Floating functions satisfying the hellmann—feynman theorem: Single floating scheme

The electrostatic calculation for molecules using approximated variational wave functions leads to well known difficulties connected with the application of the Hellmann‐Feynman (HF) theorem. This is due to the basis set inadequacies in the underlying calculations. This defect can easily be remedied by floating functions, whose centers are optimized in space. We can keep almost everything of the traditional wave function with a nuclear‐fixed basis set, but we apply single floating to ensure the HF theorem. Then, one can obtain a wave function obeying the HF theorem. This provides a great conceptual simplification and may lead to practical advantages. The single floating scheme, which retains one expansion center per nucleus, is successfully applied to a series of small molecules using SCF and CASSCF wave functions with sufficiently polarized basis sets.