subroutine potent(x,v,f,number)
c      calculates the potential energy and derivative
c      for a number of standard functions
      implicit real*8 (a-h,o-z)
      if(number.le.0) goto 100
      xi=1.0/x
      alambda = 1.1d0
      sect = 1.d0 + (alambda - 1.d0)*0.95d0
c     alambda is the extent of the square well interaction
      goto (1,2,3,4,5,6,7,8) number
1     continue
c      lennard jones 6-12 potential
c      x2=xi**2
c      x6=x2**3
c      v=4.d0*(x6-1.d0)*x6
c      f=24.d0*x6*x2*(2.d0*x6-1.d0)
      if (x.lt.alambda) then
         if (x.ge.1.d0) then
            if(x.le.sect) then
              v = (x - 1.d0)/(19.d0*(alambda - 1.d0)) -1.d0
              f = -1.d0*xi/(19.d0*(alambda - 1.d0))
            else
              v= 19.d0*(x - alambda)/(alambda - 1.d0)
              f= -19.d0*xi/(alambda - 1.d0)
            endif
        else
            v=xi**15 - 2.d0
            f=15.d0*(xi**17)
         endif 
      else
         v=0.d0
         f=0.d0
      endif      
      return
2     continue
c     coulomb potential
      v=xi
      f=v**3
      return
3     continue
      data tdsqpi/1.128379167/
      xsq=x**2
      call gammi(gm,0.5d0,xsq,sqpi)
      v=xi*sqpiv*gm
      data sqpiv/.564189584d0/
      f=xi**2*(v+tdsqpi*exp(-x**2))
      return
4     continue
c      repulsive yukawa potential
      v=exp(-x)/x
      f=xi*v*(xi+1.d0)
      return
5     continue
      v=exp(-x)
      v2=v*v
      f=xi*(2*v2-v)
      v=v2-v
      return
6     continue
c rahman stillinger lember hh force
      y6=exp(-40.*(x-2.0))
      v=20.*y6/(1.+y6)
      f=40.*v/(1.+y6)
      y6=-17.03002*exp(-7.60626*(x-1.4525)**2)
      v=v+y6
      f=xi*(f+y6*15.21252*(x-1.4525))
      return
7     continue
c rahman stillinger lemberg oh potential
      v=2.6677*xi**14.97
      f=14.97*v*xi**2
      y7=exp(-5.49305*(x-2.2))
      v7=-6.*y7/(1.0+y7)
      v=v+v7
      f=f+xi*v7*5.49305/(1.0+y7)
      return
8     continue
c rahman stillinger lemberg oo potential
      v=23401.9*xi**8.3927
      f=8.3927*v*xi*xi
      return
100   v=0.0
      f=0.0
      return
      end