FUNCTION EXPINT(X)
C     ==================
C
C     First exponential integral function E1(X)
C
      INCLUDE 'IMPLIC.FOR'
      PARAMETER (A1  =  -0.57721566,
     *           A2  =   0.99999193,
     *           A3  =  -0.24991055,
     *           A4  =   0.05519968,
     *           A5  =  -0.00976004,
     *           A6  =   0.00107857,
     *           B1  =   0.2677734343,
     *           B2  =   8.6347608925,
     *           B3  =  18.059016973,
     *           B4  =   8.5733287401,
     *           C1  =   3.9584969228,
     *           C2  =  21.0996530827,
     *           C3  =  25.6329561486,
     *           C4  =   9.5733223454,
     *           UN  =   1.0)
C
      IF(X.LE.UN) THEN
         EXPINT=-LOG(X)+A1+X*(A2+X*(A3+X*(A4+X*(A5+X*A6))))
       ELSE
         EXPINT=EXP(-X)*((B1+X*(B2+X*(B3+X*(B4+X))))/
     *                   (C1+X*(C2+X*(C3+X*(C4+X)))))/X
      END IF
      RETURN
      END