Flexible-monomer potentials

The potentials become still more complicated if the rigid monomer approximation is abandoned. Unfortunately, at the present time very little reliable information is available about flexible-monomer potentials. What is often done is to assume that the flexible potential can be obtained by simply letting the sites centered on atoms to move with atoms, but this assumption has not been truly tested so far. 

Babin, V., Leforestler, C and Paesani, F. (2013). Development of a "first principles" water potential with flexible monomers Dimer potential energy surface, VRT spectrum, and second vlrlal coefficient,. Chem. Theo. Comp. 9, 12, pp. 5395-5403, doi 10.1021/ct400863t, URL http //pubs.acs.org/doi/abs/10.1021/ct400863t. 

Leforestier C, Szalewicz K, van der Avoird A (2012) Spectra of water dimer from a new ab initio potential with flexible monomers. J Chem Phys 137 014305... 

Fig. 2 Coexistence curve for CO2 in the temperature-density plane (a), vapor pressure at coexistence (b), and surface tension versus temperature (c). Dashed curves are the experimental data, while solid curves describe the prediction of the simple (truncated and shifted) LJ model (5), where the critical temperature and density are adjusted to coincide with experiment to fix the two parameters Sgs and as for Fig. 1. Stars and crosses denote the results of [131] for the parameter q Tc) that controls the strength of the quadrupolar interaction being chosen as q Tc) = 0.387 or q(Tc) = 0.47, respectively (see Sect. 2.2). Plus symbols and triangles are the result of atomistic models called EPM and EPM2 [146]. Small circles near the pluses are the results for flexible monomers [146], which give essentially the same results for the thermodynamic properties as the model for rigid molecules. Big circles and squares are simulation results [156] for two ab initio potentials [146,150]. Note that the interaction parameters of the EPM2 models have similarly been rescaled to fit the critical density and temperature of the experiment as done in Fig. 1, and that no prediction for the liquid-vapor surface tension from the atomistic models is available. From Mognetti et al. [131]...

Murdachaew, G., Szalewicz, K., Bukowski, R. (2002). Efficient generation of flexible-monomer intermolecular potential energy surfaces. Physical Review Letters, 88,12320. 

A potential for the water dimer with flexible monomers has recently been developed [206]. The resulting potential energy surface is 12-dimensional, which is at the limits of complexity that one can deal with. About half a million points have been computed (compared to about 2500 in [30] using rigid monomers). Fitting these data required a very substantial effort. The potential will be used in calculations of the spectra of water dimer and of the second virial coefficient for water. 

Erucic acid, H00C(CH2) CH=CH(CH2)jCH, can be economically obtained from rapeseed ana crambe abyssinica oils and is potentially a major source of industrial materials. It can be ozonized to brassylic acid, HOOCCCH ), COOH, which is known to impart flexibility and moisture resistance to nylons. Here preliminary results of a study of brassylic acid as a monomer for polyester resin/melamine resin coatings are described. It is demonstrated that brassylic acid imparts good flexibility to such coatings. It is also shoim that brassylic acid is polymorphic. 

The conformational entropies of copolymer chains are calculated through utilization of semiempirical potential energy functions and adoption of the RIS model of polymers. It is assumed that the glass transition temperature, Tg, is inversely related to the intramolecular, equilibrium flexibility of a copolymer chain as manifested by its conformational entropy. This approach is applied to the vinyl copolymers of vinyl chloride and vinylidene chloride with methyl acrylate, where the stereoregularity of each copolymer is explicitly considered, and correctly predicts the observed deviations from the Fox relation when they occur. It therefore appears that the sequence distribution - Tg effects observed in many copolymers may have an intramolecular origin in the form of specific molecular interactions between adjacent monomer units, which can be characterized by estimating the resultant conformational entropy. 

The monomer-monomer correlation functions of flexible polyelectrolytes exhibit qualitatively the same behavior as those for rod-like molecules. The conformational changes, however, result in more pronounced and shifted peaks. From Fig. 8 we deduce a shift of the peaks of flexible chains to larger distances compared to those of rod-like chains. This is a consequence of a smaller overlap between flexible chains compared to the one between rodlike molecules. Naturally, the effect is most pronounced for densities larger than the overlap densities. The increased peak intensity corresponds to a more pronounced order in the system of flexible chains, and is a result of the more compact structure of a polymer coil. (The structural properties of flexible polyelectrolytes without medium-induced potential have been studied in [48].)... 

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