A twist may arise because some chiral, or optically active, molecules have been dissolved in the liquid crystal, as shown in this diagram. Such molecules are not superimposable on their mirror images (like a pair of scissors): they occur in 'right-handed' and 'left-handed' forms. The higher the concentration of these molecules, the bigger the twisting effect. This is understood in general terms, but the microscopic origin of the effect (i.e. how it is related to the structure of the molecules) still needs clarifying.
The so-called helical twisting power measures the effectiveness of a given chiral molecule at producing such a twist. We have developed ways of measuring this quantity in simulations, based on considering the free energy change that occurs when such a molecule is inserted into an already-twisted nematic phase.
M P Allen, Phys Rev E, 47, 4611-4614 (1993).
G Germano, M P Allen, A J Masters, J Chem Phys, 116, 9422-9430 (2002).