c
c
c     ###################################################
c     ##  COPYRIGHT (C)  1990  by  Jay William Ponder  ##
c     ##              All Rights Reserved              ##
c     ###################################################
c
c     ###########################################################
c     ##                                                       ##
c     ##  subroutine gradrot  --  energy and torsional derivs  ##
c     ##                                                       ##
c     ###########################################################
c
c
c     "gradrot" calls subroutines to calculate the potential
c     energy and its torsional first derivatives
c
c
      subroutine gradrot (energy,derivs)
      implicit none
      include 'sizes.i'
      include 'atoms.i'
      include 'deriv.i'
      include 'domega.i'
      include 'omega.i'
      include 'potent.i'
      include 'rotate.i'
      integer i,j,k
      integer base,partner
      real*8 energy,norm
      real*8 xatom,yatom,zatom
      real*8 xdist,ydist,zdist
      real*8 xterm,yterm,zterm
      real*8 g(3,maxatm)
      real*8 derivs(maxrot)
c
c
c     zero out individual components of torsional derivatives
c
      do i = 1, nomega
         teb(i) = 0.0d0
         tea(i) = 0.0d0
         teba(i) = 0.0d0
         teub(i) = 0.0d0
         teaa(i) = 0.0d0
         teopb(i) = 0.0d0
         teopd(i) = 0.0d0
         teid(i) = 0.0d0
         teit(i) = 0.0d0
         tet(i) = 0.0d0
         tept(i) = 0.0d0
         tebt(i) = 0.0d0
         tett(i) = 0.0d0
         tev(i) = 0.0d0
         tec(i) = 0.0d0
         tecd(i) = 0.0d0
         ted(i) = 0.0d0
         tem(i) = 0.0d0
         tep(i) = 0.0d0
         ter(i) = 0.0d0
         tes(i) = 0.0d0
         telf(i) = 0.0d0
         teg(i) = 0.0d0
         tex(i) = 0.0d0
      end do
c
c     calculate the energy and Cartesian first derivatives
c
      call gradient (energy,g)
c
c     transform Cartesian derivatives to torsional space
c
      do i = 1, nomega
         base = iomega(1,i)
         partner = iomega(2,i)
         call rotlist (base,partner)
         xdist = x(base) - x(partner)
         ydist = y(base) - y(partner)
         zdist = z(base) - z(partner)
         norm = sqrt(xdist**2 + ydist**2 + zdist**2)
         xdist = xdist / norm
         ydist = ydist / norm
         zdist = zdist / norm
         do j = 1, nrot
            k = rot(j)
            xatom = x(k) - x(base)
            yatom = y(k) - y(base)
            zatom = z(k) - z(base)
            xterm = ydist*zatom - zdist*yatom
            yterm = zdist*xatom - xdist*zatom
            zterm = xdist*yatom - ydist*xatom
            teb(i) = teb(i) + deb(1,k)*xterm + deb(2,k)*yterm
     &                              + deb(3,k)*zterm
            tea(i) = tea(i) + dea(1,k)*xterm + dea(2,k)*yterm
     &                              + dea(3,k)*zterm
            teba(i) = teba(i) + deba(1,k)*xterm + deba(2,k)*yterm
     &                              + deba(3,k)*zterm
            teub(i) = teub(i) + deub(1,k)*xterm + deub(2,k)*yterm
     &                              + deub(3,k)*zterm
            teaa(i) = teaa(i) + deaa(1,k)*xterm + deaa(2,k)*yterm
     &                              + deaa(3,k)*zterm
            teopb(i) = teopb(i) + deopb(1,k)*xterm + deopb(2,k)*yterm
     &                              + deopb(3,k)*zterm
            teopd(i) = teopd(i) + deopd(1,k)*xterm + deopd(2,k)*yterm
     &                              + deopd(3,k)*zterm
            teid(i) = teid(i) + deid(1,k)*xterm + deid(2,k)*yterm
     &                              + deid(3,k)*zterm
            teit(i) = teit(i) + deit(1,k)*xterm + deit(2,k)*yterm
     &                              + deit(3,k)*zterm
            tet(i) = tet(i) + det(1,k)*xterm + det(2,k)*yterm
     &                              + det(3,k)*zterm
            tept(i) = tept(i) + dept(1,k)*xterm + dept(2,k)*yterm
     &                              + dept(3,k)*zterm
            tebt(i) = tebt(i) + debt(1,k)*xterm + debt(2,k)*yterm
     &                              + debt(3,k)*zterm
            tett(i) = tett(i) + dett(1,k)*xterm + dett(2,k)*yterm
     &                              + dett(3,k)*zterm
            tev(i) = tev(i) + dev(1,k)*xterm + dev(2,k)*yterm
     &                              + dev(3,k)*zterm
            tec(i) = tec(i) + dec(1,k)*xterm + dec(2,k)*yterm
     &                              + dec(3,k)*zterm
            tecd(i) = tecd(i) + decd(1,k)*xterm + decd(2,k)*yterm
     &                              + decd(3,k)*zterm
            ted(i) = ted(i) + ded(1,k)*xterm + ded(2,k)*yterm
     &                              + ded(3,k)*zterm
            tem(i) = tem(i) + dem(1,k)*xterm + dem(2,k)*yterm
     &                              + dem(3,k)*zterm
            tep(i) = tep(i) + dep(1,k)*xterm + dep(2,k)*yterm
     &                              + dep(3,k)*zterm
            ter(i) = ter(i) + der(1,k)*xterm + der(2,k)*yterm
     &                              + der(3,k)*zterm
            tes(i) = tes(i) + des(1,k)*xterm + des(2,k)*yterm
     &                              + des(3,k)*zterm
            telf(i) = telf(i) + delf(1,k)*xterm + delf(2,k)*yterm
     &                              + delf(3,k)*zterm
            teg(i) = teg(i) + deg(1,k)*xterm + deg(2,k)*yterm
     &                              + deg(3,k)*zterm
            tex(i) = tex(i) + dex(1,k)*xterm + dex(2,k)*yterm
     &                              + dex(3,k)*zterm
         end do
      end do
c
c     sum up to give the total torsional first derivatives
c
      do i = 1, nomega
         tesum(i) = teb(i) + tea(i) + teba(i) + teub(i) + teaa(i)
     &                 + teopb(i) + teopd(i) + teid(i) + teit(i)
     &                 + tet(i) + tept(i) + tebt(i) + tett(i) + tev(i)
     &                 + tec(i) + tecd(i) + ted(i) + tem(i) + tep(i)
     &                 + ter(i) + tes(i) + telf(i) + teg(i) + tex(i)
         derivs(i) = tesum(i)
      end do
      return
      end
