TraPPE united-atom

Van der Waals non bonded potential parameters are obtained by fitting some experimentally observed properties. At present, the limited capacity of computers prevents us from using efficiently all atoms descriptions (AA potential), in which each atom is represented by a separate Lennard-Jones center. Consequently, several authors proposed united atom (UA) potential, such as the NERD potential (Nath et al., 1998) or the TraPPE model (Martin and Siepmann, 1998), where a group like CH2 or CHj is represented by a single force center, this approach was pioneered in the early work by Jorgensen (1984) with its OPLS model. While each force center is located on the carbon in the more classic UA potentials, it is shifted in the Anisotropic United Atoms (AUA) potential proposed by Toxvaerd (1990) for n-alkanes, so that it is placed between the carbon and the hydrogen atoms of the related group. Here, the AUA formalism is used. 

The united atom model combines each carbon and its bonded hydrogen atoms, CHx, into a single interaction site. This reduces the number of interaction sites compared to explicit atom models where all H atoms are included. Most of the polyolefin results presented here are for the united atom model known as TraPPE developed by Martin and Siepmann [55, 56]. This model has been optimized to give correct results for... 

Fig. 5. The intemiolecular radial distribution functions obtained from SC/PRISM theory (lines) and MD simulations (points) for a system of 3200 united atom polyethylene chains with 48 CHx sites per chain at a density 0.03282 at the temperatures indicated. All results are for a repulsive Lennard-Jones nonbond potential with the TraPPE parameters in Table 1. The curves were displaced vertically for clarity...

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