We have developed a theoretical diagnostic for interpreting the infrared spectroscopy of clusters containing on the order of 10² molecules. The linear variation method has been used to describe the infrared-active asymmetric stretching vibrations in large gas-phase CO₂ clusters and in monolayer CO₂ on NaCl(100). A basis set with v= 0 and v= 1 harmonic oscillator vibrational wavefunctions and a Hamiltonian containing a transition dipole–transition dipole coupling term has been chosen to describe these systems. The modelling of gas-phase CO₂ clusters was accomplished using multiples of the cubic unit cell of solid CO₂ to generate the overall dimensions of a particular cluster size. A comparison of the simulated spectra for many cluster sizes with experiments conducted by Ewing and Sheng (G. E. Ewing and De T. Sheng, J. Phys. Chem., 1988, 92, 4063) have led to a qualitative understanding of the observed absorption features. As an example of a two-dimensional cluster study, Berg and Ewing (O. Berg and G. E. Ewing, Surf. Sci., 1989, 220, 207) have determined the structure of a monolayer of CO₂ on NaCl(100). Our theoretical diagnostic has allowed us to model their infrared absorption polarization spectra successfully.