  It is commonly considered that the frustration between the curvature energy
and the chain stretching energy plays an important role in the formation of
lyotropic liquid crystals in bicontinuous cubic phases. Theoretic and numeric
calculations were performed for two extreme cases: Parallel surfaces eliminate
the variance of the chain length; constant mean curvature surfaces eliminate
the variance of the mean curvature. We have implemented a model with Brakke's
Surface Evolver which allows a competition between the two variances. The
result shows a compromise of the two limiting geometries. With data from real
systems, we are able to recover the G--D--P phase sequence which was observed
in experiments.
