Publications

1.
Tilmann Hickel, Blazej Grabowski, Fritz Körmann, and Jörg Neugebauer, "Advancing density functional theory to finite temperatures: Methods and applications in steel design," Journal of Physics: Condensed Matter 24, 053202 (2012).
2.
Blazej Grabowski, Tilmann Hickel, and Jörg Neugebauer, "Ab initio study of the thermodynamic properties of nonmagnetic elementary fcc metals: Exchange-correlation-related error bars and chemical trends," Physical Review B 76 (2), 024309 (2007).
3.
Blazej Grabowski, Lars Ismer, Tilmann Hickel, and Jörg Neugebauer, "Ab initio up to the melting point: Anharmonicity and vacancies in aluminum," Physical Review B 79 (13), 134106 (2009).
4.
Blazej Grabowski, Tilmann Hickel, and Jörg Neugebauer, "Formation energies of point defects at finite temperatures," Physica Status Solidi B 248 (6), 1295-1308 (2011).
5.
Fritz Körmann, Alexey Dick, Tilmann Hickel, and Jörg Neugebauer, "Role of spin quantization in determining the thermodynamic properties of magnetic transition metals," Physical Review B 83 (16), 165114 (2011).
6.
Alexey Dick, Fritz Körmann, Tilmann Hickel, and Jörg Neugebauer, "Ab initio based determination of thermodynamic properties of cementite including vibronic, magnetic and electronic excitations," Physical Review B 84 (12), 125101 (2011).
7.
Fritz Körmann, Blazej Grabowski, Biswanath Dutta, Tilmann Hickel, L. Mauger, Brent T. Fultz, and Jörg Neugebauer, "Temperature dependent magnon-phonon coupling in bcc Fe from theory and experiment," Physical Review Letters 113 (16), 165503 (2014).

Ab initio calculation of free energies

Ratio of the heat capacity obtained from quantum vs. classiclal Monte-Carlo simulations for two different spins S. Zoom Image
Ratio of the heat capacity obtained from quantum vs. classiclal Monte-Carlo simulations for two different spins S.

The main challenge in ab initio calculating free energies is related to the fact that DFT is originally designed to predict ground state properties only. Its extension to finite temperatures means that excitation processes carefully need to be taken into consideration [1]. Considering vibrational and electronic excitations, a remarkable agreement with data obtained from Thermo­Calc has been obtained for non-magnetic unary metals [2]. These concepts have been extended towards the consideration of anharmonic lattice vibrations [3], of entropically stabilized vacancies [4], and of thermodynamically unstable phases.

For Fe-based materials and steels, the magnetic excitations form the most challenging contribution for a computational materials design, both with respect to methodological and numerical concepts, due the long range interactions and spin-quantization. It turns out that even well above room temperature it is essential to take the spin quantization into account in order to obtain reliable heat capacities of materials (Fig.). By the development of various new techniques, spin quantum Monte-Carlo simulations could be generalized to allow for the treatment of realistic spin Hamiltonians. Using our newly developed approaches it has been possible to achieve impressive agreements of the thermodynamic properties of all unary metals [5] and compounds [6] with experiments. Another important issue is the development of methods for the influence of magnetic excitations on other thermodynamic properties such as lattice vibrations. As demonstrated for Fe it now became possible to perform phonon calculations even for the challenging case of paramagnetic disorder [7].

 
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