code: 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))
          returf _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
docs: Normalize basis function. From THO eq. 2.2