      REAL FUNCTION SIGMA(E)
C     (H2+) COLLISIONS WITH H2
C     (H_2^+)+H_2->FAST (H^+)
C     CROSS SECTION(CM^2)  :SIGMA
C     COLLISION ENERGY(KEV):E
C     T.Shirai, et al., At. Data Nucl. Data Tables 76, 1 (2000).
      REAL A(12)
      DATA EMIN,EMAX,ETH /5.62E-3,9.90E+1,5.0E-3/
      DATA A /6.34E+1,1.78,1.38E-3,4.06E-1,1.63E-1,3.27E-1,
     >        1.554E+1,3.903,1.735,1.02E+1,2*0.0/
      DATA SIGMA0,ER /1.E-16,1.361E-2/
      SIGMA=0.0
      IF(E.LT.EMIN .OR. E.GT.EMAX) GO TO 9999
      E1=E-ETH
      SIGMA=F2(SIGMA0,ER,E1,A(1),A(2),A(3),A(4))
     >     +F2(SIGMA0,ER,E1,A(5),A(6),A(7),A(8))
     >     +A(9)*F2(SIGMA0,ER,E1/A(10),A(5),A(6),A(7),A(8))
 9999 RETURN
      END
C
      REAL FUNCTION F1(SIGMA0,ER,X,C1,C2)
        F1=SIGMA0*C1*(X/ER)**C2
      RETURN
      END
C
      REAL FUNCTION F2(SIGMA0,ER,X,C1,C2,C3,C4)
        F2=SIGMA0*C1*(X/ER)**C2
     >     /(1.+(X/C3)**(C2+C4))
      RETURN
      END
C
      REAL FUNCTION F3(SIGMA0,ER,X,C1,C2,C3,C4,C5,C6)
        F3=SIGMA0*C1*(X/ER)**C2
     >     /(1.+(X/C3)**(C2+C4)+(X/C5)**(C2+C6))
      RETURN
      END
C
      REAL FUNCTION F4(SIGMA0,ER,X,C1,C2,C3,C4,C5,C6,C7,C8)
        F4=SIGMA0*C1*(X/ER)**C2
     >     *(1.+(X/C3)**(C4-C2))
     >     /(1.+(X/C5)**(C4+C6)+(X/C7)**(C4+C8))
      RETURN
      END

