Pair-potentials calculations of the 3P1↔1S0 absorption and emission energies of atomic mercury isolated in solid Ar, Kr, and Xe are conducted and compared with the spectral bands recorded in Hg/RG matrices. The Hg⋅RG pair potentials used are derived from spectroscopic studies of the mercury atom–rare gas atom diatomics and are implemented in a localized Hg⋅RG18 cluster model to simulate the spectroscopy of Hg atoms isolated in substitutional sites of the solid rare gases. The calculated absorptions are all on the red wing of the observed matrix bands and from these favorable comparisons, substitutional site occupancy is identified for ground state atomic mercury. A pairwise sum of the Hg(3P1)⋅RG [A 30+(3Π)] and [B 31] state potentials is used to examine the vibronic modes of the excited 3P1 state Hg⋅RG18 clusters which lead to stabilization. The energetics of waist and body vibronic modes, involving motion of the lattice atoms with respect to the excited state mercury atom and motion of this atom in the solid, respectively, were calculated for the three symmetry poles of the cubo-octahedral substitutional sites. Excited state stabilization was found for the waist mode of all the Hg/RG systems in the three possible coordinate systems, i.e., based on the fourfold, threefold, and twofold symmetry systems. In contrast, the body modes were stabilized only in Hg/Xe. The difference between Hg/Xe and the other Hg/RG systems is related to the larger substitutional site size presented by the former system. The three components identified in the recorded emission bands are correlated with the existence of several vibronic modes leading to stabilization. Emission energies calculated for the three stabilized vibronic modes in Ar are centered on the observed emission but exhibit a larger splitting. In Kr they are red of the observed band maximum but occur within the observed band. A curve crossing mechanism is identified which can explain the lack of emission for the strongly stabilized, fourfold symmetry modes in Hg/Xe.