  An efficient pseudo-spectral numerical method is introduced for calculating a
self-consistent field (SCF) approximation for the linear susceptibility of
ordered phases in block copolymer melts (sometimes referred to as the random
phase approximation). Our method is significantly more efficient than that used
in the first calculations of this quantity by Shi, Laradji and coworkers,
allowing for the study of more strongly segregated structures. We have
re-examined the stability of several phases of diblock copolymer melts, and
find that some conclusions of Laradji et al. regarding the stability of the
Gyroid phase were the result of insufficient spatial resolution. We find that
an epitaxial (k=0) instability of the Gyroid phase with respect to the
hexagonal phase that was considered previously by Matsen competes extremely
closely with an instability that occurs at a nonzero crystal wavevector k.
