for force/stress) are already stated inline in
density_matrix.h/.cpp and the T1/T2/T3/T8 unit tests.

Author: dyzheng

docs/nao_lcao_force_stress_derivation.md

@@ -112,7 +112,7 @@ void DensityMatrix_Tools::cal_DMR(
                     if(ik_in >= 0 && ik_in != ik) { continue; }
                     // Inverse Fourier transform: D(R) = sum_k D(k) * exp(-i*k*R)
                     // k-point weights are embedded in DMK, so there is no 1/Nk
-                    // prefactor. See Sec. 3 of nao_lcao_force_stress_derivation.md
+                    // prefactor.
                     const ModuleBase::Vector3<double> dR(R_index[0], R_index[1], R_index[2]);
                     const double arg = (dm._kvec_d[ik] * dR) * ModuleBase::TWO_PI;
                     double sinp, cosp;
@@ -268,7 +268,7 @@ void DensityMatrix_Tools::cal_DMR_td(
                     if(ik_in >= 0 && ik_in != ik) { continue; }
                     // Inverse Fourier transform: D(R) = sum_k D(k) * exp(-i*k*R)
                     // k-point weights are embedded in DMK, so there is no 1/Nk
-                    // prefactor. See Sec. 3 of nao_lcao_force_stress_derivation.md
+                    // prefactor.
                     const ModuleBase::Vector3<double> dR(R_index[0], R_index[1], R_index[2]);
                     const double arg = (dm._kvec_d[ik] * dR) * ModuleBase::TWO_PI;
                     double sinp, cosp;

source/source_estate/module_dm/density_matrix.cpp

@@ -11,7 +11,6 @@ namespace elecstate
 {
 // ---------------------------------------------------------------------------
 // Density-matrix conventions (DMK/DMR), frozen by the DM/DMK/DMR chain.
-// See docs/nao_lcao_force_stress_derivation.md for the full derivation.
 //
 // 1. DMK(μ,ν;k) = Σ_n f_nk C*_{μn}(k) C_{νn}(k) = (C f C†)^T = D_std^T.
 //    μ = row (bra) orbital, ν = column (ket) orbital. Stored as the transpose

source/source_estate/module_dm/density_matrix.h

@@ -392,7 +392,6 @@ TEST_F(DMTest, cal_DMR_blas_complex)
 // folding_HR (e^{+ikR}). This is the test that locks the Fourier sign of the
 // inverse transform: with the k-phase flipped (density_matrix.cpp: -sinp ->
 // +sinp), D~(k) = DMK(-k) = DMK(k)† and the assertions below turn red.
-// See Sec. 3 of docs/nao_lcao_force_stress_derivation.md.
 TEST_F(DMTest, T1_fourier_round_trip)
 {
     // k-grid {0, 1/4, 1/2, 3/4} along x: contains non-Gamma k-points and k and
@@ -512,7 +511,6 @@ TEST_F(DMTest, T1_fourier_round_trip)
 // ((iat1,iat2,R) and (iat2,iat1,-R)), cal_DMR must satisfy
 //   D(iat2,iat1,-R) = D(iat1,iat2,R)^T   (real DMR, nspin<4)
 //   D(iat2,iat1,-R) = D(iat1,iat2,R)^\dagger (complex DMR, nspin=4)
-// See Sec. 5 of docs/nao_lcao_force_stress_derivation.md.
 TEST_F(DMTest, T2_dmr_hermiticity)
 {
     // two k-points, both non-Gamma, k and -k distinct
@@ -634,9 +632,8 @@ TEST_F(DMTest, T2_dmr_hermiticity)
 // T8: full-direction pairing storage guard. Every (iat1,iat2,R) block of the
 // DMR HContainer built by init_DMR must have a (iat2,iat1,-R) counterpart.
 // gint_rho and the force/stress paths rely on this pairing (closed-trace
-// protection, see Sec. 5 of docs/nao_lcao_force_stress_derivation.md).
-// A future half-set
-// storage optimization will turn this test red.
+// protection). A future half-set storage optimization will turn this test
+// red.
 TEST_F(DMTest, T8_full_direction_pairing_guard)
 {
     // multi-k DM (TK=complex) keeps the R structure of init_DMR(Record_adj);

source/source_estate/module_dm/test/test_cal_dm_r.cpp

@@ -24,7 +24,6 @@
  * This holds even when O(R) is NOT symmetric within the same R (the key case
  * for overlap/kinetic derivative operators): the protection comes from the
  * closed trace + per-k Hermiticity of O(k), not from same-R symmetry.
- * See Sec. 5 of docs/nao_lcao_force_stress_derivation.md.
  ************************************************/
 
 namespace