Condensed-phase optimized molecular studies

Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies. 

A computational model based on molecular dynamics was developed to predict the miscibility of indomethacin in the carriers polyethylene oxide (PEO), glucose, and sucrose (Gupta et al. 2011). The cohesive energy density and the solubility parameters were determined by simulations using the condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) force field. The simulations predicted miscibility for indomethacin with PEO (A5 < 2), borderline miscibility with sucrose (A5 < 7), and immiscibility with glucose (A5 > 10 Table 2.2). 

COMPASS = Condensed-phase optimized molecular potentials for atomistic simulation studies... 

Looking at the structure of HMR, it is clear that it occupies several sites on the surface. Some optimized adsorption structures on Pd(lll) surface modeled by molecular mechanics (MM) with the COMPASS (Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies) force field are presented in Fig. 7.9. As the force fields cannot properly model chemisorption and, consequently, exact structures of the adsorbed molecules, the role of the metal surface in the MM calculations was mainly to act as a steric constraint. Depending on the adsorption structure, HMR covers 10-20 Pd surface atoms. 

The reason that atom-atom potentials are so popular, especially in the study of condensed phases [34] and more complex Van der Waals molecules [35], is that they contain few parameters and can be cheaply calculated, while they still describe (implicitly) the anisotropy of the intermolecular potential and they even model its dependence on the internal molecular coordinates. Moreover, they are often believed to be transferable, which implies that the same atom-atom interaction parameters in Eq. (6) can be used for the same types of atoms in different molecules. One should realize, however, that the accuracy of atom-atom potentials is limited by Eq. (5). Further inaccuracies are introduced when the atom-atom interaction parameters in Eq. (6) are transferred from one molecular environment to another. Furthermore, Eq. (6) does not include a term which represents the induction interactions and there is the intrinsic problem that these interactions are inherently not pairwise additive (see Sect. 1.4). Numerical experimentation on the C2H4-C2H4 and N2-N2 potentials, for example, has taught us [31, 33] that even when sufficient ab initio data are available, so that the terms in Eq. (6) can be fitted individually to the corresponding ab initio contributions and, moreover, the positions of the force centers for each term can be optimized, the average error in the best fit of each contribution still remains about 10%. Since the different contributions to the potential partly cancel each other... 

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