def _normalize(self):
        "Normalize basis function. From THO eq. 2.2"
        l,m,n = self.powers
        self.norm = np.sqrt(pow(2,2*(l+m+n)+1.5)*
                            pow(self.exponent,l+m+n+1.5)/
                            fact2(2*l-1)/fact2(2*m-1)/
                            fact2(2*n-1)/pow(np.pi,1.5))
        return
Normalize basis function. From THO eq. 2.2