From realistic to primitive models: a primitive model of methanol

An attempt to develop a methodology to construct a primitive model which descends directly from a parent realistic short-range model and reproduces its structural properties has been made. The realistic three-site OPLS model of methanol has been chosen as a test case. The primitive model copies the geometry of the OPLS model and thus pictures the methanol molecule as a hard heteronuclear dumbbell (representing oxygen and carbon atoms) with one embedded hydrogen site. All sites interact as hard spheres with the exception of the oxygen-hydrogen pair which may form a hydrogen bond mimicked by a square-well attraction. To determine parameters of the model two routes have been followed: (i) theoretical, based on an effective sphericalized site-site potential obtained from the parent potential, and (ii) semitheoretical which makes use of the knowledge of the structure of the dense parent fluid. Both sets of parameters provide similar results and reproduce the structure (site-site correlation functions, distribution of H-bonds and H-bond geometry) of the parent OPLS fluid reasonably well.