c
c
c     ###################################################
c     ##  COPYRIGHT (C)  1990  by  Jay William Ponder  ##
c     ##              All Rights Reserved              ##
c     ###################################################
c
c     ##############################################################
c     ##                                                          ##
c     ##  subroutine edipole  --  dipole-dipole potential energy  ##
c     ##                                                          ##
c     ##############################################################
c
c
c     "edipole" calculates the dipole-dipole interaction energy
c
c
      subroutine edipole
      implicit none
      include 'sizes.i'
      include 'atoms.i'
      include 'bound.i'
      include 'cell.i'
      include 'chgpot.i'
      include 'dipole.i'
      include 'energi.i'
      include 'group.i'
      include 'shunt.i'
      include 'units.i'
      include 'usage.i'
      integer i,j,k,i1,i2,k1,k2
      real*8 xi,yi,zi,xk,yk,zk
      real*8 xq,yq,zq,xr,yr,zr
      real*8 f,fi,fik,fgrp
      real*8 e,ri2,rk2,rirkr3
      real*8 doti,dotk,dotp
      real*8 r,r2,r3,r4,r5,taper
      logical proceed
c
c
c     zero out the overall dipole interaction energy
c     and set up the constants for the calculation
c
      ed = 0.0d0
      if (ndipole .eq. 0)  return
c
c     set conversion factor and switching function coefficients
c
      f = electric / (debye**2 * dielec)
      call switch ('DIPOLE')
c
c     calculate the pairwise dipole interaction energy term
c
      do i = 1, ndipole-1
         i1 = idpl(1,i)
         i2 = idpl(2,i)
         xi = x(i2) - x(i1)
         yi = y(i2) - y(i1)
         zi = z(i2) - z(i1)
         ri2 = xi*xi + yi*yi + zi*zi
         xq = x(i1) + xi*sdpl(i)
         yq = y(i1) + yi*sdpl(i)
         zq = z(i1) + zi*sdpl(i)
         fi = f * bdpl(i)
         do k = i+1, ndipole
            k1 = idpl(1,k)
            k2 = idpl(2,k)
c
c     decide whether to compute the current interaction
c
            proceed = .true.
            if (use_group)  call groups (proceed,fgrp,4,i1,i2,k1,k2,0)
            if (proceed)  proceed = (use(i1) .or. use(i2) .or.
     &                                 use(k1) .or. use(k2))
            if (proceed)  proceed = (k1.ne.i1 .and. k1.ne.i2 .and.
     &                                 k2.ne.i1 .and. k2.ne.i2)
c
c     compute the energy contribution for this interaction
c
            if (proceed) then
               xk = x(k2) - x(k1)
               yk = y(k2) - y(k1)
               zk = z(k2) - z(k1)
               xr = xq - x(k1) - xk*sdpl(k)
               yr = yq - y(k1) - yk*sdpl(k)
               zr = zq - z(k1) - zk*sdpl(k)
               if (use_image)  call image (xr,yr,zr,0)
               r2 = xr*xr + yr*yr + zr*zr
               if (r2 .le. off2) then
                  rk2 = xk*xk + yk*yk + zk*zk
                  rirkr3 = sqrt(ri2*rk2*r2) * r2
                  dotp = xi*xk + yi*yk + zi*zk
                  doti = xi*xr + yi*yr + zi*zr
                  dotk = xk*xr + yk*yr + zk*zr
                  fik = fi * bdpl(k)
                  e = fik * (dotp-3.0d0*doti*dotk/r2) / rirkr3
c
c     use energy switching if near the cutoff distance
c
                  if (r2 .gt. cut2) then
                     r = sqrt(r2)
                     r3 = r2 * r
                     r4 = r2 * r2
                     r5 = r2 * r3
                     taper = c5*r5 + c4*r4 + c3*r3
     &                          + c2*r2 + c1*r + c0
                     e = e * taper
                  end if
c
c     scale the interaction based on its group membership
c
                  if (use_group)  e = e * fgrp
c
c     increment the overall dipole-dipole energy component
c
                  ed = ed + e
               end if
            end if
         end do
      end do
c
c     for periodic boundary conditions with large cutoffs
c     neighbors must be found by the replicates method
c
      if (.not. use_replica)  return
c
c     calculate interaction energy with other unit cells
c
      do i = 1, ndipole
         i1 = idpl(1,i)
         i2 = idpl(2,i)
         xi = x(i2) - x(i1)
         yi = y(i2) - y(i1)
         zi = z(i2) - z(i1)
         ri2 = xi*xi + yi*yi + zi*zi
         xq = x(i1) + xi*sdpl(i)
         yq = y(i1) + yi*sdpl(i)
         zq = z(i1) + zi*sdpl(i)
         fi = f * bdpl(i)
         do k = i, ndipole
            k1 = idpl(1,k)
            k2 = idpl(2,k)
c
c     decide whether to compute the current interaction
c
            proceed = .true.
            if (use_group)  call groups (proceed,fgrp,4,i1,i2,k1,k2,0)
            if (proceed)  proceed = (use(i1) .or. use(i2) .or.
     &                                 use(k1) .or. use(k2))
c
c     compute the energy contribution for this interaction
c
            if (proceed) then
               do j = 1, ncell
                  xk = x(k2) - x(k1)
                  yk = y(k2) - y(k1)
                  zk = z(k2) - z(k1)
                  xr = xq - x(k1) - xk*sdpl(k)
                  yr = yq - y(k1) - yk*sdpl(k)
                  zr = zq - z(k1) - zk*sdpl(k)
                  call image (xr,yr,zr,j)
                  r2 = xr*xr + yr*yr + zr*zr
                  if (r2 .le. off2) then
                     rk2 = xk*xk + yk*yk + zk*zk
                     rirkr3 = sqrt(ri2*rk2*r2) * r2
                     dotp = xi*xk + yi*yk + zi*zk
                     doti = xi*xr + yi*yr + zi*zr
                     dotk = xk*xr + yk*yr + zk*zr
                     fik = fi * bdpl(k)
                     e = fik * (dotp-3.0d0*doti*dotk/r2) / rirkr3
c
c     use energy switching if near the cutoff distance
c
                     if (r2 .gt. cut2) then
                        r = sqrt(r2)
                        r3 = r2 * r
                        r4 = r2 * r2
                        r5 = r2 * r3
                        taper = c5*r5 + c4*r4 + c3*r3
     &                             + c2*r2 + c1*r + c0
                        e = e * taper
                     end if
c
c     scale the interaction based on its group membership
c
                     if (use_group)  e = e * fgrp
c
c     increment the overall dipole-dipole energy component
c
                     if (i .eq. k)  e = 0.5d0 * e
                     ed = ed + e
                  end if
               end do
            end if
         end do
      end do
      return
      end
