c
c
c     ###################################################
c     ##  COPYRIGHT (C)  1990  by  Jay William Ponder  ##
c     ##              All Rights Reserved              ##
c     ###################################################
c
c     ##################################################################
c     ##                                                              ##
c     ##  subroutine gradient  --  find energy & gradient components  ##
c     ##                                                              ##
c     ##################################################################
c
c
c     "gradient" calls subroutines to calculate the potential energy
c     and first derivatives with respect to Cartesian coordinates
c
c
      subroutine gradient (energy,derivs)
      implicit none
      include 'sizes.i'
      include 'atoms.i'
      include 'bound.i'
      include 'deriv.i'
      include 'energi.i'
      include 'inter.i'
      include 'potent.i'
      include 'rigid.i'
      include 'vdwpot.i'
      include 'virial.i'
      integer i,j
      real*8 energy,cutoff
      real*8 derivs(3,maxatm)
c
c
c     zero out each of the potential energy components
c
      eb = 0.0d0
      ea = 0.0d0
      eba = 0.0d0
      eub = 0.0d0
      eaa = 0.0d0
      eopb = 0.0d0
      eopd = 0.0d0
      eid = 0.0d0
      eit = 0.0d0
      et = 0.0d0
      ept = 0.0d0
      ebt = 0.0d0
      ett = 0.0d0
      ev = 0.0d0
      ec = 0.0d0
      ecd = 0.0d0
      ed = 0.0d0
      em = 0.0d0
      ep = 0.0d0
      er = 0.0d0
      es = 0.0d0
      elf = 0.0d0
      eg = 0.0d0
      ex = 0.0d0
c
c     zero out each of the first derivative components
c
      do i = 1, n
         do j = 1, 3
            deb(j,i) = 0.0d0
            dea(j,i) = 0.0d0
            deba(j,i) = 0.0d0
            deub(j,i) = 0.0d0
            deaa(j,i) = 0.0d0
            deopb(j,i) = 0.0d0
            deopd(j,i) = 0.0d0
            deid(j,i) = 0.0d0
            deit(j,i) = 0.0d0
            det(j,i) = 0.0d0
            dept(j,i) = 0.0d0
            debt(j,i) = 0.0d0
            dett(j,i) = 0.0d0
            dev(j,i) = 0.0d0
            dec(j,i) = 0.0d0
            decd(j,i) = 0.0d0
            ded(j,i) = 0.0d0
            dem(j,i) = 0.0d0
            dep(j,i) = 0.0d0
            der(j,i) = 0.0d0
            des(j,i) = 0.0d0
            delf(j,i) = 0.0d0
            deg(j,i) = 0.0d0
            dex(j,i) = 0.0d0
         end do
      end do
c
c     zero out the virial and the intermolecular energy
c
      do i = 1, 3
         do j = 1, 3
            vir(j,i) = 0.0d0
         end do
      end do
      einter = 0.0d0
c
c     maintain any periodic boundary conditions
c
      if (use_bounds .and. .not.use_rigid)  call bounds
c
c     remove any previous use of the replicates method
c
      cutoff = 0.0d0
      if (use_image)  call replica (cutoff)
c
c     alter bond and torsion constants for pisystem
c
      if (use_orbit)  call piscf
c
c     call the local geometry energy and gradient routines
c
      if (use_bond)  call ebond1
      if (use_angle)  call eangle1
      if (use_strbnd)  call estrbnd1
      if (use_urey)  call eurey1
      if (use_angang)  call eangang1
      if (use_opbend)  call eopbend1
      if (use_opdist)  call eopdist1
      if (use_improp)  call eimprop1
      if (use_imptor)  call eimptor1
      if (use_tors)  call etors1
      if (use_pitors)  call epitors1
      if (use_strtor)  call estrtor1
      if (use_tortor)  call etortor1
c
c     call the van der Waals energy and gradient routines
c
      if (use_vdw) then
         if (vdwtyp .eq. 'LENNARD-JONES')  call elj1
         if (vdwtyp .eq. 'BUCKINGHAM')  call ebuck1
         if (vdwtyp .eq. 'MM3-HBOND')  call emm3hb1
         if (vdwtyp .eq. 'BUFFERED-14-7')  call ehal1
         if (vdwtyp .eq. 'GAUSSIAN')  call egauss1
      end if
c
c     call the electrostatic energy and gradient routines
c
      if (use_charge)  call echarge1
      if (use_chgdpl)  call echgdpl1
      if (use_dipole)  call edipole1
      if (use_mpole .or. use_polar)  call empole1
      if (use_rxnfld)  call erxnfld1
c
c     call any miscellaneous energy and gradient routines
c
      if (use_solv)  call esolv1
      if (use_metal)  call emetal1
      if (use_geom)  call egeom1
      if (use_extra)  call extra1
c
c     sum up to get the total energy and first derivatives
c
      energy = eb + ea + eba + eub + eaa + eopb + eopd + eid + eit
     &            + et + ept + ebt + ett + ev + ec + ecd + ed + em
     &            + ep + er + es + elf + eg + ex
      do i = 1, n
         do j = 1, 3
            desum(j,i) = deb(j,i) + dea(j,i) + deba(j,i)
     &                      + deub(j,i) + deaa(j,i) + deopb(j,i)
     &                      + deopd(j,i) + deid(j,i) + deit(j,i)
     &                      + det(j,i) + dept(j,i) + debt(j,i)
     &                      + dett(j,i) + dev(j,i) + dec(j,i)
     &                      + decd(j,i) + ded(j,i) + dem(j,i)
     &                      + dep(j,i) + der(j,i) + des(j,i)
     &                      + delf(j,i) + deg(j,i) + dex(j,i)
            derivs(j,i) = desum(j,i)
         end do
      end do
      return
      end
