c
c
c     ###################################################
c     ##  COPYRIGHT (C)  2003  by  Jay William Ponder  ##
c     ##              All Rights Reserved              ##
c     ###################################################
c
c     #################################################################
c     ##                                                             ##
c     ##  subroutine epitors  --  pi-orbit torsion potential energy  ##
c     ##                                                             ##
c     #################################################################
c
c
c     "epitors" calculates the pi-orbital torsion potential energy
c
c
      subroutine epitors
      implicit none
      include 'sizes.i'
      include 'atoms.i'
      include 'bound.i'
      include 'energi.i'
      include 'group.i'
      include 'pitors.i'
      include 'torpot.i'
      include 'usage.i'
      integer i,ia,ib,ic
      integer id,ie,ig
      real*8 e,rdc,fgrp
      real*8 xt,yt,zt,rt2
      real*8 xu,yu,zu,ru2
      real*8 xtu,ytu,ztu,rtru
      real*8 v2,c2,s2,phi2
      real*8 sine,cosine
      real*8 sine2,cosine2
      real*8 xia,yia,zia
      real*8 xib,yib,zib
      real*8 xic,yic,zic
      real*8 xid,yid,zid
      real*8 xie,yie,zie
      real*8 xig,yig,zig
      real*8 xip,yip,zip
      real*8 xiq,yiq,ziq
      real*8 xad,yad,zad
      real*8 xbd,ybd,zbd
      real*8 xec,yec,zec
      real*8 xgc,ygc,zgc
      real*8 xcp,ycp,zcp
      real*8 xdc,ydc,zdc
      real*8 xqd,yqd,zqd
      logical proceed
c
c
c     zero out the pi-orbital torsion potential energy
c
      ept = 0.0d0
c
c     calculate the pi-orbital torsion angle energy term
c
      do i = 1, npitors
         ia = ipit(1,i)
         ib = ipit(2,i)
         ic = ipit(3,i)
         id = ipit(4,i)
         ie = ipit(5,i)
         ig = ipit(6,i)
c
c     decide whether to compute the current interaction
c
         proceed = .true.
         if (use_group)  call groups (proceed,fgrp,ia,ib,ic,id,ie,ig)
         if (proceed)  proceed = (use(ia) .or. use(ib) .or. use(ic) .or.
     &                              use(id) .or. use(ie) .or. use(ig))
c
c     compute the value of the pi-orbital torsion 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)
            xie = x(ie)
            yie = y(ie)
            zie = z(ie)
            xig = x(ig)
            yig = y(ig)
            zig = z(ig)
            xad = xia - xid
            yad = yia - yid
            zad = zia - zid
            xbd = xib - xid
            ybd = yib - yid
            zbd = zib - zid
            xec = xie - xic
            yec = yie - yic
            zec = zie - zic
            xgc = xig - xic
            ygc = yig - yic
            zgc = zig - zic
            if (use_polymer) then
               call image (xad,yad,zad,0)
               call image (xbd,ybd,zbd,0)
               call image (xec,yec,zec,0)
               call image (xgc,ygc,zgc,0)
            end if
            xip = yad*zbd - ybd*zad + xic
            yip = zad*xbd - zbd*xad + yic
            zip = xad*ybd - xbd*yad + zic
            xiq = yec*zgc - ygc*zec + xid
            yiq = zec*xgc - zgc*xec + yid
            ziq = xec*ygc - xgc*yec + zid
            xcp = xic - xip
            ycp = yic - yip
            zcp = zic - zip
            xdc = xid - xic
            ydc = yid - yic
            zdc = zid - zic
            xqd = xiq - xid
            yqd = yiq - yid
            zqd = ziq - zid
            if (use_polymer) then
               call image (xcp,ycp,zcp,0)
               call image (xdc,ydc,zdc,0)
               call image (xqd,yqd,zqd,0)
            end if
            xt = ycp*zdc - ydc*zcp
            yt = zcp*xdc - zdc*xcp
            zt = xcp*ydc - xdc*ycp
            xu = ydc*zqd - yqd*zdc
            yu = zdc*xqd - zqd*xdc
            zu = xdc*yqd - xqd*ydc
            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
               rdc = sqrt(xdc*xdc + ydc*ydc + zdc*zdc)
               cosine = (xt*xu + yt*yu + zt*zu) / rtru
               sine = (xdc*xtu + ydc*ytu + zdc*ztu) / (rdc*rtru)
c
c     set the pi-orbital torsion parameters for this angle
c
               v2 = kpit(i)
               c2 = -1.0d0
               s2 = 0.0d0
c
c     compute the multiple angle trigonometry and the phase terms
c
               cosine2 = cosine*cosine - sine*sine
               sine2 = 2.0d0 * cosine * sine
               phi2 = 1.0d0 + (cosine2*c2 + sine2*s2)
c
c     calculate the pi-orbital torsion energy for this angle
c
               e = ptorunit * v2 * phi2
c
c     scale the interaction based on its group membership
c
               if (use_group)  e = e * fgrp
c
c     increment the total pi-orbital torsion angle energy
c
               ept = ept + e
            end if
         end if
      end do
      return
      end
