Toxvaerd anisotropic model

Fig 4 43 The interaction energy between the two arrangements shown is equal in a traditional united atom force field but different in the Toxvaerd anisotropic model (Figure adapted from Toxvaerd S 1990 Molecular Dynamics Calculations of the Equation cf State of Alkanes The Journal of Chemical Physics 93-4290-4295 )... 

Abstract Configurational-bias Monte Carlo simulations in the Gibbs ensemble have been carried out to determine the vapor-liquid coexistence curve for a pentadecanoic acid Langmuir monolayer. Two different force fields were studied (i) the original monolayer model of Karaborni and Toxvaerd including anisotropic interactions between alkyl tails, and (ii) a modified version of this model which uses an isotropic united-atom description for the methylene and methyl groups and includes dispersive interactions between the tail segments and the water surface. 

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. 

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