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PSCF allows user to model polymer solutions by optionally including one or more "solvent" species among the components of a mixture. Each solvent molecule is treated as a mobile but structureless point-like particle that occupies some specified volume within an incompressible mixture. Solvent species can be used to model either rigid small-molecule solvent molecules or oligomeric chain molecules that are much smaller than any of the larger "polymer" species present in the same system, and smaller than any spatial feature of interest.

Each solvent species is assigned a monomer type that is selected from the same list of monomer types as that used to construct polymer species. A solvent that has properties that are distinct from those of any of the monomers used to construct the relevant polymer species can be assigned an extra monomer type that is not also used as the monomer type for any polymer block. Alternatively, a solvent species can be assigned a monomer type that is also assigned to one or block polymer blocks to represent a solvent that is a small oligomer of the specified monomer type, or another species with properties similar to those of such an oligomer. The input file formats used by PSCF require that every monomer type be assigned a statistical segment length. If a monomer type is only assigned to a solvent species, however, that segment length value is simply never used.

Each solvent species is also assigned a steric volume. This volume is specified in the PSCF input files by a dimensionless parameter named "size". The value of the size parameter is equal to the ratio of the solvent volume to the monomer reference volume \( v \). The "size" parameter for a solvent species is thus analogous to the "length" parameter for a polymer block in a thread model, since both are defined as volumes measured in units of the monomer reference volume. The size parameter for a solvent is always specified as a real (i.e., floating point) number, whether the bead or thread model is used for polymer species.

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