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About me
I am a PhD student working in the European Union funded PROPAGATE project; a collaboration between the Amsterdam based software development company Scientific Computing & Modelling and the Free University of Amsterdam. So far, this means working as a software developer at SCM, where I am currently implementing timedependent density functional based tight binding into the ADF molecular modeling suite.
Before that I was a master student in the condensed matter group of Prof. Valentí at the University of Frankfurt's Institute for Theoretical Physics, where I mostly worked with computational methods in condensed matter theory and statistical mechanics, especially Monte Carlo methods.
Publications

Tightbinding approximations to timedependent density functional theory – A fast approach for the calculation of electronically excited statesAbstract: We propose a new method of calculating electronically excited states that combines a density functional theory based ground state calculation with a linear response treatment that employs approximations used in the timedependent density functional based tight binding (TDDFTB) approach. The new method termed timedependent density functional theory TDDFT+TB does not rely on the DFTB parametrization and is therefore applicable to systems involving all combinations of elements. We show that the new method yields UV/Vis absorption spectra that are in excellent agreement with computationally much more expensive TDDFT calculations. Errors in vertical excitation energies are reduced by a factor of two compared to TDDFTB.published in: J. Chem. Phys. 144, 184103 (2016)
eprint: arXiv:1603.02571 [physics.chemph]

Efficient calculation of electronic absorption spectra by means of intensityselected TDDFTBAbstract: During the last two decades density functional based linear response approaches have become the de facto standard for the calculation of optical properties of small and mediumsized molecules. At the heart of these methods is the solution of an eigenvalue equation in the space of singleorbital transitions, whose quickly increasing number makes such calculations costly if not infeasible for larger molecules. This is especially true for timedependent density functional tight binding (TDDFTB), where the evaluation of the matrix elements is sufficiently cheap so that relatively large systems can be studied. We propose to do an oscillator strength based truncation of the singleorbital transition space to reduce the computational effort of TDDFTB based absorption spectra calculations. We show that even a sizeable truncation does not destroy the principal features of the absorption spectrum, while naturally avoiding the unnecessary calculation of excitations with small oscillator strengths. We argue that the reduced computational cost of intensityselected TDDFTB together with its ease of use compared to other methods lowers the barrier of performing optical properties calculations of large molecules, and can serve to make such calculations possible in a wider array of applications.published in: J. Chem. Theory Comput., 2015, 11 (1), pp 157–167
eprint: arXiv:1409.4521 [physics.chemph]

Phase diagram of the square lattice bilayer Hubbard model:
a variational Monte Carlo studyAbstract: We investigate the phase diagram of the square lattice bilayer Hubbard model at half filling with the variational Monte Carlo method for both the magnetic and the paramagnetic case as a function of interlayer hopping t_perp and onsite Coulomb repulsion U. With this study we resolve some discrepancies in previous calculations based on the dynamical mean field theory, and we are able to determine the nature of the phase transitions between metal, Mott insulator and band insulator. In the magnetic case we find only two phases: An antiferromagnetic Mott insulator at small t_perp for any value of U and a band insulator at large t_perp. At large U values we approach the Heisenberg limit. The paramagnetic phase diagram shows at small t_perp a metal to Mott insulator transition at moderate U values and a Mott to band insulator transition at larger U values. We also observe a reentrant Mott insulator to metal transition and metal to band insulator transition for increasing t_perp in the range of 5.5t < U < 7.5t. Finally, we discuss the obtained phase diagrams in relation to previous studies based on different manybody approaches.published in: New J. Phys. 16 (2014) 033010
eprint: arXiv:1311.6504 [condmat.strel]

Pattern formation in the dipolar Ising model on a twodimensional honeycomb latticeAbstract: We present Monte Carlo simulation results for a twodimensional Ising model with ferromagnetic nearestneighbor couplings and a competing longrange dipolar interaction on a honeycomb lattice. Both structural and thermodynamic properties are very similar to the case of a square lattice, with the exception that structures reflect the sixfold rotational symmetry of the underlying honeycomb lattice. To deal with the longrange nature of the dipolar interaction we also present a simple method of evaluating effective interaction coefficients, which can be regarded as a more straightforward alternative to the prevalent Ewald summation techniques.published in: Phys. Rev. B 86, 024431 (2012)
eprint: arXiv:1207.1864 [condmat.statmech]
M.Sc. Thesis
Variational Monte Carlo Method
for the Hubbard model
 Introduction
 The Variational Monte Carlo method
 The application of VMC to the Hubbard model
 hVMC  a free VMC code for the Hubbard model
 The bilayer Hubbard model
 Summary and conclusion
 Appendix: hVMC quick start guide
 Appendix: Parallelism in modern computers and the hVMC code
Institut für Theoretische Physik
J.W. GoetheUniversität Frankfurt
August 2013
The full text of the thesis can be downloaded here.
I've released the Variational Monte Carlo code for the Hubbard model that I wrote as a part of this thesis as free software. See below for a brief description and instructions on where to get the code.
B.Sc. Thesis
und ihre Anwendung zur Simulation von Spinsystemen
 Einleitung(Introduction)
 Grundlagen der Thermodynamik(Fundamentals of thermodynamics)
 Grundlagen der klassischen statistischen Physik(Fundamentals of classical statistical physics)
 Einführung in Monte Carlo Methoden(Introduction to Monte Carlo methods)
 Das eindimensionale IsingModell(The onedimensional Ising model)
 Das zweidimensionale IsingModell(The twodimensional Ising model)
 Ausblick: IsingModell mit DipolDipolWechselwirkung(Outlook: The Ising model with dipoledipole interaction)
 Zusammenfassung der Ergebnisse(Summary of the results)
Institut für Theoretische Physik
J.W. GoetheUniversität Frankfurt
September 2011
My thesis is available in full text and as L^{a}T_{e}X source code. Feel free to use the source code as a template for your own thesis!
I've released the source code of the simulation software SSMC that I wrote as a part of this thesis. See below for a brief description and instructions on where to get the code. Note that the code in my thesis' appendix is an old and outdated version of the released.
Software
I believe that the results of research done at public educational institutions should be freely available to the general public. This also applies to software and I have therefore released everything that I wrote during my studies as free and open source software under the GPLv3+ license. You can get the source codes on my GitHub page. Feel free to write me an email with any problems (or bugs!) that you run into!

hVMC is a free Variational Monte Carlo code for the Hubbard model that I have written as a part of my master's thesis. Read my master's thesis if you want to know how the code works or how to use it! On the right you see the double occupation density of the bilayer Hubbard model as a function of the interplane hopping. The step in the light blue curve is quite interesting as it show a metallic phase in between two insulating phases. Read the article if you want to know more!

SSMC is a Monte Carlo simulation code for classical spin systems like the Ising model that I originally started to write as a part of my bachlor's thesis. It has grown quite a bit since then and by now has some rather advanced features like the simulation of spin systems with dipolar interactions or cluster updates. Check out the README that comes with SSMC! It should give you a head start in using, understanding and modifying SSMC.

MFHUB is a very small and simple code that performs mean field calculations for the two dimensional Hubbard model on a triangular lattice. It was written as an exercise for the computational methods in solid state theory lecture. In order to understand what MFHUB does, I suggest that you take a look at the corresponding lecture notes and the exercise that MFHUB attempts to solve. There is a README that explains how to use MFHUB.

RC4 is a simple Python2 implementation of the popular game "Connect Four"! I played around a little bit with writing an AI that actually deserves this name and the result is not as dumb as one might think, considering that I have no expertise in this field whatsoever! Try it! I think it's pretty difficult to win against it, but I might just be horribly bad at the game ;) ...
The picture on the right is a screenshot of a game I had against the AI. The AI is player two and actually managed to get me into a triple bind!
Talks & Posters

Efficient calculation of electronic absorption spectra by means of intensity selected TDDFTB11.02.2014  Poster session at NWO Meeting: Chemistry, Physics and Materials Sciences20.06.2014  Poster session at WEHeraeus Physics School 201414.09.2014  Poster session at Symposium on Theoretical Chemistry 2014poster: tddftbposter.pdf

Implementation of the Variational Monte Carlo method for the Hubbard model30.08.2013  Research Group Seminar Condensed Matterslides: vmctalk.pdf

Deconvolution methods for analytic continuation13.09.2012  Research Group Seminar Condensed Matterslides: deconvtalk.pdf

GPU architecture and its impact on GPGPU programming04.07.2012  Student's talks as part of the high performance computing practical courseslides: gputalk.pdf

Pattern formation in the dipolar Ising model on a 2dim. honeycomb lattice26.04.2012  Research Group Seminar Condensed Matterslides: diphctalk.pdf05.09.2012  Poster session at correl1220.09.2012  Poster session at the annual retreat of SFB/TR49poster: diphcposter.pdf13.04.2013  DPG Spring Meeting 2013, Regensburgslides: diphctalk2.pdf

The Heisenberg model and the MerminWagner theorem: About the possibility of spontaneous symmetry breaking in lowdimensional systems25.01.2012  Student's talks as part of the lecture: Introduction to solid state theoryslides: mwttalk.pdf

The Quantum Metropolis Algorithm: An implementation of Metropolis' famous algorithm on a quantum computer13.07.2011  Student's talks as part of the lecture: Advanced solid state theoryslides: qmatalk.pdf

Monte Carlo methods in numerical integration10.02.2011  Research Group Seminar Condensed Matterslides: mcinttalk.pdf
Tutorials

First steps with Linux[Prof. Eberhard Engel, Goethe University Frankfurt, April 2013]

Introduction to Astronomy I/II[Prof. René Reifarth, Goethe Univ. Frankfurt, summer term 2010  summer term 2012]

Theoretical Physics 1+2: Classical mechanics[Prof. Marcus Bleicher, Goethe University Frankfurt, summer term 2011]