Derivation of coarse grained models for multiscale simulation of liquid crystalline phase transitions

We present a systematic derivation of a coarse grained (CG) model for molecular dynamics (MD) simulations of a liquid crystalline (LC) compound containing an azobenzene mesogen. The model aims at a later use in a multiscale modeling approach to study liquid crystalline phase transitions that are (photo)induced by the trans/cis photoisomerization of the mesogen. One of the major challenges in the coarse graining process is the development of models that are for a given chemical system structurally consistent with for example an all-atom reference model and reproduce relevant thermodynamic properties such as the LC phase behavior around the state point of interest. The reduction of number of degrees of freedom makes the resulting coarse models by construction state point dependent; that is, they cannot easily be transferred to a range of temperatures, densities, system compositions, etc. These are significant challenges, in particular if one wants to study LC phase transitions (thermally or photoinduced). In the present paper we show how one can systematically derive a CG model for a LC molecule that is highly consistent with an atomistic description by choosing an appropriate state point for the reference simulation. The reference state point is the supercooled liquid just below the smectic-isotropic phase transition which is characterized by a high degree of local nematic order while being overall isotropic. With the resulting CG model it is possible to switch between the atomistic and the CG levels (and vice versa) in a seamless manner maintaining values of all the relevant order parameters which describe the smectic A (smA) state. This model will allow us in the future to link large length scale and long time scale CG simulations of the LC state with chemically accurate QM/MM simulations of the photoisomerization process.