chpot.f

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      SUBROUTINE chpot(DP)
*
*
*       Potential energy correction due to chain.
*       -----------------------------------------
*
      include 'common6.h'
      common/chainc/  xc(3,ncmax),uc(3,ncmax),bodyc(ncmax),ich,
     &                listc(lmax)
*
*
*       Obtain current global coordinates of chain components.
      CALL xcpred(0)
*
*       Consider contributions from all active perturbers.
      dp = 0.0d0
      jdum = 0
      nnbc = listc(1) + 1
*
      DO 10 l = 2,nnbc
*       Subtract potential energy due to chain c.m.
          j = listc(l)
*       Replace any regularized c.m. body by individual components.
          IF (j.GT.n) THEN
              jdum = 2*(j - n) - 1
              j = jdum
          END IF
    2     a1 = x(1,ich) - x(1,j)
          a2 = x(2,ich) - x(2,j)
          a3 = x(3,ich) - x(3,j)
          rij2 = a1*a1 + a2*a2 + a3*a3
          dp = dp - body(ich)*body(j)/sqrt(rij2)
*
*       Add individual interactions to obtain differential correction.
          DO 5 k = 1,nch
              a1 = xc(1,k) - x(1,j)
              a2 = xc(2,k) - x(2,j)
              a3 = xc(3,k) - x(3,j)
              rij2 = a1*a1 + a2*a2 + a3*a3
              dp = dp + bodyc(k)*body(j)/sqrt(rij2)
    5     CONTINUE
*
*       Check for possible second KS component and restore dummy index.
          IF (j.EQ.jdum) THEN
              j = jdum + 1
              jdum = 0
              go to 2
          END IF
   10 CONTINUE
*
      RETURN
*
      END