Aristides D. Zdetsis

A comparative ab initio study of the Si2C4, Si3C3, and Si4C2 clusters

Various structural possibilities for the Si2C4 and Si4C2 clusters are investigated by employing a basis set of triple‐zeta plus polarization quality; electron correlation is generally accounted for by second‐order Mo/ller–Plesset and, in certain instances, by higher‐order perturbation (CASPT2) approaches. The building‐up principle recently suggested from an analysis of Si3C3 clusters is found to be fully operative for Si2C4 and Si4C2 clusters. A comparison of the structure and stability of various geometrical arrangements in the series C6, Si2C4, Si3C3, Si4C2, and Si6 shows that linear and planar structures become rapidly less stable if carbons are replaced by silicons and that the three‐dimensional bipyramidal forms become less favorable as soon as silicons are exchanged by carbons in the parent Si6 structure. The effects can be rationalized in qualitative terms based on differences in silicon and carbon bonding.

Analogy of silicon clusters with deltahedral boranes: How far can it go? Reexamining the structure of Sin and Sin2−, n=5–13 clusters

Silicon clusters of 5 up to 13 atoms, Si(n), n=5-13, and their dianions are studied in the light of an anticipated analogy with the corresponding isoelectronic boranes suggested recently by Zdetsis [J. Chem. Phys. 127, 014314 (2007)]. It is demonstrated that this analogy is a fruitful and powerful concept which allows the straightforward determination of the structures of silicon clusters, based on the structure of corresponding closo-boranes, meeting the requirements of well known structural rules. All lowest-lying structures of Si(n), n=5-13 clusters, have been obtained through a systematic way on the basis of this analogy. For magic clusters, such as Si(6) and Si(10), characterized by special stability, the analogy to boranes seems to be much stronger.

Theoretical study of the Si3C2 cluster

Abstract Using ab initio calculations based on geometry optimizations at the MP2/DZ2P level various structural and bonding features of the Si3C2 system have been investigated. The energies of the MP2 optimized structures are calculated using singles and doubles coupled cluster (CCSD) theory and the CCSD(T) method. The results show that the structure of lowest energy is a C2v pentagon. This planar structure is stabilized against competing three-dimensional geometries by strong SiC bonds, in accordance with the stability criteria we have suggested earlier. The harmonic frequencies and isotopic shifts of this planar ground state structure are also calculated at the MP2/DZ2P level.

Importance of multicenter bonding in the structure of Si3C3

Abstract We have investigated the nature of the chemical bonding in the ground state of the Si 3 C 3 cluster on the ab initio level. In this state the molecule shows a pyramid-like structure of C s geometry, which is stabilized against competing structures by the formation of the multicenter bonds. This finding is supported by two kinds of population analysis. In order to facilitate future experimental characterization, we have also calculated 1s chemical shifts, the dipole moment, harmonic vibrational frequencies and IR intersites for this system.

Stabilization of flat aromatic Si6 rings analogous to benzene: Ab initio theoretical prediction

It is shown by ab initio calculations, based on density functional (DFT/B3LYP), and high level coupled-cluster [CCSD(T)] and quadratic CI [QCISD(T)] methods, that flat aromatic silicon structures analogous to benzene (C6H6) can be stabilized in the presence of lithium. The resulting planar Si6Li6 structure is both stable and aromatic, sharing many key characteristics with benzene. To facilitate possible synthesis and characterization of these species, routes of formation with high exothermicity are suggested and several spectral properties (including optical absorption, infrared, and Raman) are calculated.

High accuracy calculations of the optical gap and absorption spectrum of oxygen contaminated Si nanocrystals

We report accurate high level calculations of the optical gap and absorption spectrum of small Si nanocrystals, with hydrogen and oxygen at the surface. Our calculations have been performed in the framework of time dependent density functional theory (TDDFT) using the hybrid nonlocal exchange and correlation functional of Becke and Lee, Yang and Parr (B3LYP). The accuracy of these calculations has been verified by the high level multi-reference second order perturbation theory. The effect of oxygen contamination is studied by considering several different bonding configurations of the surface oxygen atoms. We show that for nanocrystals of sizes smaller than 20 angstroms, the widening of the gap due to quantum confinement facilitates the stabilization of Si[double bond, length as m-dash]O double bonds. For this type of bonding, the oxygen related states determine the value of the optical gap and make it significantly lower compared to the corresponding gap of oxygen-free nanocrystals. For diameters larger than 20 angstroms, the double bonds delocalize inside the valence band. We find that for small amounts of oxygen, the size of the optical gap depends strongly on their relative distribution and bonding type, while it is practically insensitive to the exact number of oxygen atoms.

AB INITIO STUDY OF ELECTRONIC, STRUCTURAL, AND VIBRATIONAL PROPERTIES OF THE SI4C CLUSTER

Using Moller–Plesset second‐order perturbation theory (MP2) for the geometry optimizations, we have examined various structural possibilities for the Si4C cluster. The energies of the MP2‐optimized structures have been calculated using singles and doubles coupled cluster (CCSD) theory and the CCSD (T) method. The structure of lowest energy is a C3V symmetric trigonal pyramid made from four silicons and one carbon atom in a face capping position. Very close in energy (around 5 kcal/mol) lies an isomer with C2V symmetry, resembling the pyramid of the previous structure but with the carbon atom in an edge capping position this time. Both of these structures are closely related to the Si5 ground state structure. Planar and linear structures analogous to C5 and C4 lie higher in energy and they are transition states in most of the cases examined. To help future experimental tests of our present results, we have computed, at the MP2‐level, the harmonic frequencies, infrared intensities, and isotopic shifts for t...

Ab initio investigation of the stability of Si3C3 clusters and their structural and bonding features

Various structural possibilities for Si3C3 clusters are investigated by ab initio calculations employing basis sets of double- and triple-zeta quality augmented by d polarization functions. Correlation effects are included by a second-order Moeller Plesset perturbation treatment. For the two lowest-lying structures higher-order correlation corrections and multi-reference effects are also included. Bonding features are investigated by two different types of population analyses to obtain insight into the nature of chemical bonding. A total of 17 stationary points were investigated, 14 of which correspond to local minima and three being transition states. The energetically lowest-lying structures are: A “pyramidlike” structure with various multicenter bonds, followed by a Cs symmetric isomer closely related to the ground state Si6 structure. Planar structures, favoured in small carbon clusters, lie higher in energy and are transition states. The lowest-lying triplet system is found to be the linear nonsymmetric Si-C-C-C-Si-Si structure, which is calculated to lie about 38 kcal/mole above the singlet ground state. A building-up principle based on bonding criteria is suggested for the occurence of the various structural possibilities.

Structural and static electric response properties of highly symmetric lithiated silicon cages: Theoretical predictions

It is shown by density functional theory calculations that high symmetry silicon cages can be designed by coating with Li atoms. The resulting highly symmetric lithiated silicon cages (up to D5d symmetry) are low‐lying true minima of the energy hypersurface with binding energies of the order of 4.6 eV per Si atom and moderate highest occupied molecular orbital–lowest unoccupied molecular orbital gaps. Moreover, relying on a systematic study of the electric response properties obtained by ab initio (Hartree–Fock, MP2, and configuration interaction singles (CIS)) and density functional (B3LYP, B2PLYP, and CAM‐B3LYP) methods, it is shown that lithium coating has a large impact on the magnitude of their second hyperpolarizabilities resulting to highly hyperpolarizable species. Such hyperpolarizable character is directly connected to the increase in the density of the low‐lying excited states triggered by the interaction between the Si cage and the surrounding Li atoms.

The structure of C6Si

Ab initio calculations including coupled cluster CCSD(T) and multi-reference configuration interaction treatments predict the linear C6Si chain to be more stable by about 5 kcal/mol than the ring isomer with C2v symmetry. Electron correlation effects are found to be much larger for the compact ring structure than for the extended linear chain. Approximate values for the first electronically excited states and for vibrational IR frequencies are given to guide spectroscopic detection of this molecule.