c-----------------------------------------------------------------------
c Evaluate the Boltzmann distribution function
c   f(T,v)=4*pi*(xm/2/pi/xk/T)^1.5 * exp(-xm*v*v/2/xk/T)*v*v
c Given
c  v  = Speed (m/sec)
c  xm = Mass of molecule = 32 g/mole for oxygen
c  xk = Boltzmann (gas) constant = 8.319 Joule/mole-K
c  T  = Temperature (K)
c ----------------------------------------------------------------------
c Instructor: Nam Sun Wang
c-----------------------------------------------------------------------

c Some constants
      pi = 3.141593
      xk = 8.319   ! Joule/mole-K

c Print out a program header
      print *, 'Evaluate the Boltzmann distribution function'

c Input the parameters
      print *, 'Enter molecular mass in Da (e.g., 32. for oxygen): '
      read *, xm
      print *, 'Enter temperature in K (e.g., 273.): '
      read *, T
      print *, 'Enter velocity in m/sec (e.g., 500.): '
      read *, v

c Change units to be consistent with the rest.
      xm = xm/1000.

c Find the distribution function
      f = 4.*pi*(xm/2./pi/xk/T)**1.5 * exp(-xm*v*v/2./xk/T)*v*v

c Output the result
      print *, ' '
      print *, 'Boltzmann distribution function: f = ', f, ' sec/m'
      print *, 'Condition:'
      print *, '    Molecular mass (Dalton): ', xm*1000.
      print *, '    Temperatuer (K):         ', T
      print *, '    Velocity (m/sec)         ', v

      end