subroutine orig_frame_chain(istart)
C
C Define the origin and orientation of the coordinate system starting
C at residue istart and locate sites istart+1 and istart+2. The
C coordinates of site istart and the respective dc and dc_norm must be
C pre-defined
C
      implicit none
      integer i,j,istart,ichain
      double precision cost,sint,cosg,sing,aux
      include 'DIMENSIONS'
      include 'COMMON.CHAIN'
      include 'COMMON.IOUNITS'
      include 'COMMON.LOCAL'
      include 'COMMON.GEO'
      include 'COMMON.VAR'
      cost=dc_norm(1,istart)
      aux=dsqrt(dc_norm(2,istart)**2+dc_norm(3,istart)**2)
      cosg=dc_norm(2,istart)/aux
      sing=dc_norm(3,istart)/aux
      cost=dcos(theta(istart+2))
      sint=dsin(theta(istart+2))
      prod(1,1,istart)=cost
      prod(1,2,istart)=-sint
      prod(1,3,istart)=0.0d0
      prod(2,1,istart)=sint*cosg
      prod(2,2,istart)=cost*cosg
      prod(2,3,istart)=-sing
      prod(3,1,istart)=sint*sing
      prod(3,2,istart)=cost*sing
      prod(3,3,istart)=cosg
      aux=prod(1,1,istart)*(prod(2,2,istart)*prod(3,3,istart)
     & -prod(3,2,istart)*prod(2,3,istart))
     & -prod(1,2,istart)*(prod(2,1,istart)*prod(3,3,istart)
     & -prod(3,1,istart)*prod(2,3,istart))
     & +prod(1,3,istart)*(prod(2,1,istart)*prod(3,2,istart)
     & -prod(3,1,istart)*prod(2,2,istart))
c      write (iout,*) "orig_frame_chain prod",istart
c      do i=1,3
c        write(iout,'(i5,3f10.5)') i,(prod(i,j,istart),j=1,3)
c      enddo
c      write (iout,*) "orig_frame_chain: prod",istart," determinant",aux
      t(1,1,istart)=cost
      t(1,2,istart)=-sint 
      t(1,3,istart)= 0.0D0
      t(2,1,istart)=sint
      t(2,2,istart)=cost
      t(2,3,istart)= 0.0D0
      t(3,1,istart)= 0.0D0
      t(3,2,istart)= 0.0D0
      t(3,3,istart)= 1.0D0
      r(1,1,istart)= 1.0D0
      r(1,2,istart)= 0.0D0
      r(1,3,istart)= 0.0D0
      r(2,1,istart)= 0.0D0
      r(2,2,istart)= 1.0D0
      r(2,3,istart)= 0.0D0
      r(3,1,istart)= 0.0D0
      r(3,2,istart)= 0.0D0
      r(3,3,istart)= 1.0D0
      do i=1,3
        do j=1,3
          rt(i,j,istart)=t(i,j,istart)
        enddo
      enddo
      call matmult(prod(1,1,istart),rt(1,1,istart),prod(1,1,istart+1))
c      aux=prod(1,1,istart+1)*(prod(2,2,istart+1)*prod(3,3,istart+1)
c     & -prod(3,2,istart+1)*prod(2,3,istart+1))
c     & -prod(1,2,istart+1)*(prod(2,1,istart+1)*prod(3,3,istart+1)
c     & -prod(3,1,istart+1)*prod(2,3,istart+1))
c     & +prod(1,3,istart+1)*(prod(2,1,istart+1)*prod(3,2,istart+1)
c     & -prod(3,1,istart+1)*prod(2,2,istart+1))
c      write (iout,*) "orig_frame_chain prod",istart+1
c      do i=1,3
c        write(iout,'(i5,3f10.5)') i,(prod(i,j,istart+1),j=1,3)
c      enddo
c      write (iout,*)"orig_frame_chain: prod",istart+1," determinant",aux
      do j=1,3
        dc_norm(j,istart+1)=prod(j,1,istart+1)
        dc(j,istart+1)=vbld(istart+2)*prod(j,1,istart+1)
        c(j,istart+2)=c(j,istart+1)+dc(j,istart+1)
      enddo
      call locate_side_chain(istart+1)
      return
      end