diff --git a/src/constrain_pot.f90 b/src/constrain_pot.f90
index 1ce0cddb8..f45f7e349 100644
--- a/src/constrain_pot.f90
+++ b/src/constrain_pot.f90
@@ -173,7 +173,7 @@ subroutine constrain_dist(fix,n,at,xyz,g,e)
    real(wp),intent(inout) :: e
    real(wp),intent(inout) :: g(3,n)
 
-   integer i,j,k,l,m,mm
+   integer i,j,k,l,m,mm,idx
    real(wp)rij(3),dum,r0,r,vp(3),ra(3),rb(3)
    real(wp)va(3),vb(3),vc(3),vab(3),vcb(3),deda(3),dedc(3),dedb(3)
    real(wp)dda(3),ddb(3),ddc(3),ddd(3)
@@ -198,8 +198,10 @@ subroutine constrain_dist(fix,n,at,xyz,g,e)
       e=e+fix%fc*dum
       ff=fix%fc*fix%expo(m)*dum2
       dum=ff/r
-      g(:,j)=g(:,j)+dum*rij
-      g(:,i)=g(:,i)-dum*rij
+      do idx = 1, 3
+        g(idx,j)=g(idx,j)+dum*rij(idx)
+        g(idx,i)=g(idx,i)-dum*rij(idx)
+      end do
 
    enddo