C FONCF     SOURCE    CHAT      05/01/13    00:06:02     5004           
C
C*********************************************************************
C
      FUNCTION FONCF(Q,X,Y,Z,X0)
      IMPLICIT INTEGER(I-N)
      IMPLICIT REAL*8(A-H,O-Z)
      EPSIL2=1.D-8
      Q2=SQRT(Q*Q+1.)
      D=Z+Q*X0
      IF(D.EQ.0.) THEN
         Z=Z+EPSIL2
      ENDIF
      DELTA=SQRT((X-X0)**2+Y**2+(Q*X+Z)**2)
C****
      DELTA1=MAX(X*Q2+(Q*Z-X0)/Q2+DELTA,EPSIL2)
      DELTA2=MAX(Y+DELTA,EPSIL2)
      FONCF=Y*LOG(DELTA1)
     &     +(X*Q2+(Q*Z-X0)/Q2)*LOG(DELTA2)
     &     -(Z+Q*X0)/Q2*ATAN(Y*(X*(Q*Q+1.)+Q*Z-X0)/(Z+Q*X0)/DELTA)
C****
      RETURN
      END