Prediction of pitch in twisted nematics: Puzzling cases

The correlation between structure of chiral molecules and macroscopic prope rties of cholesteric phases is not obvious, but often the experimental beha viour can be rationalised by resorting to simple rules. However, there are cases in which solvent / temperature changes or introduction of substituent s in the chiral molecules can produce dramatic and unexpected effects, incl uding helix inversion. Some significant examples will be presented and disc ussed on the basis of theoretical predictions derived by the 'surface tenso r' mean field model for molecular structures optimised with quantum-mechani cal methods.