From e5cff91a5d41824397f96316a56ac5d60c20abe8 Mon Sep 17 00:00:00 2001
From: Paolo Giannozzi <p.giannozzi@gmail.com>
Date: Mon, 3 Aug 2020 19:20:31 +0200
Subject: [PATCH] Incorrect detection of inversion symmetry in phonons fixed
 (Phil Wang, JHU). I think a better fix would be to move the detection of the
 inversion symmetry into function "copy_sym" (not making assumptions on which
 operation is the inversion, and transforming the function into a subroutine)
 but this must be done with care and far from a release. The current fix seem
 to be 100% safe.

---
 LR_Modules/set_small_group_of_q.f90 | 15 ++++++++++-----
 TDDFPT/src/lr_smallgq.f90           | 13 +++++++++----
 2 files changed, 19 insertions(+), 9 deletions(-)

diff --git a/LR_Modules/set_small_group_of_q.f90 b/LR_Modules/set_small_group_of_q.f90
index 5f5bc2068..0d9196924 100644
--- a/LR_Modules/set_small_group_of_q.f90
+++ b/LR_Modules/set_small_group_of_q.f90
@@ -30,7 +30,7 @@ SUBROUTINE set_small_group_of_q(nsymq, invsymq, minus_q)
   LOGICAL, INTENT(INOUT) :: minus_q, invsymq
   !
   REAL(DP), ALLOCATABLE :: rtau(:,:,:)
-
+  INTEGER :: isym
   LOGICAL :: sym(48)
   !
   sym(1:nsym)=.true.
@@ -55,11 +55,16 @@ SUBROUTINE set_small_group_of_q(nsymq, invsymq, minus_q)
   !
   CALL inverse_s ( )
   !
-  ! check if inversion (I) is a symmetry. If so, there should be nsymq/2
-  ! symmetries without inversion, followed by nsymq/2 with inversion
-  ! Since identity is always s(:,:,1), inversion should be s(:,:,1+nsymq/2)
+  ! Check if inversion (I) is a symmetry
+  ! Note that the first symmetry operation is always the identity (E)
   !
-  invsymq = ALL ( s(:,:,nsymq/2+1) == -s(:,:,1) )
+  invsymq =.FALSE.
+  DO isym = 1, nsymq
+     IF ( ALL ( s(:,:,isym) == -s(:,:,1) ) ) THEN
+        invsymq = .TRUE.
+        EXIT
+     END IF
+  END DO
   !
   !  Since the order of the s matrices is changed we need to recalculate:
   !
diff --git a/TDDFPT/src/lr_smallgq.f90 b/TDDFPT/src/lr_smallgq.f90
index 70c0c220a..30b38740d 100644
--- a/TDDFPT/src/lr_smallgq.f90
+++ b/TDDFPT/src/lr_smallgq.f90
@@ -139,12 +139,17 @@ SUBROUTINE lr_smallgq (xq)
      !
   ENDDO
   !
-  ! Check if inversion (I) is a symmetry. If so, there should be nsymq/2
-  ! symmetries without inversion, followed by nsymq/2 with inversion
-  ! Since identity is always s(:,:,1), inversion should be s(:,:,1+nsymq/2)
+  ! Check if inversion (I) is a symmetry
+  ! Note that the first symmetry operation is always the identity (E)
   ! IT: it seems that invsymq is useless (used nowhere)...
   !
-  invsymq = ALL ( s(:,:,nsymq/2+1) == -s(:,:,1) )
+  invsymq =.FALSE.
+  DO isym = 1, nsymq
+     IF ( ALL ( s(:,:,isym) == -s(:,:,1) ) ) THEN
+        invsymq = .TRUE.
+        EXIT
+     END IF
+  END DO
   !
   ! The order of the s matrices has changed.
   ! Transform symmetry matrices s from crystal to cartesian axes.