Mauro Stener

From clusters to bulk: A relativistic density functional investigation on a series of gold clusters Aun, n=6,…,147

A series of gold clusters spanning the size range from Au6 through Au147 (with diameters from 0.7 to 1.7 nm) in icosahedral, octahedral, and cuboctahedral structure has been theoretically investigated by means of a scalar relativistic all-electron density functional method. One of the main objectives of this work was to analyze the convergence of cluster properties toward the corresponding bulk metal values and to compare the results obtained for the local density approximation (LDA) to those for a generalized gradient approximation (GGA) to the exchange-correlation functional. The average gold–gold distance in the clusters increases with their nuclearity and correlates essentially linearly with the average coordination number in the clusters. An extrapolation to the bulk coordination of 12 yields a gold–gold distance of 289 pm in LDA, very close to the experimental bulk value of 288 pm, while the extrapolated GGA gold–gold distance is 297 pm. The cluster cohesive energy varies linearly with the inverse o...

Time dependent density functional theory of core electrons excitations

Abstract The Time Dependent Density Functional Theory (TD-DFT) method implemented in the ADF program has been extended to treat core electrons excitations. The scheme consists to reduce the complete one-electron excited configurations space to the subspace where only the core electrons are excited. The scheme has been applied to the Ti 1s, Ti 2p and Cl 2p core excitations of TiCl4, employing different basis sets and exchange correlation potentials. The comparison with the experimental data is good, especially for the Ti 2p shell which cannot be described even qualitatively by the too simple Kohn–Sham method. Also the Cl 2p shell, dominated by Rydberg features, is properly described. The method is computationally economic, and can be applied to larger and less symmetric systems. Further extensions to the relativistic case, with explicit spin–orbit treatment, are suggested to improve the description of spin–orbit splitting and intensity redistributions.

Au24(SAdm)16 Nanomolecules: X-ray Crystal Structure, Theoretical Analysis, Adaptability of Adamantane Ligands to Form Au23(SAdm)16 and Au25(SAdm)16, and Its Relation to Au25(SR)18

Here we present the crystal structure, experimental and theoretical characterization of a Au24(SAdm)16 nanomolecule. The composition was verified by X-ray crystallography and mass spectrometry, and its optical and electronic properties were investigated via experiments and first-principles calculations. Most importantly, the focus of this work is to demonstrate how the use of bulky thiolate ligands, such as adamantanethiol, versus the commonly studied phenylethanethiolate ligands leads to a great structural flexibility, where the metal core changes its shape from five-fold to crystalline-like motifs and can adapt to the formation of Au(24±1)(SAdm)16, namely, Au23(SAdm)16, Au24(SAdm)16, and Au25(SAdm)16. The basis for the construction of a thermodynamic phase diagram of Au nanomolecules in terms of ligands and solvent features is also outlined.

Convergence of the multicenter B-spline DFT approach for the continuum

A multicenter approach for the calculation of the electronic continuum spectrum based on the B-spline functions and employing a Kohn–Sham density functional hamiltonian is introduced. The method is based on a large expansion on the origin, supplemented by a limited number of off-center functions located on the positions of the nuclei. The method has been applied to study the photoionization of Cl2, of the model system ðCOÞ 2 and of CrðCOÞ 6 . The method has proven very efficient: the convergence to the exact results is obtained with matrices which are much smaller than those involved in one center expansion calculations and the algorithm is numerically stable up to very high photoelectron energies (200 eV). The importance of the asymptotic moment basis set requirement is pointed out and rationalized with the help of a simple model. Preliminary calculations on CrðCOÞ 6 are then presented and their convergence discussed. 2002 Elsevier Science B.V. All rights reserved.

Valence photoionization dynamics in circular dichroism of chiral free molecules: The methyl-oxirane

The dynamical behavior of circular dichroism for valence photoionization processes in pure enantiomers of randomly oriented methyl-oxirane molecules has been studied by circularly polarized synchrotron radiation. Experimental results of the dichroism coefficient obtained for valence photoionization processes as a function of photon energy have been compared with theoretical values predicted by state-of-the-art ab initio density-functional theory. The circular dichroism measured at low electron kinetic energies was as large as 11%. Trends in the experimental dynamical behavior of the dichroism coefficients D(i)(omega) have been observed. Agreement between experimental and theoretical results permits unambiguous identification of the enantiomer and of the individual orbitals.

Time-dependent density-functional theory for molecular photoionization with noniterative algorithm and multicenter B-spline basis set: CS2 and C6H6 case studies

In this work a new direct (noniterative) algorithm to solve the time-dependent density-functional theory equations for molecular photoionization has been proposed and implemented, using a multicentric basis set expansion of B-spline functions and complete exploiting of the molecular point-group symmetry. The method has been applied to study the photoionization dynamics of CS2 and C6H6: the results confirmed the expectation of large screening effects in CS2. For C6H6 the screening effects have been found to play a minor role than in CS2, however, also in this case the quality of the final results is definitely improved. The method has proven suitable to study with confidence molecules of medium size, and there is still room for further improvement working on more elaborate treatment of the exchange-correlation functional.

Density functional study on the circular dichroism of photoelectron angular distribution from chiral derivatives of oxirane

The linear combination of atomic orbitals B-spline density functional method has been successfully applied to a series of four chiral derivatives of oxirane, to calculate the photoionization dynamical parameters, the circular dichroism in the angular distribution effect, and to identify trends along the series. The computational algorithm has proven numerically stable and computationally competitive. The photoionization cross section, asymmetry, and dichroic parameter profiles relative to valence orbitals have been systematically studied for the states which retain their nature along the series: the identified trends have been ascribed to the different electronic properties of the substituents. A rather unexpected sensitivity of the dichroic parameter to changes in the electronic structure has been found in many instances, making this dynamical property suitable to investigate the electronic structure of chiral compounds. The magnitude of the circular dichroism in the angular distribution effect does not seem to be associated with the initial state chirality, but rather to be governed by the ability of the delocalized photoelectron wave function to probe the asymmetry of the molecular effective potential.

NaxAu and CsxAu bimetal clusters: Finite size analogs of sodium–gold and cesium–gold compounds

Mixed metal clusters of sodium and cesium with gold have been generated in a supersonic expansion from the mixed vapor phase. Their tendency towards binary cluster formation, relative thermodynamic stability, and ionization potentials have been experimentally and computationally investigated. The properties of the NaxAu clusters may be understood within an electronic shell model based on delocalized cluster orbitals, whereas the characteristics of CsxAu are indicative of substantial ionic interactions. Relativistic density functional calculations have been performed to elucidate the cluster electronic structure and to rationalize observed properties which may not be accounted for by the jellium model. The properties of these finite‐size clusters are shown to be related to the known bulk intermetallic compounds sodium–gold and cesium–gold (cesium aurid), respectively.

Time-dependent density functional calculations of molecular photoionization cross sections: N2 and PH3

A method based on the time-dependent density functional theory (TD-DFT) is proposed to calculate the photoionization cross section employing the explicit continuum wave-function, within a One Center Expansion and B-Splines radial basis set. The LB94 exchange-correlation potential with correct asymptotic behavior is employed. The results obtained for N2 and PH3 are in excellent agreement with the experimental data and are of comparable accuracy of ab initio methods. A deterioration is still present in the inner valence. For PH3 the effect of TD-DFT is dramatic and recovers completely the Kohn–Sham deficiency. The method has proven efficient for both valence and core ionization.

Density functional-time-dependent local density approximation calculations of autoionization resonances in noble gases

Absolute photoionization cross section profiles and asymmetry parameters of Ne, Ar and Kr have been calculated at the time-dependent local density approximation level. We employed a very accurate B-spline finite basis set and the modified Sternheimer approach, which is a first-order perturbative scheme particularly suited to finite basis set calculations. The gradient-dependent van Leeuwen and Baerends (VLB) exchange-correlation potential has been used, since it has the correct Coulombic behaviour at large distances which is a necessary condition for the existence of the Rydberg states. A simple modification of the boundary conditions considerably improves the number of terms of the Rydberg series. The resonance parameters obtained by fitting the calculated Fano profiles with a suitable analytic expression are compared with the experimental parameters.