Microscopic origins of first-order Sm-A-Sm-C phase behavior in de Vries smectic liquid crystals

We explore the phase behavior of tilted hard rods as a model of de Vries smectic behavior and the first order smectic-C (Sm-C) to smectic-A (Sm-A) phase transition. The free energy cost of azimuthal rotation of a molecule away from the local tilt direction is calculated via umbrella sampling. This calculation is used to map the hard rod system onto a lattice spin system which shows a cross-over from a continuous to first-order phase transition as the tilt of the rods is increased. This analysis offers a natural explanation of the first-order Sm-A-Sm-C phase transition common to de Vries smectics.