Dale A. Braden

Pseudospectral time-dependent density functional theory

Time-dependent density functional theory (TDDFT) is implemented within the Tamm-Dancoff approximation (TDA) using a pseudospectral approach to evaluate two-electron repulsion integrals. The pseudospectral approximation uses a split representation with both spectral basis functions and a physical space grid to achieve a reduction in the scaling behavior of electronic structure methods. We demonstrate here that exceptionally sparse grids may be used in the excitation energy calculation, following earlier work employing the pseudospectral approximation for determining correlation energies in wavefunction-based methods with similar conclusions. The pseudospectral TDA-TDDFT method is shown to be up to ten times faster than a conventional algorithm for hybrid functionals without sacrificing chemical accuracy.

Inelastic neutron scattering spectra of the longitudinal acoustic modes of the normal alkanes from pentane to pentacosane

The inelastic neutron scattering spectra of the polycrystalline n-alkanes, 5<n<25, have been obtained at ∼25 K. At this temperature the n-alkane molecules are extended in their all-trans configuration. The spectral region from 200 to 600 cm−1 is reported with emphasis on the higher frequency longitudinal acoustic modes (LAM). These modes are identified by their nominal translational symmetry eigenvector, κ. A complete assignment of experimental frequencies out to κ≈0.7 is presented for the first time. This assignment is consistent with the known optical data and ab initio Hartree–Fock and DFT calculations. On the basis of these theoretical results, the lower frequency, higher κ LAM modes are also assigned. The LAM sequences exhibit bandhead behavior, i.e., a maximum frequency value, vmax, at some value of κ. The maximum frequency is consistently found at κmax=0.36±0.01 [cf. polyethylene (PE), κmax=0.34] for all chain lengths but the value of the maximum frequency, vmax, increases roughly linearly with n f...

Pseudospectral Local Second-Order Møller-Plesset Methods for Computation of Hydrogen Bonding Energies of Molecular Pairs.

We present a methodology for computing the binding energy of molecular dimers based on extrapolation of pseudospectral local second-order Moller-Plesset (MP2), or PS-LMP2, energies to the basis set limit. The extrapolation protocol is based on carrying out PS-LMP2 calculations with the Dunning cc-pVTZ (-f) and cc-pVQZ (-g) basis sets and then using a simple two-parameter function to compute the final basis set limit results. The function is parametrized to ultralarge basis set MP2 calculations for 5 molecular pairs taken from the literature and then tested by calculating results for a set of formamide dimers for which such calculations have also been carried out. The results agree to within ca. 0.2 kcal/mol with the conventional MP2 large basis set calculations. A specialized, but relatively simple, protocol is described for eliminating noise due to overcompleteness of the basis set. Timing results are presented for the LMP2 calculations, and comparisons are made with the LMP2 methodology of the QChem program. CPU time required by each of the methods scales as N(3), where N is the number of the basis functions, with the PS-LMP2 approach displaying a 2- to 3-fold advantage in the prefactor. We also discuss one set of test cases for which the PS-LMP2 results disagree with those obtained from an alternative type of MP2 calculation, N-methyl acetamide (NMA) dimers, and show that the results for liquid-state simulations using polarizable parameters derived by fitting to the PS-LMP2 binding energies appear to produce better results when compared with experimental data. The convergence issues associated with the alternative MP2 formulation remain to be investigated.

Nuclear-magnetic-resonance shielding constants calculated by pseudospectral methods

We have developed an algorithm based upon pseudospectral (PS) ab initio electronic structure methods for evaluating nuclear magnetic shielding constants using gauge-including atomic orbitals (GIAOs) in the spin-restricted and spin-unrestricted formalisms of Hartree-Fock (HF) theory and density-functional theory (DFT). The nuclear magnetic shielding constants for both 1H and 13C calculated using PS methodology for 21 small molecules have absolute mean errors of less than 0.3 ppm in comparison with analytic integral results. CPU timing comparisons between PS methods and conventional methods carried out for seven large molecules ranging from 510 to 1285 basis functions demonstrate that the PS methods are an order of magnitude more efficient than the conventional methods. PS-HF was between 9 and 26 times faster than conventional integral technology, and PS-DFT (Becke three-parameter Lee-Yang-Parr) was between 6 and 21 times faster.

The Vibrational Inelastic Neutron Scattering Spectrum of Dodecahedrane: Experiment and DFT Simulation

Vibrational frequencies that are forbidden in Raman and IR absorption can be observed by inelastic neutron scattering. In the case of the I(h)-symmetrical molecule dodecahedrane (shown schematically), the resulting spectrum agrees with a DFT calculation.

Aqueous Coordination Chemistry of H2: Why is Coordinated H2 Inert to Substitution by Water in trans-Ru(P2)2(H2)H+-type Complexes (P2 = a Chelating Phosphine)?

The reactivity of a series of trans-Ru(P(2))(2)Cl(2) complexes with H(2) was explored. The complexes reacted with H(2) via a stepwise H(2) addition/heterolysis pathway to form the trans-[Ru(P(2))(2)(H(2))H](+) dihydrogen complexes. Some of the resulting eta(2)-H(2) complexes were surprisingly inert to substitution by water, even at concentrations as high as 55 M; however, the identity of the bidentate phosphine ligand greatly influenced the lability of the coordinated eta(2)-H(2) ligand. With less electron-donating phosphine ligands, the H(2) ligand was susceptible to substitution by H(2)O, whereas with more electron-rich phosphine ligands, the H(2) ligand was inert to substitution by water. Density functional theory (DFT) calculations of the ligand substitution reactions showed that the Ru-H(2) and Ru-H(2)O complexes are very close in energy, and therefore slight changes in the donor properties of the bidentate phosphine ligand can inhibit or promote the substitution of H(2)O for H(2).

The inelastic incoherent neutron spectrum of crystalline oxamide: experiment and simulation of a solid

Abstract The inelastic neutron scattering spectrum of crystalline oxamide, CO(NH 2 )–CO(NH 2 ), is reported and compared with the results of Hartree–Fock and density functional theory calculations on a pentamer cluster model and the results of Car–Parrinello molecular dynamics calculations for the periodic crystal. All four hydrogen atoms of this centrosymmetric, planar sheet molecular crystal are involved in CO–H–N hydrogen bonds. It is shown that all three of these simulations provide reasonable descriptions of the inelastic neutron scattering spectra and thus of the hydrogen bond dynamics. There is no evidence for unusual structural or dynamic effects beyond those typically associated with H-bond formation. It is argued that a polymolecular approach is needed in order to provide an adequate treatment of such strongly hydrogen bonded systems.

Experimental test of the validity of the use of the n-alkanes as model compounds for polyethylene

n-Alkanes are widely used as model compounds for the dynamics of the crystalline region of polyethylene; this use relies on the assumption of the transferability of the forcefield of the n-alkanes, a stringent test of this assumption is a comparison of the dispersion curves of the n-alkanes with those of polyethylene; the only direct measurements of the dispersion curves of polyethylene are for the ν5 acoustic branch, the longitudinal acoustic mode (LAM), an in-plane skeletal mode of perdeuterated polyethylene; in the present work we have observed (by inelastic neutron scattering spectroscopy) and assigned (by the use of density functional theory calculations) the LAM modes of perdeuterated n-hexadecane in order to provide the first experimental test of the validity of the assumption.

On the Unusual Synclinal Conformations of Hexafluorobutadiene and Structurally Similar Molecules

An explanation is presented for the unusual conformations of some molecules that contain the C═C-C═C core, namely, butadienes, biphenyls, and styrenes. Small substituents often induce a synclinal conformation, which brings the substituents into close proximity, and sometimes, there is no anticlinal minimum at all. This would not be predicted from steric repulsion arguments nor would it be expected that atoms that are nonbonded in a Lewis structure would approach closer than the sum of their van der Waals radii. Atomic energies calculated according to the quantum theory of atoms in molecules (QTAIM) do not show a consistent pattern for these structurally similar molecules, nor are intersubstituent bond paths consistently found, nor favorable diatomic interaction energies calculated using the interacting quantum atoms (IQA) scheme. Instead, the synclinal conformations are found to be driven by the attraction energy of the electron distribution of the carbon atoms and the nuclei of the molecule.