code: def M200(self, Rs, rho0, c):
          
          return 4*np.pi*rho0*Rs**3*(np.log(1.+c)-c/(1.+c))
docs: M(R_200) calculation for NFW profile
  
          :param Rs: scale radius
          :type Rs: float
          :param rho0: density normalization (characteristic density)
          :type rho0: float
          :param c: concentration
          :type c: float [4,40]
          :return: M(R_200) density