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Title:Embedded-fragment studies of ice, water, and molecular cluster cations
Author(s):Salim, Michael A
Director of Research:Hirata, So
Doctoral Committee Chair(s):Hirata, So
Doctoral Committee Member(s):Makri, Nancy; Murphy, Catherine; Trinkle, Dallas
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Electronic structure theory
Molecular dynamics
Molecular crystals
Abstract:The embedded-fragment scheme known as the binary interaction method (BIM) was extended to enable ab initio second-order many-body perturbation (MP2) theory simulations of molecular liquids and solids with nonorthogonal lattice vectors. Three related anomalies of water ice Ih were investigated with BIM-MP2. The negative thermal expansion and pressure-induced amorphization of ice Ih were computationally observed and linked to hydrogen bond-bending acoustic phonons that soften with increasing pressure. The anomalous volume isotope effect, where D2O possesses a larger molar volume than H2O ice, was determined to be a more complex phenomenon relying on cancellations between volume-shrinking and expanding contributions from several anharmonic mode groups. In application of BIM-MP2 to the Gibbs free energy of other ice phases, it was found that basis sets larger than aug-cc-pVDZ are essential to predict reasonable lattice energies and bulk moduli of the ices. While the smaller aug-cc-pVDZ basis set predicts a nearly backwards lattice energy ordering among the ices Ih, II, III/IX, V, and VI, the order is largely corrected by moving towards the complete basis set limit (CBS). The bulk modulus is also systematically reduced to accurate values, although density continues to be overestimated by BIM-MP2 with the atomic embedding field. The phase diagram of solid water proves too difficult even for a qualitatively correct prediction by BIM-MP2/CBS, although the small free energy differences between phases may be improved by a simple increase of BIM lattice truncation radii or disorder-averaging lattice energy over proton configurations of the H-disordered ices. BIM-MP2 with spin component scaling was used to generate on-the-fly forces for ab initio molecular dynamics of liquid water at constant volume and temperature. The simulated structure of the first and second coordination shells, as judged by the oxygen-oxygen radial distribution, is in excellent agreement with experiment. This requires a slightly lower than ambient temperature of 250 K and fixed density, however, resulting in an average pressure of -0.6 GPa. The significant negative pressure exerted to maintain the experimental density of water is related to the overestimated dispersion interactions of MP2. The simulated diffusivity, infrared, and Raman spectra (especially intensity and line width) were in reasonable agreement with the observed. The difference between isotropic and anisotropic Raman components was traced to the diverse hydrogen-bonding environments experienced by the waters, which is corroborated by the broad distribution of monomer geometries and induced molecular dipole moments. The strong dispersion interaction of BIM-MP2 stabilizes the occupation of a fifth non-hydrogen-bonded location relative to each tetra-coordinated water molecule. Hence, the water structure is significantly less ice-like and the probability of tetra- and penta-coordination were found to be nearly equal. The transient attachment, detachment, and exchange of this weakly bound fifth molecule seems to to play a large role in fluctuations of the hydrogen bonding network. Because the viability of BIM and related fragment methods depends on weakly interacting monomers with integer electron counts, the methodology has been restricted to molecular crystals, leaving systems with significant electron delocalization out of reach. As a first step towards handling significant charge transfer in a BIM-like framework, the valence bond charge transfer (VBCT) method was proposed for molecular cluster cations. VBCT approaches construction of an effective Hamiltonian in a basis of charge-localized configurations, which are intuitively amenable to fragmentation. The Hamiltonian matrix elements are directly defined in terms of embedded-fragment calculations of monomers and dimers with the goal of approximating the cation ground state energy. VBCT was applied to the UHF and UMP2 energies of several helium, argon, ethylene, water, and water-sodium clusters to evaluate its performance. The preliminary results are generally encouraging, although erratic behaviors are observed and related to the UHF symmetry breaking in significantly multireference systems. In regions of the potential energy surface where charge is delocalized or localized without the existence of near-degenerate UHF solutions, VBCT can reproduce UHF and UMP2 ground state energies quite reliably.
Issue Date:2017-07-10
Rights Information:Copyright 2017 Michael Salim
Date Available in IDEALS:2017-09-29
Date Deposited:2017-08

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