  The low-T part of the phase diagram in self-assembling systems is correctly
predicted by the known versions of the density functional theory (DFT). The
high-T part obtained in DFT, however, does not agree with simulations even on
the qualitative level. In this work a new version of the DFT is developed. The
contribution to the grand thermodynamic potential associated with mesoscopic
fluctuations is explicitly taken into account. The expression for this
contribution is obtained by the methods known from the Brazovskii field theory.
Apart from developing the approximate expression for the grand thermodynamic
potential that contains the fluctuation contribution and is ready for numerical
minimization, we develop a simplified version of the theory valid for weakly
ordered phases, i.e. for the high -T part of the phase diagram. The simplified
theory is verified by a comparison with the results of simulations for a
particular version of the short-range attraction long-range repulsion (SALR)
interaction potential. Except from the fact that in our theory the ordered
phases are stable at lower T than in simulations, a good agreement for the
high-T part of the phase diagram is obtained for the range of density that was
considered in simulations. In addition, the equation of state and
compressibility isotherms are presented. Finally, the physical interpretation
of the fluctuation-contribution to the grand potential is discussed in detail.
