Atomistic Force Field for Azobenzene Compounds Adapted for QM/MM Simulations with Applications to Liquids and Liquid Crystals.

An atomistic force field has been adapted for use in molecular dynamics simulations of molecular materials that contain azobenzene (AB) functional groups. Force field parameters for bonded interactions and partial charges in the AB unit have been derived from ab initio molecular dynamics reference calculations. First applications of the new force field to liquid trans- and cis-AB are presented, both using a purely classical approach (MM) and a hybrid quantum-classical (QM/MM) simulation scheme. Detailed structural analysis confirms that QM/MM and purely MM simulations yield results that are in good agreement with each other. The force field of the AB core has been extended to include aliphatic chains that are attached via ether bridges to the two AB benzene rings. This allows for studying temperature induced phase transitions in the liquid-crystalline 8AB8 system. Using replica exchange techniques the new force field has successfully reproduced the smectic to isotropic-phase transition.