docs: def Pgas(rho,T,mu):
      ''' 
      P = R/mu * rho * T
  
      Parameters
      ----------
      mu : float
          Mean molecular weight
      rho : float
          Density [cgs]
      T : float
          Temperature [K]
  
      '''
      R = old_div(boltzmann_constant, atomic_mass_unit)
      return (old_div(R,mu)) * rho * T