As united atoms

The OPLS model is an example of pair potential where non-bonded interactions are represented through Coulomb and Lennard-Jones terms interacting between sites centred on nuclei (equation (51). Within this model, each atomic nucleus has an interaction site, except CH groups that are treated as united atoms centered on the carbon. It is important to note that no special functions were found to be needed to describe hydrogen bonding and there are no additional interaction sites for lone pairs. Another important point is that standard combining rules are used for the Lennard-Jones interactions such that An = (Ai As )1/2 and Cu = (C Cy)1/2. The A and C parameters may also be expressed in terms of Lennard-Jones o s and e s as A = 4ei Oi and C ... 

Lynden-Bell and coworkers examined the validity of this approximation by simulating systems with fully explicit methyl groups (i.e. aU hydrogens) as well as united atom models. They found that the united atom approximation leads to a higher density in both the liquid and crystalline states. As a consequence, the dynamics of the united atom system are somewhat slower than for the case of the explicit atom model. 

To apply the aufbauprinzip to molecules, two sorts of questions called for clarification. The first concerned the natiue of quantum niunbers appropriate to characterize electrons in molecules and the natiue of closed shells, molecular states, and multiplets. The second concerned binding energies and the type of energy relations resulting from the imion of two atoms. To address the first set of questions, the relation between a molecule and a molecule-as-united-atoms was emphasized. To address the second set of questions, the relation between a molecule and the separated atoms was all important. 

One can almost marvel at the ability and way of those who worked on molecular orbitals to make sense of the molecular spectral data. And, yet, the apparently simplistic approach to "duplicate" Bohr s aufbau program for molecules turned out to be a great success. Furthermore, it adhered to a basic chemical characteristic that of being visualizable. The molecule conceived as "united atoms" helped in visualizing the new bonding mechanisms. Notably, one did not need to make use of the Schrodinger equation, even though the Pauli principle was absolutely crucial. 

The final constant density system considered involves a comparison of SD simulation results for an UA and an EA tridecane system. The systems studied were identical to the system of Ref. 24, with the exception of the structure of the surfaces. Here the surface topography, instead of being structureless, reflected a (111) face of an fee solid with the nearest-neighbor distance between particles being 0.4 nm. The surface particles interacted with the tridecane atoms as united atoms in the UA simulations and as... 

The final constant chemical potential configuration considered represents a film of alkane melts sandwiched between two solid plates in Refs 27 and 28. The system was periodic in the x and y directions, but only a portion of the surfaces in the y direction was occupied by the solid substrate. Constant chemical potential was maintained using the reservoir method, where the liquid bubbles which form at the edge of the substrates are in equilibrium with a vapor phase which interacts across the periodic boundaries. The surfaces were modeled as static surfactant crystalline monolayers which interacted with the alkanes as united atom CH2 and CH3 groups (weakly attractive surface). Both /i-octane and 2-methylheptane systems were studied as a function of surface separation. 

Currently, the GROMOS force field treats aliphatic carbon atoms with the attached hydrogen atoms as united atoms. 

It is useful to define the tenns coverage and monolayer for adsorbed layers, since different conventions are used in the literature. The surface coverage measures the two-dimensional density of adsorbates. The most connnon definition of coverage sets it to be equal to one monolayer (1 ML) when each two-dimensional surface unit cell of the unreconstructed substrate is occupied by one adsorbate (the adsorbate may be an atom or a molecule). Thus, an overlayer with a coverage of 1 ML has as many atoms (or molecules) as does the outennost single atomic layer of the substrate. 

CIIARMM was first developed as a united atom force field and parameters for some amino acids have been published B. R. Brooks et al.. 1 Comp. ( hem.. 4, 1H7 fl9K3). Siihseqiient changes to the functional form and param eters h ave been published W. Reiher, Ph.D.. TIarvard but most recent parameter develop-... 

AMBER was first developed as a united atom force field [S. J. Weiner et al., J. Am. Chem. Soc., 106, 765 (1984)] and later extended to include an all atom version [S. J. Weiner et al., J. Comp. Chem., 7, 230 (1986)]. HyperChem allows the user to switch back and forth between the united atom and all atom force fields as well as to mix the two force fields within the same molecule. Since the force field was developed for macromolecules, there are few atom types and parameters for small organic systems or inorganic systems, and most calculations on such systems with the AMBER force field will fail from lack of parameters. 

Liquid Injection. Liquid injection units are the most common type of incinerator today for the destmction of Hquid hazardous wastes such as solvents. Atomizers break the Hquid into fine droplets (100—150 microns) which allows the residence time to be extremely short (0.5—2.5 s). The viscosity of the waste is very important the waste must be both pumpable and capable of being atomized into fine droplets. Both gases and Hquids can be incinerated in Hquid injection units. Gases include organic streams from process vents and those from other thermal processes in the latter case, the Hquid injection incinerator operates as an afterburner. Aqueous wastes containing less than 75% water can be incinerated in Hquid injection units. 

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