  We investigated the nematic to smectic transition undergone by parallel hard
spherocylinders in the framework provided by the residual multi-particle
entropy (RMPE) formalism. The RMPE is defined as the sum of all contributions
to the configurational entropy of the fluid which arise from density
correlations involving more than two particles. The vanishing of the RMPE
signals the structural changes which take place in the system for increasing
pressures. Monte Carlo simulations carried out for parallel hard
spherocylinders show that such a one-phase ordering criterion accurately
predicts also the nematic-smectic transition threshold notwithstanding the
almost continuous character of the transition. A similar quantitative
correspondence had been already noted in the case of an isotropic fluid of
freely rotating hard spherocylinders undergoing a transition to a nematic,
smectic or solid phase. The present analysis confirms the flexibility of the
RMPE approach as a practical and reliable tool for detecting the formation of
mesophases in model liquid-crystal systems.
