Fix CELL_OPT ignoring CONSTRAINT for convergence criterion

The internal pressure is calculated including dimensions fixed
with the CONSTRAINT keyword. This can result in CELL_OPT
calculations that never converge because the direction with
remaining stress is not allowed to change. With this patch
only directions that are not constrained contribute to the
internal pressure.

src/motion/cell_opt_utils.F

@@ -272,7 +272,7 @@ SUBROUTINE get_dg_dh(gradient, av_ptens, pres_ext, cell, mtrx, keep_angles, &
 
       CHARACTER(LEN=*), PARAMETER :: routineN = 'get_dg_dh', routineP = moduleN//':'//routineN
 
-      INTEGER                                            :: i, my_constraint_id
+      INTEGER                                            :: my_constraint_id
       LOGICAL                                            :: my_keep_angles, my_keep_symmetry
       REAL(KIND=dp), DIMENSION(3, 3)                     :: correction, pten_hinv_old, ptens
 
@@ -292,10 +292,22 @@ SUBROUTINE get_dg_dh(gradient, av_ptens, pres_ext, cell, mtrx, keep_angles, &
       ptens = av_ptens
 
       ! Evaluating the internal pressure
-      pres_int = 0.0_dp
-      DO i = 1, 3
-         pres_int = pres_int + ptens(i, i)
-      ENDDO
+      SELECT CASE (my_constraint_id)
+      CASE (fix_x)
+         pres_int = ptens(2, 2) + ptens(3, 3)
+      CASE (fix_y)
+         pres_int = ptens(1, 1) + ptens(3, 3)
+      CASE (fix_z)
+         pres_int = ptens(1, 1) + ptens(2, 2)
+      CASE (fix_xy)
+         pres_int = ptens(3, 3)
+      CASE (fix_xz)
+         pres_int = ptens(2, 2)
+      CASE (fix_yz)
+         pres_int = ptens(1, 1)
+      CASE (fix_none)
+         pres_int = ptens(1, 1) + ptens(2, 2) + ptens(3, 3)
+      END SELECT
       pres_int = pres_int/3.0_dp
 
       ptens(1, 1) = av_ptens(1, 1) - pres_ext