  Composition fluctuations in disordered melts of symmetric diblock copolymers
are studied by Monte Carlo simulation over a range of chain lengths and
interaction strengths. Results are used to test three theories: (1) the random
phase approximation (RPA), (2) the Fredrickson-Helfand (FH) theory, which was
designed to describe large fluctuations near an order-disorder transition
(ODT), and (3) a more recent renormalized one-loop (ROL) theory, which reduces
to FH theory near the ODT, but which is found to be accurate over a much wider
range of parameters.
