Force concept for predicting the geometries of molecules in an external electric field

An intuitive Hellmann–Feynman (H–F) force concept is presented for predicting the geometries of molecules in external electric fields of various strengths. The field-induced H–F force consists of internal and external forces and their balance determines the molecular geometry. Our previous model is valid in both strong and weak fields except for non-protic hydrogens. This exception is explained to being due to the local nature of the field-induced density reorganization, i.e. H-to-bond and bond-to-H electron flow. The force concept including this bond effect is applicable to various molecules in electric fields of various strengths.