We describe quantum-chemical calculations on dimers of CO(2) and use the results to develop first-principles models for Gibbs ensemble Monte Carlo simulations of the phase coexistence curve. Isotropic pairwise potentials are insufficient to model the phase behavior and overestimate the binding in liquid CO(2) by 4 kJ mol(-1). An anisotropic treatment of the atoms in the pairwise potential reduces the strength of the binding by approximately 0.5 kJ mol(-1). We use ab initio calculations on trimers of CO(2) to assess the strength of nonadditive interactions. Including nonadditive dispersion in Gibbs ensemble simulations gives an enthalpy of vaporization within 1.5 kJ mol(-1) of the experimental value over a wide range of temperatures.