Berlin 2018 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 37: Electronic-Structure Theory: General II

O 37.6: Vortrag

Dienstag, 13. März 2018, 15:45–16:00, MA 141

Vibrational Anharmonic Raman Spectra of Polymorphic Crystals from Density-Functional-Perturbation Theory — •Nathaniel Raimbault, Honghui Shang, and Mariana Rossi — Fritz Haber Institute of the Max Planck Society, Berlin, Germany

Vibrational Raman spectroscopy is widely used for characterizing different molecular-crystal polymorphs and their phase transitions. Few theoretical studies take anharmonic effects into account, despite their acknowledged importance in particular in organic and flexible compounds. In this work we compute anharmonic Raman spectra through the calculation of polarizability autocorrelation functions in thermodynamic equilibrium, obtaining polarizability tensors from our recent implementation of Density Functional Perturbation Theory (DFPT) in the all-electron FHI-aims code [1,2]. We focus here on different polymorphic forms of the paracetamol and aspirin crystals. Our anharmonic calculations show important effects in the low-frequency range of these spectra which prove essential for obtaining agreement with experimental data. Furthermore, in order to reduce the cost of DFPT simulations we use Kernel Ridge Regression (KRR) to obtain DFPT-level polarizabilities at a reduced cost. Training our model on a few thousand points, we can reproduce Raman spectra that would typically take the calculation of hundreds of thousands of points. [1] H. Shang et al., CPC 215, 26 (2017) [2] H. Shang, N. Raimbault, et al., submitted (2017)