Abstract
In this work, suitable mathematical relationships to compute isobaric heat capacities from molecular simulations in the Grand Canonical (GC) ensemble are derived and tested via Monte Carlo methods. Using atomistic classical force fields, the residual isobaric heat capacities of pure carbon dioxide (CO2) and pure methanol (MeOH) were obtained at supercritical conditions (with critical properties estimated from a finite-size scaling analysis). The total isobaric heat capacity was determined by combining the residual isobaric heat capacity obtained from molecular simulations with the ideal gas contributions obtained from experimental correlations. Isobaric heat capacities generated from both GC and Isothermal–Isobaric ensemble simulations were compared to predictions from accurate equations of state (EOS)s for CO2 and MeOH at corresponding reduced temperatures and pressures. Isobaric heat capacities calculated from both ensembles were in good agreement with those obtained from the Span and Wagner EOS for CO2 and the IUPAC EOS for MeOH. For comparable computation times, simulations run in the GC ensemble generate results with significantly lower statistical uncertainty than those run in the Isothermal–Isobaric ensemble.
Acknowledgements
The authors are grateful to computing cycles provided by the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the National Science Foundation grant number ACI-1053575. We also acknowledge the support provided by the National Science Foundation’s IGERT program grant.