Brett I. Dunlap

The Gaussian-Type Orbitals Density-Functional Approach to Finite Systems

The linear combination of Gaussian-type orbitals (LCGTO) approach to Xa and density functional theory is reviewed, with particular emphasis on applications to large molecules and clusters. Fitting the potential is central to the LCGTO approach, and efficient and accurate ways to do so are described. Model cluster calculations apply these methods to the adsorption of alkali atoms and carbon monoxide on transition metal surfaces as well as the problem of CO vibrational shifts upon alkali coadsorption.

Fitting the Coulomb potential variationally in Xα molecular calculations

The method of Mintmire and Dunlap which variationally fits the Coulomb potential rather than the charge density is extended to linear combination of Gaussian‐type orbitals Xα calculations on molecules. This approach is more efficient than fitting the charge density since the same integrals are used to treat the Coulomb and exchange potentials. Furthermore, the required Fock matrix elements are simple overlap rather than Coulomb integrals. However, the method is somewhat less accurate than fitting the charge density in a test on H2.

Ion molecule reactions of carbon cluster ions with D2 and O2

The ion/molecule chemistry of laser‐generated carbon cluster ions (C+n; n=3–19) has been studied using Fourier transform mass spectrometry. The ion/molecule reactions of mass‐selected carbon cluster ions with D2 and O2 have been studied and reaction rates measured. Reactions of the primary product ions with D2 and O2 were also studied. The change in reactivity observed as a function of cluster size suggests a structural change from linear to monocyclic rings in the cluster ions between n=9 and 10. Evidence for the presence of two structural isomers for the C+7 cluster ion has also been observed and has been attributed to the existence of both a linear and cyclic form of the ion. MNDO calculations have also been used to obtain structural and thermodynamic information on possible reactant and product ions in an attempt to explain the differences observed in the ion/molecule reactions. The results from collision induced dissociation studies of the carbon cluster ions are also discussed.

Variational fitting methods for electronic structure calculations

We review the basics and the evolution of a powerful and widely applicable general approach to the systematic reduction of computational burden in many-electron calculations. Variational fitting of electron densities (either total or partial) has the great advantage, for quantum mechanical calculations, that it respects the stationarity property, which is at the heart of the success of the basis set expansion methods ubiquitous in computational chemistry and materials physics. The key point is easy. In a finite system, independent of whether the fitted charge distribution is constrained to contain the proper amount of charge, variational fitting guarantees that the quantum mechanical total energy retains the stationarity property. Thus, many-electron quantum mechanics with variational fitting of an electronic density in an incomplete density-fitting basis set behaves similarly as the exact quantum mechanical energy does when evaluated with an incomplete basis set to fit wavefunctions or spin-orbitals. Periodically bounded systems are a bit more subtle but the essential stationarity is preserved. This preservation of an exact property is quite distinct from truncation of the resolution of the identity in a basis. Variational fitting has proven to have benefits far beyond the original objective of making a Gaussian-orbital basis calculation of an early density functional computationally feasible. We survey many of those developments briefly, with guidance to the pertinent literature and a few remarks about the connections with Quantum Theory Project.

Fractional occupation numbers and density functional energy gradients within the linear combination of Gaussian-type orbitals approach

Abstract Gradients of the total energy with respect to nuclear displacements using fractionally occupied orbitals within a linear combination of Gaussian-type orbitals density-functional theoretical appraoch are discussed. Expressions for the gradient in the fractional occupation number solution as well as for Fermi and Gaussian broadening are presented and related to previously described methods.

The role of cluster ion structure in reactivity and collision‐induced dissociation: Application to cobalt/oxygen cluster ions in the gas phase

Ion/molecule reaction products of cobalt cluster ions have been characterized using mass spectrometric techniques. Atomic and bare‐metal cluster ions were desorbed from foils by particle bombardment within a high‐pressure (0.1–0.2 Torr) ion source. Sputtered metal cluster ions react with O2 to produce abundant stoichiometric or nearly stoichiometric cobalt(II) oxide cluster ions. The positive cluster product ions consist of three types: oxygen‐deficient [Co(CoO)x]+ clusters, oxygen‐equivalent [(CoO)x]+ clusters, and (in less abundance) metal‐deficient [(CoO)xO]+ clusters. Tandem mass spectrometry and collision spectroscopy provide structural information about the more abundant cobalt cluster product ions. A major collision‐induced fragmentation pathway for the oxygen‐equivalent [(CoO)x]+ clusters is the loss of a CoO moiety to form [(CoO)x−1]+ fragments. A major collision‐induced fragmentation pathway for the oxygen‐deficient [Co(CoO)x]+ clusters is the loss of a cobalt atom to yield [(CoO)x]+ fragments. ...

Basis set effects on spectroscopic constants for C2 and Si2 and the symmetry dilemma in the Xα model

In an attempt to perform thorough and accurate linear combination of atomic orbitals (LCAO) Xα, calculations on C2 and Si2, the effects of basis set composition on the computed spectroscopic constants are investigated for the first time. The questions of how to tell if an adequate basis set has been found and what Xα state corresponds to the lowest state of 1Σ+g symmetry which involves strong electron correlation effects are addressed. With the larger bases, general agreement with experiment and previous density functional calculations are obtained. Remarkable agreement with the configuration interaction (CI) calculations of Bruna et al. on Si2 is obtained if the fractional occupation number method of Slater et al. is used.

Basis sets in the LCAO Xα method. On the use of bond-centered basis functions in second-row homonuclear diatomics

Abstract A series of LCAO (GTO) Xα calculations on the model system Si 2 has been performed in an attempt to establish standardized basis sets for molecules containing second-row atoms. In contrast to previous investigations, bond-centered orbital basis functions turned out to be unnecessary. The effect of bond-centered auxiliary basis functions was found to be rather minor. Spectroscopic constants for the lowest state of certain symmetries were obtained with an accuracy comparable to that of current CI investigations. Calculations on Si 2 + , Al 2 and Al 2 + confirmed this finding.

Static dielectric response of icosahedral fullerenes from C60 to C2160 characterized by an all-electron density functional theory

The static dielectric response of C60, C180, C240, C540, C720, C960, C1500, and C2160 fullerenes is characterized by an all-electron density-functional method. First, the screened polarizabilities of C60, C180, C240, and C540, are determined by the finite-field method using Gaussian basis set containing 35 basis functions per atom. In the second set of calculations, the unscreened polarizabilities are calculated for fullerenes C60 through C2160 from the self-consistent Kohn-Sham orbitals and eigen-values using the sum-over-states method. The approximate screened polarizabilities, obtained by applying a correction determined within linear response theory show excellent agreement with the finite-field polarizabilities. The static dipole polarizability per atom in C2160 is (4 Angstrom^3) three times larger than that in C60 (1.344 Angstrom^3). Our results reduce the uncertainty in various theoretical models used previously to describe the dielectric response of fullerenes and show that quantum size effects in polarizability are significantly smaller than previously thought.

Secondary ion mass spectrometry (SIMS) of metal halides. IV. The envelopes of secondary cluster ion distributions

The relative secondary ion intensities of cluster ions of the type [M(MI)n]+ and [I(MI)n]− from the fast atom bombardment mass spectrometry (FABMS) of alkali iodides (MI) for n<100 are presented, and mass-resolved mass spectra of cesium iodide (CsI) clusters exceeding m/z 25 000 are reported. The FABMS spectra are qualitatively consistent with previously reported secondary ion mass spectrometry (SIMS) spectra. The envelopes of the extended mass spectra are fitted to the distributions predicted by a model based on random bond breaking. The bond-breaking model (BBM) distributions fit well on the overall envelope of both the positive and negative ion sodium iodide (NaI, simple cubic) and CsI (body-centered cubic) cluster ion distributions, when Bethe lattices of the appropriate coordination number are used to approximate the true lattices. Ion abundance enhancements at certain “magic numbers” in the envelopes can be predicted by the BBM; however, these enhancements and the reduced ion abundance immediately following the “magic number” are consistent with experimentally observed unimolecular, or more appropriately, unicluster decompositions. Finally, we contrast secondary cluster ion distributions with those obtained using other cluster sources and we discuss in detail possible origins of the large cluster ions observed in these studies.