Internuclear forces in a molecule, as an integral over a geometrical factor and the electron density, can be decomposed into a part having as origin promolecular densities of the participating atoms, and a contribution from the deformation density. At the hand of homo-nuclear diatomic molecules we show that the binding energy is linked to the deformation force except for transition metal dimers. However, vibrational frequencies involving pure bond-length variations are rather well reproduced, even for heteroatomic diatomics. For larger assemblies, frequencies for bending modes are underestimated, but the model may serve for a rough analysis of a vibrational spectrum.