An ab initio strategy for handling induction phenomena in metal ion complexes

Abstract

A strategy that incorporates the macromolecular, non-polarisable CHARMM force field models of atomic charges and polarisabilities determined from high-level quantum mechanical calculations, is probed in the paradigmatic case of metal ion complexes. Although the ab initio polarisable potential energy function is capable of hierarchising correctly the affinity of a divalent calcium ion for three distinct electron–donor chelating agents, severe inaccuracies in the underlying contributions to the total binding energy, compared to a reference symmetry-adapted perturbation theory (SAPT) expansion, illuminate the incompleteness of the interaction model. The reported calculations suggest that mapping faithfully the signature electrostatic potential and induction energy of the isolated species is a necessary, albeit not sufficient condition to guarantee that intermolecular interactions be described accurately when key physical phenomena are evidently missing from a force field to which explicit induction effects have been introduced. They further suggest that in the present strategy, naive attempts to correct the flawed reproduction of the electrostatic and induction terms of the perturbative expansion–rooted, for instance, in an inappropriate representation of electron-cloud penetration–through de novo parametrisation of the mathematically questionable ‘6–12’ form of the van der Waals potential, common to many macromolecular force fields, are unavoidably bound to failure.

Keywords:

• polarisation phenomena
• ab initio calculations
• macromolecular force field
• symmetry-adapted perturbation theory
• metal ion complexes

Acknowledgements

János G. Ángyán is gratefully acknowledged for stimulating discussions. The authors are indebted to the Centro Nacional de Supercomputación, Barcelona, Spain, for provision of generous amounts of computational time.