Improved Bond-Stretching-Energy Curves from the Integral Hellmann–Feynman Method

The connection between the local-energy concept and the integral Hellmann–Feynman (IHF) method is used to obtain an improved bond-stretching-energy curve from short one-center expansions of HeH+. An alternative method of analyzing the IHF method, based on spectral impurities, is developed. It is shown how this formalism also enables one to rationalize and predict IHF results. IHF calculations on single-center expansions for H2 are reported. It is concluded that, for single-center expansions (especially when centered at a nucleus), the IHF method is a very promising way to get improved bond-stretching energies.