1,403 research outputs found
Structures and Stabilities of Doubly-charged (MgO)nMg2+ (n=1-29) Cluster Ions
Ab initio perturbed ion plus polarization calculations are reported for
doubly-charged nonstoichiometric (MgO)nMg2+ (n=1-29) cluster ions. We consider
a large number of isomers with full relaxations of the geometries, and add the
correlation correction to the Hartree-Fock energies for all cluster sizes. The
polarization contribution is included at a semiempirical level also for all
cluster sizes. Comparison is made with theoretical results for neutral (MgO)n
clusters and singly-charged alkali-halide cluster ions. Our method is also
compared to phenomenological pair potential models in order to asses their
reliability for calculations on small ionic systems. The large
coordination-dependent polarizabilities of oxide anions favor the formation of
surface sites, and thus bulklike structures begin to dominate only after n=24.
The relative stabilities of the cluster ions against evaporation of a MgO
molecule show variations that are in excellent agreement with the experimental
abundance spectra.Comment: Final version accepted in Journal of Chemical Physics; 8 pages plus 8
figures (6 GIFs and 2 PSs). The main difference with respect to the original
submission is the inclusion of coordination-dependent polarizabilities for
oxide anions. That results in substantial changes in the result
Determination of the lowest energy structure of Ag from first-principles calculations
The ground-state electronic and structural properties, and the electronic
excitations of the lowest energy isomers of the Ag cluster are calculated
using density functional theory (DFT) and time-dependent DFT (TDDFT) in real
time and real space scheme, respectively. The optical spectra provided by TDDFT
predict that the D dodecahedron isomer is the structural minimum of
Ag cluster. Indeed, it is borne out by the experimental findings.Comment: 4 pages, 2 figures. Accepted in Physical Review A as a brief repor
Breaking Bonds with the Left Eigenstate Completely Renormalized Coupled-Cluster Method
The recently developed [P. Piecuch and M. Wloch, J. Chem. Phys.123, 224105 (2005)] size-extensive left eigenstate completely renormalized (CR) coupled-cluster (CC) singles (S), doubles (D), and noniterative triples (T) approach, termed CR-CC(2,3) and abbreviated in this paper as CCL, is compared with the full configuration interaction (FCI) method for all possible types of single bond-breaking reactions between C, H, Si, and Cl (except H2) and the H2SiSiH2 double bond-breaking reaction. The CCL method is in excellent agreement with FCI in the entire region R=1â3Re for all of the studied single bond-breaking reactions, whereR and Re are the bond distance and the equilibrium bond length, respectively. The CCL method recovers the FCI results to within approximately 1mhartree in the region R=1â3Reof the HâSiH3, HâCl, H3SiâSiH3, ClâCH3, HâCH3, and H3CâSiH3bonds. The maximum errors are â2.1, 1.6, and 1.6mhartree in the R=1â3Re region of the H3CâCH3, ClâCl, and H3SiâClbonds, respectively, while the discrepancy for the H2SiSiH2 double bond-breaking reaction is 6.6 (8.5)mhartree at R=2(3)Re. CCL also predicts more accurate relative energies than the conventional CCSD and CCSD(T) approaches, and the predecessor of CR-CC(2,3) termed CR-CCSD(T)
Accurate <i>ab initio</i> ro-vibronic spectroscopy of the X<sup>2</sup>∏ CCN radical using explicitly correlated methods
Explicitly correlated CCSD(T)-F12b calculations have been carried out with systematic sequences of correlation consistent basis sets to determine accurate near-equilibrium potential energy surfaces for the X<sup>2</sup>∏ and a<sup>4</sup>Σ<sup>â</sup> electronic states of the CCN radical. After including contributions due to core correlation, scalar relativity, and higher order electron correlation effects, the latter utilizing large-scale multireference configuration interaction calculations, the resulting surfaces were employed in variational calculations of the ro-vibronic spectra. These calculations also included the use of accurate spin-orbit and dipole moment matrix elements. The resulting ro-vibronic transition energies, including the Renner-Teller sub-bands involving the bending mode, agree with the available experimental data to within 3 cm<sup>â1</sup> in all cases. Full sets of spectroscopic constants are reported using the usual second-order perturbation theory expressions. Integrated absorption intensities are given for a number of selected vibronic band origins. A computational procedure similar to that used in the determination of the potential energy functions was also utilized to predict the formation enthalpy of CCN, ΔH<sub>f</sub>(0K) = 161.7 ± 0.5 kcal/mol
Unexpected Magnetism of Small Silver Clusters
The ground-state electronic, structural, and magnetic properties of small
silver clusters, Ag (2n22), have been studied using a linear
combination of atomic Gaussian-type orbitals within the density functional
theory. The results show that the silver atoms, which are diamagnetic in bulk
environment, can be magnetic when they are grouped together in clusters. The
Ag cluster with icosahedral symmetry has the highest magnetic moment per
atom among the studied silver clusters. The cluster symmetry and the reduced
coordination number specific of small clusters reveal as a fundamental factor
for the onset of the magnetism.Comment: 4 pages, 4 figure
Emergence of Bulk CsCl Structure in (CsCl)nCs+ Cluster Ions
The emergence of CsCl bulk structure in (CsCl)nCs+ cluster ions is
investigated using a mixed quantum-mechanical/semiempirical theoretical
approach. We find that rhombic dodecahedral fragments (with bulk CsCl symmetry)
are more stable than rock-salt fragments after the completion of the fifth
rhombic dodecahedral atomic shell. From this size (n=184) on, a new set of
magic numbers should appear in the experimental mass spectra. We also propose
another experimental test for this transition, which explicitely involves the
electronic structure of the cluster. Finally, we perform more detailed
calculations in the size range n=31--33, where recent experimental
investigations have found indications of the presence of rhombic dodecahedral
(CsCl)32Cs+ isomers in the cluster beams.Comment: LaTeX file. 6 pages and 4 pictures. Accepted for publication in Phys.
Rev.
First principles study of the adsorption of C60 on Si(111)
The adsorption of C60 on Si(111) has been studied by means of
first-principles density functional calculations.
A 2x2 adatom surface reconstruction was used to simulate the terraces of the
7x7 reconstruction.
The structure of several possible adsorption configurations was optimized
using the ab initio atomic forces, finding good candidates for two different
adsorption states observed experimentally.
While the C60 molecule remains closely spherical, the silicon substrate
appears quite soft, especially the adatoms, which move substantially to form
extra C-Si bonds, at the expense of breaking Si-Si bonds.
The structural relaxation has a much larger effect on the adsorption
energies, which strongly depend on the adsorption configuration, than on the
charge transfer.Comment: 4 pages with 3 postscript figures, to appear in Surf. Science.
(proceedings of the European Conference on Surface Science ECOSS-19, Sept
2000
Density-Functional-Based Determination of the CH3-CH4 Hydrogen Exchange Reaction Barrier
Due to the overbinding that is inherent in existing {\em local}
approximations to the density-functional formalism, certain reaction energies
have not been accessible. Since the generalized gradient approximation
significantly decreases the overbinding, prospects for density-functional-based
reaction dynamics are promising. Results on the generalized-gradient based
determination of the CH3-CH4 hydrogen exchange reaction are presented.
Including all Born-Oppenheimer effects an energy barrier of 9.5 kcal/Mole is
found which is a very significant improvement over the local-density
approximation.Comment: 5 twocolumn pages (needs twocolumn.sty), revtex, 3 figures, To appear
in Chem.Phys.Let
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