where r i = (x i , y i , z i ) is the center of the basis function, n xi , n yi , n zi ∈ N 0 define its angular momentum and β i ∈ R + is the orbital exponent parameter. The cutoff function, (1-e -αr 2 ) q , with α ∈ R + ensures that the integrals exist for k > 2 provided the power of the cutoff function q satisfies