  It is shown that a bare simple fluid model (SFM) proposed some years ago for
studying adsorption between two semi-infinite solid walls can be improved by
modifying the surface term in the grand potential for the film phase. Such a
correction substantially improves the agreement between the predictions for
phase transitions provided by that SFM and results obtained from calculations
carried out for $^4$He with the density-functional method at zero temperature.
The corrective term depends on the strength of the adsorption potential and
observables of bulk helium.
