  A renormalized one-loop theory (ROL) is used to calculate corrections to the
random phase approximation (RPA) for the structure factor $\Sc(q)$ in
disordered diblock copolymer melts. Predictions are given for the peak
intensity $S(q^{\star})$, peak position $q^{\star}$, and single-chain
statistics for symmetric and asymmetric copolymers as functions of $\chi N$,
where $\chi$ is the Flory-Huggins interaction parameter and $N$ is the degree
of polymerization. The ROL and Fredrickson-Helfand (FH) theories are found to
yield asymptotically equivalent results for the dependence of the peak
intensity $S(q^{\star})$ upon $\chi N$ for symmetric diblock copolymers in the
limit of strong scattering, or large $\chi N$, but yield qualitatively
different predictions for symmetric copolymers far from the ODT and for
asymmetric copolymers. The ROL theory predicts a suppression of $S(q^\star)$
and a decrease of $q^{\star}$ for large values of $\chi N$, relative to the RPA
predictions, but an enhancement of $S(q^{\star})$ and an increase in
$q^{\star}$ for small $\chi N$ ($\chi N < 5$). By separating intra- and
inter-molecular contributions to $S^{-1}(q)$, we show that the decrease in
$q^{\star}$ near the ODT is caused by the $q$ dependence of the intermolecular
direct correlation function, and is unrelated to any change in single-chain
statistics, but that the increase in $q^{\star}$ at small values of $\chi N$ is
a result of non-Gaussian single-chain statistics.
