Gaussian Expansions of Orbitals.

Using numerical calculations and analytic theory, we examine the convergence behavior of Gaussian expansions of several model orbitals. By following the approach of Kutzelnigg, we find that the errors in the energies of the optimal n-term even-tempered Gaussian expansions of s-type, p-type, and d-type exponential orbitals are εn s ∼ exp(−π(3n)), εn ∼ exp(−π(5n)), and εn ∼ exp(−π(7n)), respectively. We show that such “root-exponential” convergence patterns are a consequence of the orbital cusps at r = 0, rather than the over-rapid decay of Gaussians at large r. We find that even-tempered expansions of the cuspless Lorentzian orbital also exhibit root-exponential convergence but that this is a consequence of its fat tail.