c
c
c     ###################################################
c     ##  COPYRIGHT (C)  1993  by  Jay William Ponder  ##
c     ##              All Rights Reserved              ##
c     ###################################################
c
c     ########################################################
c     ##                                                    ##
c     ##  subroutine eimprop  --  improper dihedral energy  ##
c     ##                                                    ##
c     ########################################################
c
c
c     "eimprop" calculates the improper dihedral potential energy
c
c
      subroutine eimprop
      implicit none
      include 'sizes.i'
      include 'atoms.i'
      include 'energi.i'
      include 'group.i'
      include 'improp.i'
      include 'math.i'
      include 'usage.i'
      integer i,ia,ib,ic,id
      real*8 e,ideal,force
      real*8 dt,cosine,sine
      real*8 angle,fgrp
      real*8 xia,yia,zia
      real*8 xib,yib,zib
      real*8 xic,yic,zic
      real*8 xid,yid,zid
      real*8 xba,yba,zba
      real*8 xcb,ycb,zcb,rcb
      real*8 xdc,ydc,zdc
      real*8 xt,yt,zt,rt2
      real*8 xu,yu,zu,ru2
      real*8 xtu,ytu,ztu,rtru
      logical proceed
c
c
c     zero out improper dihedral energy
c
      eid = 0.0d0
c
c     calculate the improper dihedral angle energy term
c
      do i = 1, niprop
         ia = iiprop(1,i)
         ib = iiprop(2,i)
         ic = iiprop(3,i)
         id = iiprop(4,i)
c
c     decide whether to compute the current interaction
c
         proceed = .true.
         if (use_group)  call groups (proceed,fgrp,4,ia,ib,ic,id,0)
         if (proceed)  proceed = (use(ia) .or. use(ib) .or.
     &                              use(ic) .or. use(id))
c
c     compute the value of the improper dihedral angle
c
         if (proceed) then
            xia = x(ia)
            yia = y(ia)
            zia = z(ia)
            xib = x(ib)
            yib = y(ib)
            zib = z(ib)
            xic = x(ic)
            yic = y(ic)
            zic = z(ic)
            xid = x(id)
            yid = y(id)
            zid = z(id)
            xba = xib - xia
            yba = yib - yia
            zba = zib - zia
            xcb = xic - xib
            ycb = yic - yib
            zcb = zic - zib
            xdc = xid - xic
            ydc = yid - yic
            zdc = zid - zic
            xt = yba*zcb - ycb*zba
            yt = zba*xcb - zcb*xba
            zt = xba*ycb - xcb*yba
            xu = ycb*zdc - ydc*zcb
            yu = zcb*xdc - zdc*xcb
            zu = xcb*ydc - xdc*ycb
            xtu = yt*zu - yu*zt
            ytu = zt*xu - zu*xt
            ztu = xt*yu - xu*yt
            rt2 = xt*xt + yt*yt + zt*zt
            ru2 = xu*xu + yu*yu + zu*zu
            rtru = sqrt(rt2 * ru2)
            if (rtru .ne. 0.0d0) then
               rcb = sqrt(xcb*xcb + ycb*ycb + zcb*zcb)
               cosine = (xt*xu + yt*yu + zt*zu) / rtru
               sine = (xcb*xtu + ycb*ytu + zcb*ztu) / (rcb*rtru)
               cosine = min(1.0d0,max(-1.0d0,cosine))
               angle = radian * acos(cosine)
               if (sine .lt. 0.0d0)  angle = -angle
c
c     set the improper dihedral parameters for this angle
c
               ideal = vprop(i)
               force = kprop(i)
               dt = angle - ideal
               dowhile (dt .gt. 180.0d0)
                  dt = dt - 360.0d0
               end do
               dowhile (dt .lt. -180.0d0)
                  dt = dt + 360.0d0
               end do
               dt = dt / radian
c
c     calculate the improper dihedral energy
c
               e = force * dt**2
c
c     scale the interaction based on its group membership
c
               if (use_group)  e = e * fgrp
c
c     increment the total improper dihedral energy
c
               eid = eid + e
            end if
         end if
      end do
      return
      end
