function HCTIon(ion,nelem)
c     ion is stage of ionization, 1 for atom
c     nelem is atomic number of element, 2 up to 30
c     Example:  O + H+ => O+ + H is HCTIon(1,8)
c      integer ion , nelem
      INCLUDE 'IMPLIC.FOR'
      common/CTRTEMP/ te
      common/CTIon/ CTIon(7,4,30)
c
c     local variables
c      real tused 
c      integer ipIon
c
      ipIon = ion
c
c     Make sure te is between temp. boundaries; set constant outside of range
      tused = max( te,CTIon(5,ipIon,nelem) )
      tused = min( tused , CTIon(6,ipIon,nelem) )
      tused = tused * 1e-4
c
c     the interpolation equation
      HCTIon = CTIon(1,ipIon,nelem)* 1e-9 * 
     1 (tused**CTIon(2,ipIon,nelem)) *
     2 (1. + 
     3 CTIon(3,ipIon,nelem) * exp(CTIon(4,ipIon,nelem)*tused) ) *
     4 exp(-CTIon(7,ipIon,nelem)/tused)
c
      end