Do partial atomic charges matter for the adsorption of molecules on a substrate when using Molecular Mechanics? It depends.

Molecular Mechanics (MM) calculations offer powerful tools to explore adsorption geometries and energies. Various force fields can be used for different combinations of adsorbates and substrates. These MM calculations can be advantageous over purely quantum chemistry approaches due to requirements of less computational power. Suitable MM calculations rely on the charge distributions within a molecule. In order to study several configurations, the molecules are optimized using quantum mechanical calculations and various charge schemes.

An interesting system for the investigation of adsorption are various merocyanine molecules and their conformers on an insulating NaCl template. This system offers a rich complexity of molecular adsorbates together with a polar substrate. The results show that stable conformers of merocyanine have nearly identical adsorption configurations for various charge schemes of the molecules and force field parameters. The adsorption energies are similar for equivalent polar substrates and show an increasing trend with increased polarity of the substrate. However, other less stable conformers adsorb in different configurations depending on charge settings. Overall, dependence of partial atomic charges on adsorption geometries and energies is limited.