EXC!TiNG Tasks

Task 2: Codes

In addition to what has been already mentioned in connection with Task 1, significant progress could be achieved in terms of code developments. Details can be found below.

Milestone 7 (Code development in TDDFT):

The coding of TDDFT in the computer code ABINIT has been improved by the Louvain group. In particular, the TDDFT part of ABINIT is now parallelized. Recent developments are ground-state calculations based on the ACFD formalism and GW respectively. For the latter a collaboration with L. Reining (Nanophase network) has been started. The realization of a computer code based on the linear-response Sham-Schlüter equation is presently being examined.

Between June 2003 and now, there have been two releases of ABINIT; see the release notes at

A variety of new features have been implemented to the OCTOPUS code (partly in cooperation with the group of Angel Rubio in San Sebastian: They include parser improvements, a single precision version, support for FFTW3 and support for NetCDF output to OpenDX, an openDX program to plot wavefunctions, densities, and potentials; new mixing schemes: GRPulay, potential mixing; incipient support for Optimum Control, a small tutorial, triple excitation spectrum, dicroism, incipient support for full OEP, preconditioned Lanczos to solve the eigenvalue problem, semicode HGH pseudopotentials, noncollinear spin using the GGAs, new propagator methods: Lanzos, Suzuki-Trotter, Magnus integrators, spin-orbit corrections, working ELF function, improvements to the manual; octopus runs in 1, 2 and 3D. There have been

Milestone 8 (GW within different methods):

The GW part of ABINIT is now more general, in that it is able to treat non-symmorphic symmetry operations. The Modena group has implemented the GW approach in reciprocal space into their computer code based on a plane-wave basis set. The algorithms employed are also applicable to the general-purpose code that is a final goal of the network. A substantial part of the GW implementation in the framework of the LAPW method has been done in Graz. Hot electron lifetimes in metals have been calculated by the GW-LAPW approach (see Milestone 3) [G5]. Concerning the calculation of the band gap corrections (collaboration with Nanophase) the main part of the programming has already been carried out. Code development of GW for nano-particles in a real space approach has been done in Kaiserslautern.

Milestone 9 (BSE and density-matrix approaches):

The implementation of the BSE formalism for the LAPW method (Milestone 4) based on the WIEN2k codem, was carried out in Graz [P. Puschnig and C. Ambrosch-Draxl, Phys. Rev. B 66, 165105 (2002)] and applied to different organic materials [P. Puschnig and C. Ambrosch-Draxl, Phys. Rev. Lett., 89, 056405 (2002), G7]. In addition to the planned work, the Aarhus node got involved in this Milestone. R. Laskowski extended the existing version to work also for materials without inversion symmetry. Moreover, he made the code more efficient leading to a speed-up of a factor 3-5 depending on the material under investigation. This Milestone is thus fulfilled prior to the schedule.

The new scheme developed with the aim to calculate excitonic effects in nanotubes, has been realized in a new computer code which has been applied so far to a (4,2) carbon nanotube: the binding energy of the lowest optically active states turned out to be sizeable (approximately 0.8eV), where the corresponding exciton wavefunctions are delocalized along the circumference of the tube and localized in the direction of the tube axis [M16].

Milestone 10 (Raman spectroscopy):

A computer code to calculate Raman spectra from first-principles had been developed in Graz before. It is currently implemented into the general purpose code EXC!TING. As a first step towards the calculation of the Raman spectra phonon frequencies and eigenvectors of different materials have been investigated [EX-9, G2, G3] and the phonon dispersion of solids has been studied by means of the linear response to lattice vibrations [EX-2, EX-12].

Milestone 11 (Nonlinear optical processes):

Nonlinear optical spectroscopy is one of the major tasks in the groups of Kaiserslautern, Uppsala [U3], and Graz. Whereas UKL and UU had developed codes prior to the commencement of the network, the Graz node has developed a NLO package [G1, G6] within the framework of the EXCITING network. This code also includes linear and non-linear magneto-optics. In collaboration with the Graz node, Aarhus has started to study NLO effects in high-pressure phases of metals.

Milestone 12 (Photoemission and inverse photoemission):

The low-energy electronic excitations in photoemission and inverse photoemission spectroscopy were simulated by a theoretical method that includes many-body effects and a realistic description of the band structure. The so obtained k-dependent self-energies, hole spectral function and quasi-particle energies can be compared to photoemission spectra. The calculated spectral functions were obtained for prototypical systems such as transition metals and were found to be in very good agreement with experiments [M1]. The many-body corrections in Co turned out to be much stronger in the majority-spin channel and drastically affect the spin polarization of the spectra.

A joint work with an experimental group on hydrides was recently established by interpreting ARPES spectra with the help of the generalized susceptibility [G13]. The inclusion of matrix elements will be a common future task of Graz and Modena.

The work of the Berlin group regarding photoemission is described in Milestone 17.

Milestone 13 (General purpose code EXCITING):

A general-purpose code with all the implementations for the desired excited state properties is being developed throughout the whole runtime of the network. Some of the tools - as the BSE and the Raman code, or NLO packages - are ready as add-ons to the WIEN2k code, which is a full-potential LAPW programme. This EXC!TiNG@WIEN package will be made available to the more than 600 user groups of the WIEN2k code.

R. Laskowski, has developed a non-collinear magnetic (NCM) version of the WIEN2k package (WIENNCM) during his postdoc in Vienna. WIENNCM is based on WIEN2k, although considerably changed due to the spinor form of the basis set. The full symmetrization of the spinor wave functions together with a symmetry analysis of the NCM structures has been developed in Aarhus. WIENNCM is in a final stage, and so far it is used by selected groups.

The formalism for the linear-optical properties as calculated in the random phase approximation has been submitted for publication [G11]. Despite the fact that a major part of the EXCITING developments concerning the LAPW method has been done starting from the WIEN2k code a new augmented planewave [G9] package has been written by the Graz node. Being designed from scratch and fully documented, it shall be the basis for future code developments. This code has proven its applicability for battery materials where the change of the bonding character as a function of the lithiation process has been demonstrated by the electron localization function (ELF) [G14]. Currently the possibilities to define interfaces between different codes, packages, and tools are being explored.