This is a bit of a sensitive topic as the grand potential is the most important property describing open inhomogeneous systems. Therefore it should work in all circumstances including heterosegmented DFT (whether the chains are fused or not is irrelevant). What I wonder is: is there ever a need for a residual grand potential density? I assume to factor out some ideal gas pressure in a non-equilibrium application? In any way we shouldn't have two functions for the same purpose.

For the functions in DFTProfile: There is no need to instantiate a new Convolver. For the entropy function, this is only required because we need a Dual64 convolver. For the grand potential you can just use self.convolver. Also, to reduce redundancy, grand_potential should just call grand_potential_density and integrate it. To have it as a function of DFTProfile is nice to have, because right now the grand potential is calculated e.g. as

// calculate grand potential density
let omega = self
    .profile
    .integrate(&self.profile.dft.grand_potential_density(
        self.profile.temperature,
        &self.profile.density,
        &self.profile.convolver,
    )?);

These instances should be changed, when there is a simpler way available.