  We apply the minimum energy paths (MEPs) approach to study the helix
unwinding transition in chiral nematic liquid crystals. A mechanism of the
transition is determined by a MEP passing through a first order saddle point on
the free energy surface. The energy difference between the saddle point and the
initial state gives the energy barrier of the transition. Two starting
approximations for the paths are used to find the MEPs representing different
transition scenarios: (a) the director slippage approximation with in-plane
helical structures; and (b) the anchoring breaking approximation that involves
the structures with profound out-of-plane director deviations. It is shown
that, at sufficiently low voltages, the unwinding transition is solely governed
by the director slippage mechanism with the planar saddle point structures.
When the applied voltage exceeds its critical value below the threshold of the
Freedericksz transition, the additional scenario through the anchoring breaking
transitions is found to come into play. For these transitions, the saddle point
structure is characterized by out-of-plane deformations localized near the
bounding surface. The energy barriers for different paths of transitions are
computed as a function of the voltage and the anchoring energy strengths.
