An accurate acetylene intermolecular potential for phase behavior predictions from quantum chemistry

We have developed an ab initio potential for acetylene by computing interaction energies for a range of orientations and center-of-mass separation distances. These energies are initially fit with a simple weighting scheme to a pairwise-additive, site-site Morse-C-6 intermolecular potential. Additional interaction energies were then calculated at separation distances determined to be important from the center-of-mass radial distribution function calculated from molecular simulation with use of the initial potential. The expanded set of interaction energies is then fit using Boltzmann-like weighting to obtain an improved intermolecular potential. The phase behavior calculated from Gibbs ensemble Monte Carlo simulations using this improved potential is in excellent agreement with experimental data. Also, the results of NVT ensemble Monte Carlo calculations show good agreement with experimental data at supercritical temperatures and pressures, and results are presented for conditions that would be hazardous experimentally. Additionally, the second virial coefficients calculated using this potential indicate that one set of experimental data reported in the literature is likely to be erroneous. The prescription described here for obtaining the optimum potential from quantum chemical methods should be applicable to other systems.