def _surfdens(self,R,z,phi=0.,t=0.):
        """
        NAME:
           _surfdens
        PURPOSE:
           evaluate the surface density for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           the density
        HISTORY:
           2018-08-04 - Written - Bovy (UofT)
        """
        if R > self.a: return 0.
        h= nu.sqrt(self.a2-R**2)
        if z < h: return 0.
        else: return 1./(2.*nu.pi*self.a*h)