A theoretical model based on long-range dispersion corrections of the charge density functional is proposed for model Hg₂Cl₂ calomel crystals, typical representatives of molecular inorganic compounds where the intermolecular interaction is found to play an important role. This model successfully describes the electronic state and the phonon spectrum of the above crystal, predicts the earlier unstudied phase transition at high hydrostatic pressure. Study of the baric behavior of the phonon spectrum with Raman spectroscopy observes the soft mode in the low-symmetry orthorhombic phase with the frequency softening as the pressure rises. Pressures above 9 GPa considerably transform the Raman spectra, indicating a structural phase transition.