Methylene group force fields

Because of the restricted availability of computational resources, some force fields use United Atom types. This type of force field represents implicitly all hydrogens associated with a methyl, methylene, or methine group. The van der Waals parameters for united atom carbons reflect the increased size because of the implicit (included) hydrogens. 

Yet another way to minimize the number of parameters required is to adopt a so-called united-atom (UA) model. That is, instead of defining only atoms as the fundamental units of the force field, one also defines certain functional groups, usually hydrocarbon groups, e.g., methyl, methylene, aryl CH, etc. The group has its own single set of non-bonded and other parameters - effectively, this reduces the total number of atoms by one less than the total number incorporated into the united atom group. 

All-atom force fields consider every atom as an individual interaction site, while united-atom force fields gather different atoms of a functional group into one interaction site, e.g., as is often done to model methyl or methylene groups. To describe chain-Uke polymers or proteins, coarse grained force fields are also employed, where the interaction sites usually represent a larger number of atoms. 

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. 

The direction of the equilibrium isotope shifts in the averaged spectra can be correlated with the shift and intensity ratio of the two types of methylene in the frozen out spectra. The upheld shift of the averaged CHj-carbons in the deuteriated cations towards the one methylene group at —2.8 ppm and the corresponding downfield shift of the deuteriated methylene carbon shows that the mean of the force constants of the two C—H bonds at the high field carbon ( — 2.8 ppm) must be lower than those of the C—H bonds at the two low field methylene carbons. 

The simulations in Ref. 22 were performed using the UA approximation (see above) with the methods and the force field already described. The same 6-12 potential was used for the mutual interactions of the methyl and methylene groups and for their interactions with the atomic units comprising the solid surfaces. A total of 1297 tridecane molecules was introduced in the basic cell one at a time in such a way that two nonbonded units (including those of the solid surfaces) could not come within 0.32 run from each other. The system was then equilibrated at 300 K using the reptation technique. The results shown in the next sections represent the average over 10 different equilibrium models obtained in sequence and separated by more than 800000 accepted reptations. It was verified that the tridecane molecules were able to diffuse during the simulation from the narrow slits to the wider slits and vice versa. 

Both SD and MD simulations of alkanes melts confined by solid surfaces (solid/liquid interfaces) and MD simulations of liquid alkanes at free surfaces (liquid/vapor interfaces) have been performed. The alkane molecules were represented by realistic atomistic force fields with constrained bond lengths. In all cases except Refs 29 and 30, the bond angle flexibility was maintained and in all cases the torsional flexibility was maintained. In most simulations the methyl and methylene groups were represented by single, spherically symmetric Lennard-Jones (LJ) force centers, i.e., the united atom (UA) approximation. Results from simulations which explicitly include the pendant hydrogen atoms as individual force centers, which we refer to as the explicit atom (EA) representation, will also be discussed. 

---