c------------------------------------------------------------------------
c only update bond-length r(i,i+1) and ddx which will be used to update
c forces and 
c setup common/ddx/d_x(maxa,3),r(i,i+1)
c for linear dna, r(n,1) have no use, but it's ok to update it as well.
c also update u(maxa,3) (useful for force evaluation)

	subroutine update_bond(n)
	
	 implicit double precision(a-h,o-z)
         parameter(maxa=500)
         integer n
         double precision l,l2
        common/matrix/x(maxa,3),bangle(maxa)
        common/matrix2/ r(maxa,maxa)
        common/ddx/d_x(maxa,3)
        common/bf_coords/f(maxa,3),v(maxa,3),u(maxa,3)

        do i=1,n-1
	  l2=0.0
	  ii=i+1
	  do j=1,3
	   d_x(i,j)=x(ii,j)-x(i,j)
	   l2=l2+d_x(i,j)**2
	  enddo
	  r(i,ii)=dsqrt(l2)
	  do j=1,3
	     u(i,j)=d_x(i,j)/r(i,ii)
	  enddo
	enddo

	  l2=0.0
	  do j=1,3
	   d_x(n,j)=x(1,j)-x(n,j)
	   l2=l2+d_x(n,j)**2
	  enddo
	  r(n,1)=dsqrt(l2)
	  do j=1,3
	     u(n,j)=d_x(n,j)/r(n,1)
	  enddo
            
	 return
	 end