erel4.f

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      SUBROUTINE erel4(IM,EB,SEMI)
*
*
*       Dominant two-body energy in four-body system.
*       ---------------------------------------------
*
      IMPLICIT REAL*8  (a-h,m,o-z)
      common/creg/  m(4),x(12),xd(12),p(12),q(12),time4,energy,epsr2,
     &              xr(9),w(9),r(6),ta(6),mij(6),cm(10),rmax4,tmax,
     &              ds,tstep,eps,nstep4,name4(4),kz15,kz27,nreg,nfn
      common/savep/  pi(12)
      common/ksave/  k1,k2
*
*
*       Obtain physical momenta of pericentre configuration.
      CALL trans4
*
*       Form kinetic energy terms of dominant c.m. (K1 + K2) and the rest.
      zk = 0.0d0
      p0 = 0.0d0
      DO 10 k = 1,3
          ps = 0.0
          DO 8 i = 1,4
*       Note PI is associated with the bodies (PI sequential in CHAIN code).
              IF (i.NE.k1.AND.i.NE.k2) THEN
                  j = 3*(i - 1)
                  ps = ps + pi(j+k)
                  zk = zk + pi(j+k)**2/m(i)
              END IF
    8     CONTINUE
          p0 = p0 + ps**2
   10 CONTINUE
*
*       Evaluate potential energy due to non-singular terms.
      pot = 0.0d0
      DO 20 i = 1,6
          IF (i.EQ.im) go to 20
          pot = pot + mij(i)/r(i)
   20 CONTINUE
*
*       Obtain binding energy from total energy and perturbing function.
      mb = m(k1) + m(k2)
      vp = 0.5d0*(p0/mb + zk) - pot
      eb = energy - vp
*
*       Set semi-major axis.
      semi = -m(k1)*m(k2)/(2.0d0*eb)
*
      RETURN
*
      END