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The synthesis and structure of a neutral SiA114 cluster is described.

A new study shows that molecular electronic circular dichroism can be investigated using adiabatic time-dependent density functional theory. A summary of the theory is included and its extension for the calculation of rotatory strengths is outlined.

Research was conducted to determine the applicability of time dependent density functional theory in the optical absorption measurements of fullerenes. Specifically, the approach was used to study electronic transitions for C60(Ih), C70(D5h), C76(D2), C78(C2v), C78(C2v'), C78(D3), C78(D3h), C78(D3h') and C80(D2). Results indicated that the level of agreement is adequate to permit the assignment of dominant spectral attributes.

Dynamic electron correlation and other ab initio calculations help determine the structure of various small copper-selenium clusters and their energetics. These exist as isomers with linear or trigonal planar coordination among the molecules bonded to Cu. The steric shielding and the kinetic stability depend on the ligands covering the clusters. These clusters are not energetically stable, and this is evident from bonding energy values per monomer unit. The structure of the clusters depends on the bonding between ligands. The steric interactions influence the formation of these isomers.

Ab initio investigation using dynamical electron correlation is employed to determine the geometrics and energetics of ethylene insertion in the homogeneous Ziegler-Natta catalysis. The result is compared with the results from Molher-Plesset second-order perturbation method and the local density function method. Agostic interactions influence the results, and the mechanism suggested by Brookhart and Green is supported by the results.

Diphosphanylcarbene prefers the asymmetric PE multiple-bond structure over the symmetric carbene structure. A multiple-bond equilibrium structure accompanied by carbene reactivity should result from the comparatively negligible energy difference between the carbene and PC multiple-bond structures along with vast differences in geometry and electronic structure. Disphosphanylcarbenes are distinct alkylidynephosphoranes.

Theoretical NMR shift calculations were carried out for a variety of Al compounds at the SCF and MP2 levels. The computed values were rationalized in terms of a molecular orbital picture of the bonding and compared with available experimental data for Al compounds. Results indicate that chemical shift calculations are very useful for the interpretation of experimental spectra of unknown compounds if there are no other experimental NMR data available.

Detailed studies on 32 phosphorus and 7 phosphane species, ranging in size up to P28, reveal that many large clusters are energetically stable with reference to P4. The characterization of clusters on the basis of equilibrium structures and NMR chemical shieldings is completed by vibrational spectra studies. The potential structural units of red phosphorus are determined, and a possible route of its formation from white phosphorus suggested.

The structures of ruthenium cluster anions have been investigated using a combination of trapped ion electron diffraction and density functional theory computations in the size range from eight to twenty atoms. In this size range, three different structural motifs are found: Ru8--Ru12 - have simple cubic structures, Ru13--Ru16- form double layered hexagonal structures, and larger clusters form close packed motifs. For Ru17-, we find hexagonal close packed stacking, whereas octahedral structures occur for Ru18--Ru20-. Our calculations also predict simple cubic structures for the smaller clusters Ru4--Ru7-, which were not accessible to electron diffraction measurements [ABSTRACT FROM AUTHOR]

A systematic and unbiased structure search based on a genetic algorithm in combination with density functional theory (DFT) procedures has been carried out to locate low-energy isomers of Gan up to n = 25. For the smaller clusters up to n = 8 results are checked by coupled cluster singles and doubles with perturbative triples corrections (CCSD(T)) employing a quadruple zeta type basis set. The CCSD(T) calculations confirm a 3Πu ground state for the dimer. Ga3 has a doublet ground state 0.2 eV below two quartet states, whereas two isoenergetic triplet states are predicted for Ga4 with D4h and a rhombus structure (D2h). Three low-lying isomers with doublet electronic states are found for Ga5: a W-structure (C2v), a planar envelope (Cs) at 0.015 eV, and a non-planar envelope (C1) 0.086 eV above the ground state. A triplet state for a trigonal prism (D3h) and a singlet for an open prism (C2v) are computed with virtually identical energy for Ga6. The global minimum for Ga7 is a capped trigonal prism (Cs) and that for Ga8 a distorted cube in D2h. DFT provides a fair agreement with CCSD(T), deviations in dissociation energies are up to 0.2 eV for n <= 8. The structures for Gan are mostly irregular for n >= 9, those for Ga12 to Ga17 can be derived from the truncated decahedron with D5h symmetry though highly distorted by Jahn-Teller effects, for example. For Ga18 to Ga23 we find stacks of five- and six-membered rings as global minima, e.g., 5-1-5-1-6 for Ga18. Ga24 and Ga25 consist of layers with packing sequence ABCBA similar to those found for clusters of aluminum. The most important feature of computed cohesive energies is a rapid increase with n: for Ga25 it reaches 2.46 eV, the experimental bulk value is 2.84 eV. Particularly stable clusters for Gan are seen for n = 7, 14, and 20. [ABSTRACT FROM AUTHOR]

We present results of molecular electronic structure treatments of multireference configuration interaction (MRCI) type for clusters Aln and Snn in the range up to n = 4, and of coupled cluster singles and doubles with perturbative triples corrections (CCSD(T)) type in the range up to n = 10. Basis sets of quadruple zeta size are employed, computed energy differences, such as cohesive energies, Ecoh, or dissociation energies for the removal of a single atom, De, differ from the complete basis set limit by only a few 0.01 eV. MRCI and CCSD(T) results are then compared to those obtained from density functional theory (DFT) treatments, which show that all computational procedures agree with the general features of De and Ecoh. The best agreement of DFT with CCSD(T) is found for the meta-GGA (generalized gradient approximation) TPSS (Tao, Perdew, Staroverov, Scuseria) for which De differs from CCSD(T) by at most 0.15 eV for Aln and 0.21 eV for Snn. The GGA PBE (Perdew, Burke, Ernzerhof) is slightly poorer with maximum deviations of 0.23 and 0.24 eV, whereas hybrid functionals are not competitive with GGA and meta-GGA functionals. A general conclusion is that errors of De and/or energy differences of isomers computed with DFT procedures may easily reach 0.2 eV and errors for cohesive energies Ecoh 0.1 eV. [ABSTRACT FROM AUTHOR]

By a combination of gas phase ion mobility measurements and relativistic density functional theory calculations with inclusion of spin-orbit coupling, we assign structures of lead cluster cations and anions in the range between 4 and 15 atoms. We find a planar rhombus for the tetramer, a trigonal bipyramid for the pentamer, and a pentagonal bipyramid for the heptamer, independent of charge state. For the hexamer, the cation and anion structures differ: we find an octahedron for the anion while the cation consists of fused tetrahedra. For the octamer, we find in both cases structures based on the pentagonal bipyramid motif plus adatom. For the larger clusters investigated we always find different structures for cations and anions. For example, Pb12- is confirmed to be a hollow icosahedron while Pb12+ is a truncated filled icosahedron. Pb13+ is a filled icosahedron but Pb13- is a hollow icosahedron with the additional atom capping a face. In order to get experimental information on the relative stabilities, we investigated the collision induced dissociation mass spectra for the different cluster sizes and charge states, and observe a strong correlation with the calculated fragmentation energies. Up to n = 13 the main fragmentation channel is atom loss; for the larger cluster sizes we observe fission into two large fragments. This channel is dominant for larger anions, less pronounced but clearly present for the cations. [ABSTRACT FROM AUTHOR]

We employ a combination of ion mobility measurements and an unbiased systematic structure search with density functional theory methods to study structure and energetics of gas phase tin cluster cations, Snn+, in the range of n = 3-15. For Sn13+ we also carry out trapped ion electron diffraction measurements to ascertain the results obtained by the other procedures. The structures for the smaller systems are most easily described by idealized point group symmetries, although they are all Jahn-Teller distorted: D3h (trigonal bipyramid), D4h (octahedron), D5h (pentagonal bipyramid) for n = 5, 6, and 7. For the larger systems we find capped D5h for Sn8+ and Sn9+, D3h (tricapped trigonal prism) and D4d (bicapped squared antiprism) plus adatoms for n = 10, 11, 14, and 15. A centered icosahedron with a peripheral atom removed is the dominant motif in Sn12+. For Sn13+ the calculations predict a family of virtually isoenergetic isomers, an icosahedron and slightly distorted icosahedra, which are about 0.25 eV below two C1 structures. The experiments indicate the presence of two structures, one from the Ih family and a prolate C1 isomer based on fused deltahedral moieties. [ABSTRACT FROM AUTHOR]

The gas phase structures of tin cluster anions Snn- have been studied by a combination of trapped ion electron diffraction and density functional theory calculations. In the size range of n=18–25 these clusters comprise dimers of stable subunits. In particular Sn18- and Sn20- are homodimers of Sn9 and Sn10 subunits, respectively. In Sn23- two Sn10 units are linked by three additional bridging atoms and Sn25- is a heterodimer of Sn10 and Sn15 subunits. This rather unexpected growth mode is rationalized by the extraordinary stability of the building blocks Sn9, Sn10, and Sn15. [ABSTRACT FROM AUTHOR]

A systematic density functional study has been performed for neutral and singly charged clusters of aluminum with up to 34 atoms. A thorough search for global minimum structures has been carried out for Aln employing genetic algorithm and basin-hopping procedures. For Aln this confirms results of previous investigations up to n=22; new global minima have been located for n=23–31, 33. Structures for singly charged cations and anions have been obtained by reoptimization of the pool of 40 low-energy structures of the neutral clusters. The global minima of charged and neutral clusters are always low-spin states with the possible exception of a triplet state of Al28, which is isoenergetic with a singlet. The cluster structures are mostly quite irregular and do not resemble fractions of the fcc bulk phase. High symmetries are found only for the global minimum of Al23 and the triplet state of Al28. [ABSTRACT FROM AUTHOR]

We compare two procedures to gain efficiency by approximating two-electron integrals in molecular electronic structure calculations. The first one is based on a Cholesky decomposition (CD) of two-electron integrals, the second one on the use of preoptimized auxiliary or fitting basis sets employed in a “resolution of the identity” (RI) technique. We present and test auxiliary bases for approximating the Coulomb term, which further improves accuracy over previously proposed fitting bases. It is shown that RI methods lead to insignificant errors only, which are partly comparable to or even better than that of CD treatments; but RI procedures are superior in speed. CD methods have certain advantages, however, particularly for extended basis sets. [ABSTRACT FROM AUTHOR]

The structures and energetics of small tin cluster Snn- anions up to n=15 were determined by a combination of density-functional theory and three different experimental methods: Ion mobility spectrometry, trapped ion electron diffraction, and collision induced dissociation. We find compact, quasispherical structures up to n=12. Sn12- is a slightly distorted hollow icosahedron while Sn13- to Sn15- have prolate structures, consisting of merged, hollow, in part incomplete, deltahedral subunits: Sn13- consists of a face-sharing pentagonal bipyramid and tricapped trigonal bipyramid, Sn14- comprises a face-sharing dicapped trigonal prism and capped square-antiprism, and Sn15- consists of two face-sharing tricapped trigonal prisms. [ABSTRACT FROM AUTHOR]

An approach to describe heteroatomic clusters AnBN-n as perturbed homoatomic ones is presented. By first treating the homoatomic systems AN or/and BN and subsequent application of first-order perturbation theory it is possible to estimate relative stabilities of the 2N possible distributions of the atom types A and B at the N atomic sites in a very efficient manner. The approach was tested considering IrnPt13-n as an example (treated with density functional methods). One observes good correlation between relative stabilities estimated from the homoatomic cases and those obtained from explicit treatments of the binary systems. Moreover we rationalize the observed correlation of atom type and atom position in IrnPt13-n. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Electronic (one-photon) photodepletion spectra were recorded for gold cluster anions complexed with one xenon atom over the photon energy range 2.1–3.4 eV. Clusters were generated by pulsed laser vaporization and probed under collisionless molecular beam conditions. The spectra obtained are highly structured with the narrowest features—assigned to individual electronic transitions—having bandwidths of less than 40 meV. Time-dependent density functional theory predictions of optically allowed transitions for the most stable—planar—isomers of the corresponding bare metal cluster anions are generally consistent with the experimental observation. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present Gaussian basis sets of quadruple zeta valence quality with a segmented contraction scheme for atoms H to Kr. This extends earlier work on segmented contracted split valence (SV) and triple zeta valence (TZV) basis sets. Contraction coefficients and orbital exponents are fully optimized in atomic Hartree–Fock (HF) calculations. As opposed to other quadruple zeta basis sets, the basis set errors in atomic ground-state HF energies are less than 1 mE[sub h] and increase smoothly across the Periodic Table, while the number of primitives is comparably small. Polarization functions are taken partly from previous work, partly optimized in atomic MP2 calculations, and for a few cases determined at the HF level for excited atomic states nearly degenerate with the ground state. This leads to basis sets denoted QZVP for HF and density functional theory (DFT) calculations, and for some atoms to a larger basis recommended for correlated treatments, QZVPP. We assess the performance of the basis sets in molecular HF, DFT, and MP2 calculations for a sample of diatomic and small polyatomic molecules by a comparison of energies, bond lengths, and dipole moments with results obtained numerically or using very large basis sets. It is shown that basis sets of quadruple zeta quality are necessary to achieve an accuracy of 1 kcal/mol per bond in HF and DFT atomization energies. For compounds containing third row as well as alkaline and earth alkaline metals it is demonstrated that the inclusion of high-lying core orbitals in the active space can be necessary for accurate correlated treatments. The QZVPP basis sets provide sufficient flexibility to polarize the core in those cases. All test calculations indicate that the new basis sets lead to consistent accuracies in HF, DFT, or correlated treatments even in critical cases where other basis sets may show deficiencies. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A new computational approach is presented that allows for an accurate and efficient treatment of the electronic Coulomb term in density functional methods. This multipole accelerated resolution of identity for J (MARI-J) method partitions the Coulomb interactions into the near- and far-field parts. The calculation of the far-field part is performed by a straightforward application of the multipole expansions and the near-field part is evaluated employing expansion of molecular electron densities in atom-centered auxiliary basis sets (RI-J approximation). Compared to full RI-J calculations, up to 6.5-fold CPU time savings are reported for systems with about 1000 atoms without any significant loss of accuracy. Other multipole-based methods are compared with regard to reduction of the CPU times versus the conventional treatment of the Coulomb term. The MARI-J approach compares favorably and offers speedups approaching two orders of magnitude for molecules with about 400 atoms and more than 5000 basis functions. Our new method shows scalings as favorable as N[SUP1.5], where N is the number of basis functions, for a variety of systems including dense three-dimensional molecules. Calculations on molecules with up to 1000 atoms and 7000 to 14 000 basis functions, depending on symmetry, can now be easily performed on single processor work stations. Details of the method implementation in the quantum chemical program TURBOMOLE are discussed. [ABSTRACT FROM AUTHOR]

This work presents theory, implementation, and validation of excited state properties obtained from time-dependent density functional theory (TDDFT). Based on a fully variational expression for the excited state energy, a compact derivation of first order properties is given. We report an implementation of analytic excited state gradients and charge moments for local, gradient corrected, and hybrid functionals, as well as for the configuration interaction singles (CIS) and time-dependent Hartree-Fock (TDHF) methods. By exploiting analogies to ground state energy and gradient calculations, efficient techniques can be transferred to excited state methods. Benchmark results demonstrate that, for low-lying excited states, geometry optimizations are not substantially more expensive than for the ground state, independent of the molecular size. We assess the quality of calculated adiabatic excitation energies, structures, dipole moments, and vibrational frequencies by comparison with accurate experimental data for a variety of excited states and molecules. Similar trends are observed for adiabatic excitation energies as for vertical ones. TDDFT is more robust than CIS and TDHF, in particular, for geometries differing significantly from the ground state minimum. The TDDFT excited state structures, dipole moments, and vibrational frequencies are of a remarkably high quality, which is comparable to that obtained in ground state density functional calculations. Thus, yielding considerably more accurate results at similar computational cost, TDDFT rivals CIS as a standard method for calculating excited state properties in larger molecules. [ABSTRACT FROM AUTHOR]

A combined experimental and theoretical study of small gold cluster anions is performed. The experimental effort consists of ion mobility measurements that lead to the assignment of the collision cross sections for the different cluster sizes at room temperature. The theoretical study is based on ab initio molecular dynamics calculations with the goal to find energetically favorable candidate structures. By comparison of the theoretical results with the measured collision cross sections as well as vertical detachment energies (VDEs) from the literature, we assign structures for the small Au[SUP-,SUBn] ions (n < 13) and locate the transition from planar to three-dimensional structures. While a unique assignment based on the observed VDEs alone is generally not possible, the collision cross sections provide a direct and rather sensitive measure of the cluster structure. In contrast to what was expected from other metal clusters and previous theoretical studies, the structural transition occurs at an unusually large cluster size of twelve atoms. [ABSTRACT FROM AUTHOR]

We have performed ion mobility measurements on gold cluster cations Au[sub n][sup +] generated by pulsed laser vaporization. For clusters with n<14, experimental cross sections are compared with theoretical results from density functional calculations. This comparison allows structural assignment. We find that room temperature gold cluster cations have planar structures for n=3–7. Starting at n=8 they form three dimensional structures with (slightly distorted) fragments of the bulk phase structure being observed for n=8–10. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The results of density functional theory potential surface investigations are presented for small ZnS, ZnSe, CdS, and CdSe clusters, up to the heptamer. Clusters of CdSe are investigated further by geometry optimizations of larger crystal structure sections with up to 198 atoms that correspond to the wurtzite modification. In addition to structures and energies of these clusters, we present electron affinities, ionization potentials as well as electronic excitation spectra, obtained from time dependent density functional theory. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Infrared (IR) absorption and ab initio studies probe the bonding mechanism of the cocondensation between copper(I) chloride (CuCl) and carbon monoxide in cryogenic argon matrix. Two IR bands at 2157 and 362 (cycles)/cm suggest a linear Cu(CO)Cl molecule. Polarization function (CPF) and second-level Moller-Plesset (MP2) calculations of the Cu-C, Cu-Cl, Cu-O and C-O distances and of the reaction energies tend to confirm this geometry. The Cu-C bond is formed lagely by electron transfer from the ligand's highest sigma orbital to the d-sigma orbital of copper, with a little pi-back-bonding from Cu to C.

Interesting features of the nitrogenase cofactor, which includes short Fe-Fe distances in an electron deficient MoFe7S9 core, are presented. Density functional theory (DFT) calculations were carried out on model clusters to explore the Fe-Fe interactions in the cofactor.

Density functional theory (DFT) studies of the nitrogenase, to explore the origin and determine the nature of the central light atom and its importance as a structure-stabilizing feature are presented. The results of these studies and a possible mechanism of nitrogenase function are examined.

Analyzes the structure and bonding in neutral and cationic sodium monoxides and dioxides. Calculations on structural isomers, adiabatic ionization potentials and sodium bond energies; Details on the factors leading to outer shell sodium; Analysis on monoxide and superoxide properties; Rationalization of the dioxides.

Examines the automatic generation of natural internal coordinates in all cases of fused ring systems based on geometric and mathematical optimization. Achievement of the automatic generation by a relatively simple code; Treatment of most geometrical cases with a single subroutine; Combination of natural internal coordinates with delocalized coordinated.

The calculations on the crystal structure of magnesium alanate are presented. By quantum chemical calculations on the products of hydrogen desorption MgH2, Al, and H2, the hydrogen desorption enthalpy as well as the formation enthalpy were determined.

Accurate geometries and harmonic force fields for trans- and cis-azobenzene determined by second-order Moller-Plesset perturbation theory is reported. Both isomers report UV spectra calculated using the second-order approximate couple-cluster singles-and-doubles model CC2 with accurate basis sets.

The stability conditions for restricted closed shell Kohn–Sham density functional treatments are derived. In close analogy to the Hartree–Fock method we obtain the equivalent of singlet and triplet instabilities. The nonreal instability now becomes trivial and just expresses the aufbau principle. Demonstrative applications confirm the expected increased stability of density functional treatments as compared to Hartree–Fock. This is an improvement but the unphysical bifurcations do persist. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]