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Title:A theoretical study of molecular crystals
Author(s):Sode, Olaseni
Director of Research:Hirata, So
Doctoral Committee Chair(s):Hirata, So
Doctoral Committee Member(s):Abbamonte, Peter M.; Dykstra, Clifford E.; Makri, Nancy
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):electronic structure theory
molecular crystals
Abstract:A linear-scaling electron-correlation method based on a truncated, electrostatically embedded many-body expansion of energies, named the binary-interaction method (BIM), has been proposed for molecular clusters and molecular crystals. An infinitely extended, periodic, one-dimensional zigzag hydrogen fluoride chain was studied with this method and the energies, structures, harmonic, and anharmonic frequencies of the infrared- and/or Raman-active vibrations, phonon dispersions, and inelastic neutron scattering (INS) were calculated. A systematic hierarchy of methods was applied from Hartree–Fock (HF) to coupled-cluster singles and doubles (CCSD), as well as combining the aug-cc-pVDZ and aug-cc-pVTZ basis sets. Corrections of the basis-set superposition errors (BSSE) were made to increase the accuracy, generating computed structural parameters that agreed very well with the observed parameters. The anharmonic frequencies obtained by vibrational second-order Møller–Plesset (MP2) reproduced the observed frequencies correctly. In a three-dimensional framework of BIM, two configurations of solid hydrogen fluoride are explored. The en- ergies, equilibrium atomic positions, lattice constants, and dipole moments of the two solid structures (polar and nonpolar) were determined. The longer-range two-body Coulomb interactions were included to an infinite distance by computing the Madelung constant. The MP2 method was used in conjunction with the Dunning basis sets to account for electron-correlation, along with BSSE-corrections. The predicted relative energies showed that the non- polar arrangement was considerably more stable than the polar one and the computed lattice constants of the nonpolar configuration also agreed with the observed to within 0.3 Å. A direct extension of BIM to solids under pressure was applied to the antiparallel structure of solid hydrogen fluoride and deuterium fluoride under 0–20 GPa of ambient pressure. The optimized structures, including the lattice parameters and molar volume, and phonon dispersion as well as phonon density of states (DOS), obtained at the MP2 level with aug-cc-pVDZ and aug-cc-pVTZ basis sets, were all determined at finite pressure. The non-coincidence of the infrared and Raman mode pairs, explained as factor-group (Davydov) splitting, was in good agreement with the observed and also largely justified previous vibrational band assignments based on one-dimensional chain models. The hydrogen-amplitude-weighted phonon DOS at 0 GPa was compared to the one-dimensional analogue as well as the observed INS spectra. All major observed peaks were straightforwardly assigned to the calculated peaks in the DOS. The three-dimensional, proton-disordered phase of ice Ih at the MP2 level with an aug-cc-pVDZ basis set and corrections to the BSSE was calculated. The structural and dynamical properties were explained, including the con- troversial hypothesis of two distinct types of hydrogen bonds with strengths differing by a factor of two. The reason for this explanation, two distinct hydrogen-bond-stretching peaks in the INS spectra, was investigated, and it was sug- gested that directionality of the collective hydrogen-bond stretching vibrations lead to the observed spectral features. Infrared and Raman spectra were computed for ice Ih, as well as the variation of INS with deuterium concentration. Low-temperature heat capacities were also computed for the molecular crystal. The solid-phase of carbon dioxide was treated with BIM using MP2 and the aug-cc-pVDZ and aug-cc-pVTZ basis sets at finite pressure. The zero-pressure solid structure agreed to within 0.03 Å for the C–O bond length and to within 0.1 Å for the translational period of the cubic lattice. The infrared, Raman and INS spectra were calculated, and the agreement with the observed is very accurate at nonzero pressures. Anharmonic frequencies were also obtained and the Fermi resonance between of the bending overtone and symmetric stretching fundamental was observed for the theoretical solid. The agreement with the experimental Fermi dyad peak was reproduced correctly at pressures reaching 10 GPa. An embedded fragmentation of vibrational energies was also studied. BIM and its counterpart the unary-interaction method (UIM) were applied to the harmonic zero-point vibrational energies (ZPVE) of clusters and a crystal of hy- drogen fluoride and water clusters. The ZPVE was reproduced accurately by both fragmentation schemes within a few percent of the exact values or a few tenths of 1 kcal mol−1 per molecule. As well, both the monomer- and dimer- based fragmentations were nearly equally accurate and useful for the absolute values of ZPVE, but the latter was more reliable than the former in reproducing the relative ZPVE of cluster isomers of the same size. The embedding field, which renders nonzero frequencies to the translational and rotational motions of monomers and dimers, is essential as it mimics the pseudo-translational and librational motions of the entire clusters or crystals. Imaginary frequencies were not ignored, and in fact, they were treated as estimates of the errors in the real part.
Issue Date:2013-02-03
Rights Information:Copyright 2012 Olaseni Sode
Date Available in IDEALS:2013-02-03
Date Deposited:2012-12

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