def normal(self):
        """
        Calculates the surface normal vector of the slab
        """
        normal = np.cross(self.lattice.matrix[0], self.lattice.matrix[1])
        normal /= np.linalg.norm(normal)
        return normal
def normal ( self ) : normal = np . cross ( self . lattice . matrix [ 0 ] , self . lattice . matrix [ 1 ] ) normal /= np . linalg . norm ( normal ) return normal
Calculates the surface normal vector of the slab