Theoretical methods for computational spectroscopies and microscopies

Many-Body Perturbation Theory
(Paolo Umari, Geoffrey Stenuit, Stefano Baroni)

We are working on accelaration schemes for very accurate many-body perturbation theory calculations of excitation properties and optical spectra. For this research line, we have already introduced a new approach for accelerating first-principles GW calculations of quasi-particle energies (GWW homepage). This allows to address large model structures. This approach is now used for simulating photoemission spectra of organic molecules (in collaboration A. Goldoni).

Time-Dependent Density Functional Theory
(Brent Walker, Ralph Gebauer, and Stefano Baroni)

Simulating excited-state properties and optical spectroscopic processes is still a major challenge to modern computational physics and chemistry. Traditional quantum-chemistry methods are limited by the small size of the systems they can cope with, and lighter approaches based on TDDFT are becoming popular because of their accuracy and ability to be applied to systems consisting of several tens of atoms. We aim at developing new new theoretical and computational tools suitable for complex molecular systems as well as for condensed matter. We have devised a new computational tool, based on a super-operator formulation of linearized time-dependent density-functional theory. The new method expresses the dynamical polarizability of a system of interacting electrons in terms of a matrix continued fraction, whose coefficients are obtained from the non-symmetric block-Lanczos method. The resulting algorithm allows for the calculation of the full spectrum of a system with a computational workload which is only a few times larger than that needed for static polarizabilities within time-independent density-functional perturbation theory, thus opening the way to large-scale excited-state simulations.

Dario Rocca, Ralph Gebauer, Yousef Saad, and Stefano Baroni
Turbo charging time-dependent density-functional theory with Lanczos chains
J. Chem. Phys. 128, 154105 (2008)

Dario Rocca
Time-Dependent Density Functional Perturbation Theory: New algorithms with applications to molecular spectra
PhD Thesis, SISSA (2007)

B. Walker, A. M. Saitta, R. Gebauer, and S. Baroni,
Efficient Approach to Time-Dependent Density-Functional Perturbation Theory for Optical Spectroscopy
Phys. Rev. Lett. 96, 113001 (2006)

Simulation of IR, Raman and Hyper-Raman spectra
(Paolo Umari)

The simulation of vibrational spectroscopies such as the Infrared, the Raman and the non-linear Hyper-Raman responses is a powerful tool for the characterization of complex materials. We have developed a set of computational tools for the simulation of such spectra in large model structures. Now, we are using these methods for the study of amorphous silicon nitride (in collaboration with L. Giacomazzi).

P. Umari and A. Pasquarello,
Hyper-Raman Spectrum of Vitreous Silica from First Principles
Phys. Rev. Lett. 98, 176402 (2007)

Simulation of ultra-violet photoemission spectroscopy
(Nataša Stojić, Andrea Dal Corso, and Stefano Baroni)

Despite the great importance of photoemission techniques and numerous related calculational methods, there is not yet a fully ab initio scheme for calculating photoemission spectra for low-energy photons. We have developed a calculational approach for the simulation of photoemission spectra at low photon energies based on the density-functional theory (DFT). The method is implemented in the Quantum-ESPRESSO computer package, in the framework of pseudopotentials and a plane-wave basis set. In this approach, both initial and final states, together with the transition dipole matrix elements and transmission factors are calculated from first-principles.

In collaboration with:

  • TASC - APE beamline (B. Zhou and I. Vobornik)

Nataša Stojić, Andrea Dal Corso, Bo Zhou, and Stefano Baroni,
Ab initio simulation of photoemission spectroscopy in solids: Plane-wave pseudopotential approach with applications to normal-emission spectra of Cu(001) and Cu(111)
Phys. Rev. B 77, 195116 (2008)

Simulating x-ray absorption spectroscopy with DFT methods
(Yansun Yao and Stefano Fabris)

We have developed a computer program for the calculation of x-ray absorption spectroscopy (XAS) and of Electron Energy Loss Spectroscopy (EELS) within the framework of density functional theory, employing the projector augmented wave method. The code is implemented in the Quantum ESPRESSO package and take advantage of the reconstructed all electron wave functions. Presently, XAS and EELS are simulated by using the dipole approximation, but higher-order multipoles can be easily accounted for and included in the calculation.

Gas-phase electronic spectroscopies
(D. Toffoli, G. Fronzoni, M. Stener, D. Di Tommaso, M. De Francesco,
and P. Decleva)

This activity focuses on the theoretical description of electronic spectroscopies, notably core absorption, valence and core photoemission spectroscopies in finite systems (molecules and clusters). One research line involves the development of time-dependent density-functional-theory (TDDFT) methods for a description of molecular photoemission that includes relativistic effects. The method is applied to the analysis of Mg(C5H5)2 (in collaboration with the ELETTRA GasPhase beamline), C6F6, CF4, SiF4, SF6, and C60. Moreover we focus on circular dichroism in the photoemission spectra of chiral systems and analyse of the electronic structure of transition-metal and carbonyl compounds, as well as organometallic systems.

In collaboration with:

  • ELETTRA - GasPhase beamline (M de Simone and S. Turchini)