Intermolecular potential quantum mechanical calculation

The application of ab initio quantum mechanical calculations to determine the guest-host intermolecular potential parameters was performed in a parallel effort by the group of Sandler et al. (Klauda and Sandler, 2000, 2003) and the groups of Trout and Tester et al. (Anderson et al., 2004, 2005). Klauda and Sandler (2005) extended their model to predict in-place hydrate formation in nature. 

Other flexible molecular models of nitromethane were developed by Politzer et al. [131,132]. In these, parameters for classical force fields that describe intramolecular and intermolecular motion are adjusted at intervals during a condensed phase molecular dynamics simulation until experimental properties are reproduced. In their first study, these authors used quantum-mechanically calculated force constants for an isolated nitromethane molecule for the intramolecular interaction terms. Coulombic interactions were treated using partial charges centered on the nuclei of the atoms, and determined from fitting to the quantum mechanical electrostatic potential surrounding the molecule. After an equilibration trajectory in which the final temperature had been scaled to the desired value (300 K), a cluster of nine molecules was selected for a density function calculation from which... 

Strong H2-framework interaction at low H2 loading. The Rietveld profile analysis of NPD on D2 loaded MOF-74 (Fig. 3b) revealed that the D2-D2 intermolecular distances in the first layer vary from 2.85 to 4.2 A with an average of 3.4 A, which is shorter than the 3.6 A found for solid D2 [99], indicating that the packing density of D2 on the surface of MOF-74 can be higher than that of sohd D2. Quantum mechanical calculations also suggested that such short D2-D2 distances are reasonable in the potential well created in MOF-74 (see also Sect. 2.3.6). 

The pair potential functions for the description of the intermolecular interactions used in molecular simulations of aqueous systems can be grouped into two broad classes as far as their origin is concerned empirical and quantum mechanical potentials. In the first case, all parameters of a model are adjusted to fit experimental data for water from different sources, and thus necessarily incorporate effects of many-body interactions in some implicit average way. The second class of potentials, obtained from ab initio quantum mechanical calculations, represent purely the pair energy of the water dimer and they do not take into account any many-body effects. However, such potentials can be regarded as the first term in a systematic many-body expansion of the total quantum mechanical potential (dementi 1985 Famulari et al. 1998 Stem et al. 1999). 

Monte Carlo studies of a dilute aqueous solution of benzene have been reported by Beveridge and coworkers [50] and by Linse et al. [64]. Intermolecular pairwise potential functions determined from quantum mechanical calculations were used for water-water interactions both groups have used pairwise Matsuoka-Clementi-Yoshimine potential. Interesting solvation patterns around benzene have been found. 

VAMP can be used with Tsar to provide quantum mechanically calculated descriptors for QSAR. Apart from molecular properties such as dipole, quadrupole, and octupole moments, ionization potential, electron affinity, polarizability (calculated as a default in VAMP by a variational method), etc., atomic properties such as Coulson-, Mulliken-, or MEP-derived charges, chemical shifts and atomic dipoles and quadrupoles, VAMP can also calculate surface electrostatic descriptors introduced by Politzer, and is useful for QSARs and QSPRs involving intermolecular interactions. Many of these descriptors can be exported directly into Tsar for analysis by classical regression techniques or artificial neural nets. ... 

A key question about the use of any molecular theory or computer simulation is whether the intermolecular potential model is sufficiently accurate for the particular application of interest. For such simple fluids as argon or methane, we have accurate pair potentials with which we can calculate a wide variety of physical properties with good accuracy. For more complex polyatomic molecules, two approaches exist. The first is a full ab initio molecular orbital calculation based on a solution to the Schrddinger equation, and the second is the semiempirical method, in which a combination of approximate quantum mechanical results and experimental data (second virial coefficients, scattering, transport coefficients, solid properties, etc.) is used to arrive at an approximate and simple expression. 

Solvent effects can significantly influence the function and reactivity of organic molecules.1 Because of the complexity and size of the molecular system, it presents a great challenge in theoretical chemistry to accurately calculate the rates for complex reactions in solution. Although continuum solvation models that treat the solvent as a structureless medium with a characteristic dielectric constant have been successfully used for studying solvent effects,2,3 these methods do not provide detailed information on specific intermolecular interactions. An alternative approach is to use statistical mechanical Monte Carlo and molecular dynamics simulation to model solute-solvent interactions explicitly.4 8 In this article, we review a combined quantum mechanical and molecular mechanical (QM/MM) method that couples molecular orbital and valence bond theories, called the MOVB method, to determine the free energy reaction profiles, or potentials of mean force (PMF), for chemical reactions in solution. We apply the combined QM-MOVB/MM method to... 

Because of their importance to nucleation kinetics, there have been a number of attempts to calculate free energies of formation of clusters theoretically. The most important approaches for the current discussion are harmonic models, " Monte Carlo studies, and molecular dynamics calcula-tions. In the harmonic model the cluster is assumed to be composed of constituent atoms with harmonic intermolecular forces. The most recent calculations, which use the harmonic model, have taken the geometries of the clusters to be those determined by the minimum in the two-body additive Lennard-Jones potential surface. The oscillator frequencies have been obtained by diagonalizing the Lennard-Jones force constant matrix. In the harmonic model the translational and rotational modes of the clusters are treated classically, and the vibrational modes are treated quantum mechanically. The harmonic models work best at low temjjeratures where anharmonic-ity effects are least important and the system is dominated by a single structure. 

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