Molecular dynamics simulations and integral equation theory of alkane chains: Comparison of explicit and united atom models

Molecular dynamics (MD) simulations and self-consistent polymer reference interaction site model (PRISM) calculations were performed on C20H42 and C48H98 liquids using an explicit atom (EA) model. Good agreement between theory and simulation was found for the intermolecular pair correlation functions gHH (r)and gCH(r), but the PRISM theory tended to underestimate gCC(r). Good agreement was likewise found between theory and simulation with neutron diffraction measurements of gHH(r). MD and PRISM theory calculations were also performed on a united atom (UA) potential obtained from the EA potential by the UA/EA mapping procedure of McCoy and Curro [Macromolecules 1998, 31, 9362], Excellent agreement between the liquid structure and chain dimensions from the UA and EA models was found. However, the full UA potential obtained from the UA/EA mapping exhibited attractions that were somewhat too strong as evidenced by a negative pressure and high compressibility. An empirical modification of the attractive tail of this UA potential leads to a modified UA potential that reproduces the EA pressure.