Molecular pair method

Molecular mechanics methods achieve good structural accuracy for classical molecules, whereas their reliability for species with particular combinations of atoms may be questionable, particularly for molecules containing heteroatoms, which affect geometry and conformation via the position of their lone-pairs. Force-field programs, for example, often fail to calculate the geometry... 

More advanced semiempirical molecular orbital methods have also been used in this respect in modeling, e.g., the structure of a diphosphonium extractant in the gas phase, and then the percentage extraction of zinc ion-pair complexes was correlated with the calculated energy of association of the ion pairs [29]. Semiempirical SCF calculations, used to study structure, conformational changes and hydration of hydroxyoximes as extractants of copper, appeared helpful in interpreting their interfacial activity and the rate of extraction [30]. Similar (PM3, ZINDO) methods were also used to model the structure of some commercial extractants (pyridine dicarboxylates, pyridyloctanoates, jS-diketones, hydroxyoximes), as well as the effects of their hydration and association with modifiers (alcohols, )S-diketones) on their thermodynamic and interfacial activity [31 33]. In addition, the structure of copper complexes with these extractants was calculated [32]. 

In typical organic crystals, molecular pairs are easily sorted out and ab initio methods that work for gas-phase dimers can be applied to the analysis of molecular dimers in the crystal coordination sphere. The entire lattice energy can then be approximated as a sum of pairwise molecule-molecule interactions examples are crystals of benzene [40], alloxan [41], and of more complex aziridine molecules [42]. This obviously neglects cooperative and, in general, many-body effects, which seem less important in hard closed-shell systems. The positive side of this approach is that molecular coordination spheres in crystals can be dissected and bonding factors can be better analyzed, as examples in the next few sections will show. 

DFT for the pair energies in the coordination shell of the nitroguanidine crystal. The picture is instructive because the molecular pairs where uncorrected DFT gives the worst errors (pairs E and L) are dispersion-dominated stacked pairs. Not only are total energies nearly identical in DFT/D and PIXEL, but also the dispersion contributions are nearly identical, lending mutual support to the evaluation of the sum of Coulombic-polarization and repulsion terms in the two methods, as well as further validation to the PIXEL parameterization. 

Condon s treatment is the prototype of the molecular-orbital method of discussing the electronic structure of molecules. In this method a wave function is formulated that involves the introduction of a pair of electrons in an electron orbital that extends about two or more atomic nuclei. 

Theoretical chemistry at UBC was further strengthened with the arrival of Delano Chong and Keith Mitchell in 1965 and 1966, respectively. Chong s interests in quantum chemistry have spanned the full range from semiempirical to ab initio molecular orbital methods. His long-standing interests in perturbation methods and constrained variations have figured prominently in his publications. He is probably best known for his attempts to calculate the X-ray and UV photoelectron spectra of molecules, often by means of perturbation corrections to Koopmans theorem.40 More recently he has shifted his focus to coupled pair functional methods and density functional methods, with a special interest in polarizabilities and hyperpolarizabilities.41... 

Classical molecular simulation methods such as MC and MD represent atomistic/molecular-level modeling, which discards the electronic degrees of freedom while utilizing parameters transferred from quantum level simulation as force field information. A molecule in the simulation is composed of beads representing atoms, where the interactions are described by classical potential functions. Each bead has a dispersive pair-wise interaction as described by the Lennard-Jones (LJ) potential, ULj(Ly) ... 

In this article, we shall discuss studies of phase changes in solids carried out by the application of the generalized Monte Carlo and the molecular dynamics methods. This topic is of particular significance because of the recent modifications of the method to include both variation in size and shape of the simulation cell. What is especially gratifying is that we are now able to make meaningful predictions of phase transitions in real solids by employing reliable pair potentials. Besides phase transitions of molecular solids, we shall examine phase transi-... 

COMPARISON OF THE ELECTRON PAIR AND MOLECULAR ORBITAL TREATMENTS The essential feature of the molecular orbital method is the complete freedom of movement of the electrons in the molecular orbitals. This means that one electron has no influence on the location of another electron and hence the probability of finding simultaneously one electron at a point 1, 1, -Cl and another at the point - 2 product of the... 

I think the problem was that nobody understood the two-electron bond until Heitler and London s research in 1926. Then Hund and Mulliken developed the molecular orbital method. It s true that Lewis lived much beyond 1926, but Lewis himself did not understand the nature of the electron-pair bond in his early research. Failing that, I think his time sort of passed. It may have been a good idea to give him the prize but I can understand why they didn t, because Lewis himself certainly didn t formulate the physical basis of the bond. 

Preferred geometry of the benzene oxide-oxepin system can be predicted by molecular orbital methods. Thus benzene oxide la is predicted to be markedly non-planar (with the epoxide ring at an angle of 73° to the benzene ring), while the oxepin lb has been predicted to prefer a shallow boat structure (MINDO/3) or a planar structure ab initio) As previously mentioned, the proportion of each tautomer present at equilibrium is both temperature and solvent-dependent. Molecular orbital calculations have been used to rationalize the solvent effects, both in terms of the more polar character of the arene oxide that is favored in polar solvents and the strengthening of the oxirane C-C bond upon coordination of the oxygen atom lone pair in polar solvents. Thus values in the range 1.5-2.0 D and 0.76-1.36 D for the dipole moments of arene oxide la and oxepin lb have been calculated. 

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