G. Fronzoni

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.

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.

Theoretical determination of the indirect nuclear spin–spin coupling tensors, nuclear screening tensors, and magnetic susceptibilities of multiply bonded systems via the equations‐of‐motion method

In this paper we document the possible performances of the equations‐of‐motion method for predicting second‐order magnetic properties (indirect nuclear spin–spin coupling tensors, nuclear screening tensors, and magnetic susceptibilities). Nonempirical calculations for a series of molecules containing conjugated or cumulated multiple bonds and most of them suffering from UHF triplet instability are reported. In spite of the limitation of the 631G** basis set used, the results fit reasonably well with the main features of the experimental data in all cases but some geminal J’s and the σ’s in diacetylene. Inclusion of the (2p‐2h) interactions brings the theoretical results to a closer agreement with observation. Effects of electron correlation at (1p‐1h) and (1p‐1h) +(2p‐2h) levels of approximation are analyzed by comparison with corresponding results obtained by standard CHF procedures. They are found to be of remarkable importance for the paramagnetic component of σ and χ and for the Fermi‐contact contribu...

Ab initio study of the nuclear spin—spin coupling constants and magnetic shielding constants in silicon derivatives via the equations-of-motion method

Abstract Non-empirical equations-of-motion calculations of the nuclear spin—spin coupling constants and magnetic shielding constants in a representative series of molecules featuring siliconsilicon or siliconcarbon single, double and triple bonds are presented. The EOM results, which include the main portion of the electron correlation effects, are in resonable agreement with the available experimental data. On passing from single to double and triple bonding situation the pattern for the 1 J (SiY) parameters resembles that exhibited by 1 J (CY) in the structurally related carbocompounds, whereas an inversion in the relative position of the triply bonded atom is predicted in the case of the 29 Si resonance relative to the 13 C sequence.

Theoretical study of resonances in the metal core photoionization of M@C60 (M = Li, Na, K)

The cross section and angular distribution photoemission profiles relative to core levels of the central alkali atom (Li 1s, Na 1s, K 1s and 2p) in the endohedral compounds have been computed by a large-scale one-centre expansion employing a density functional Hamiltonian and an explicit continuum wavefunction calculation. The computed spectra show the presence of sharp structures induced by the C60 cage and a strong deviation from free-atom profiles. Anisotropy of the full molecular potential proves essential to obtain the observed structures. The cross section resonances show significant changes with the change in metal atom and prove much more sensitive than the corresponding outer valence spectra. The predicted features should be easily detected experimentally.

Molecular orbital description of core excitation spectra in transition metal compounds. An ab initio CI calcultaion on TiCl4 and isoelectronic molecules

Abstract Ab initio calculation of 1s and 2p exicitation spectra are performed in the relaxed 1h-1p CI scheme for a series of isoelectronic and isostructural halides and oxohalide (TiF 4 , TiCl 4 , TiBr 4 , VOF 3 , VOCl 3 , CrO 2 F 2 , CrO 2 Cl 2 , MnO 3 F, MnO 3 Cl). Agreement with experimental spectra available for TiCl 4 is very satisfactory, indicating the reliabilty and accuracy of the present model to describe core excitation spectra even in highly ionic transition metal compounds. The Calculated spectra show significant and well detectable variation upon halogen or metal substitution, and are therefore a sensitive probe to slight electronic structure changes along the series. Significant configuration mixing are observed among different excitation channels in the case of the degenerate 2p core hole, and prevent full account of the spectra in a pure one-electron model.

Time dependent density functional photoionization of CH4, NH3, H2O and HF

Abstract The time dependent density functional theory (TD-DFT) and the Kohn–Sham (KS) schemes have been employed to calculate the cross section and the asymmetry parameter profiles of all the orbitals (from outer valence to core 1s) of CH 4 , NH 3 , H 2 O and HF, employing a one centre expansion of B-spline functions. The comparison between the KS and TD-DFT results shows that the screening effects play an important role even for the first row hydrides. The comparison of the TD-DFT results with the available experimental data shows that TD-DFT gives quantitative accuracy. Well defined trends are identified along the series, with respect to the nature of the ionized orbitals. Inner valence cross section is the most difficult to be described even at the TD-DFT level, and this has been attributed to shake-up processes not considered at the present level of the theory. In general, the good accuracy of the TD-DFT method and its computational economy makes it a good candidate to application to more extended and complicated molecules.