Geometry optimization in the presence of external forces: a theoretical model for enforced structural changes in molecules

Recently, we proposed a very simple quantum chemical model to simulate the effect of external forces acting on a single molecule [Mol. Phys. 107, 2403 (2009)]. It is based on optimizing the geometry of a molecule with an external force applied to selected pairs of nuclei. In this study we extend this model by considering interactions of external forces not only with the nuclei but also with their electrons, in particular their core electrons, which can be viewed as ‘rigidly’ connected to a nucleus. In the proposed revised model an external force acts on an object which consists of the nucleus of an atom and its 1s core electrons. It is shown in this study that such a model predicts the same conformational (structural) changes in a molecule as our simpler model where the external forces interact with the nuclei only. However, the magnitude of the forces required to cause these changes is now lower and within the range of forces used in real AFM experiments.