Browsing by Subject "X-Pol"
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Item Development of fragment-based quantum mechanical methods and combined quantum mechanical and molecular mechanical methods(2014-08) Wang, YingjieThis thesis is dedicated to develop fragment-based QM methods and QM/MM schemes: For the first part, the fragment-based explicit polarization (X-Pol) method has been extended in three aspects: I. the inclusion of exchange repulsion terms in the X-Pol model is examined by antisymmetrizing the X-Pol Hartree-product wave function; this yields X-Pol with full eXchange, called X-Pol-X; II. the original X-Pol method, where all fragments are treated using the same electronic structure theory, is extended to a multilevel representations, called multilevel X-Pol, in which different electronic structure methods are used to describe different fragments; III. a fragment-based variational many-body (VMB) expansion method is described to directly account for exchange repulsion, charge delocalization (charge transfer) and dispersion interactions in the X-Pol method. For the second part, a universal QM/MM scheme, the projected hybrid orbital (PHO) method, is proposed to handle the covalent boundary at QM/MM interface at ab initio level with arbitrary basis sets. As an extension to the generalized hybrid orbital (GHO) method in which hybrid orbitals are constructed using the valence orbitals on the boundary atom, the PHO method further represents the core and valence electrons with a secondary, minimal basis set by projecting the original (primary) basis set used in the QM system. The PHO method is then validated on several aspects: geometry optimization, charge population and proton-affinity calculation. Comparison with standard QM results shows that PHO is a robust and balanced QM/MM scheme that yields satisfactory structural, electronic, and energetic properties.Item The explicit polarization theory as a quantum mechanical force field and the development of coarse-grained models for simulating crowded systems of many proteins(2014-01) Mazack, Michael John MorganThis dissertation consists of two parts. The first part concerns the use of explicit polarization theory (X-Pol), the semiempirical polarized molecular orbital (PMO) method, and the dipole preserving, polarization consistent (DPPC) charge model as a quantum mechanical force field (QMFF). A detailed discussion of Hartree-Fock theory and X-Pol is provided, along with expressions for the energy and the analytical first derivative of this QMFF. Test cases for this QMFF with extensive comparisons to experimental data and other models are provided for water (XP3P) and hydrogen fluoride (XPHF), showing that the PMO/X-Pol/DPPC approach discussed in this dissertation is competitive with the most accurate models for those two chemical species over a wide range of chemical and physical properties.The second part of this dissertation concerns the development and application of coarse-grained models for protein dynamics. First, a coarse-grained force field (CGFF) for macromolecules in crowded environments is introduced and described along with a visualization environment for the cartoon-like rendering of biomolecules in vivo. This CGFF is tested against experimental diffusion coefficients for myoglobin (Mb) at a wide range of concentrations, including volume fractions as high as 40%, finding it to be surprisingly accurate for its simplicity and level of coarseness. Second, an analytical coarse-grained (ACG) model for mapping the internal dynamics of proteins into a spherical harmonic expansion is described.