FUNCTION EXPINT(X)
C     ==================
C
C     First exponential integral function E1(X)
C
      INCLUDE 'PARAMS.FOR'
C
      IF(X.LE.1.0) THEN
         EXPINT=-LOG(X)-0.57721566+X*(0.99999193+X*(-0.24991055
     *          +X*(0.05519968+X*(-0.00976004+X*0.00107857))))
       ELSE
         EXPINT=EXP(-X)*((0.2677734343+X*(8.6347608925+X*
     *          (18.059016973+X*(8.5733287401+X))))/
     *          (3.9584969228+X*(21.0996530827+X*
     *          (25.6329561486+X*(9.5733223454+X)))))/X
      END IF
      RETURN
      END