Pressure (P) and temperature (T) effects on (quasi)homogeneous optical spectra of dyes in solvent glasses and polymers were investigated by hole burning. The frequency-dependent P- and T-induced shift and broadening of zero-phonon holes burned in the inhomogeneous spectra are rationalized using two-body Lennard-Jones potentials. The difference between the excited-state potential (U*) and the ground-state potential (U-g) yields the absolute vacuum-to-matrix shift as a function of the intermolecular coordinate. The P- and T-shifts can be described in terms of differences between the derivatives U*' and U-g' and the ratios of second derivatives U*" and U-g", respectively. The P-shift increases and the T-shift decreases as the optical transition energy increases. The Lennard-Jones model reproduces this opposite frequency dependence very well, by assuming that, in the excited state, the potential well minimum is displaced to shorter distances (sigma* < sigma(g)). The force constant of a 6-12 potential vanishes at a distance exceeding the equilibrium value only by 11%. This feature explains qualitatively the phonon mode softening in the presence of a free volume in glasses or defect solids.