Bereiche | Tage | Auswahl | Suche | Aktualisierungen | Downloads | Hilfe
MM: Fachverband Metall- und Materialphysik
MM 34: Methods in Computational Materials Modelling (methodological aspects, numerics)
MM 34.6: Vortrag
Donnerstag, 4. April 2019, 11:45–12:00, H44
Anharmonic Raman Spectra in Polymorphic Crystals from Density-Functional-Perturbation Theory and Machine Learning — Nathaniel Raimbault and •Mariana Rossi — Fritz Haber Institute of the Max Planck Society, Berlin, Germany
Anharmonic contributions to vibrational Raman spectra of molecular crystals can be decisive to identify the structure of different polymorphs. In this work we characterise the low-frequency Raman spectral region of different polymorphs of the flexible aspirin and paracetamol crystals. We include anharmonicities though the time-correlation formalism, combining ab initio molecular dynamics and density-functional perturbation theory (DFPT) implemented in a full-potential, all-electron framework [1]. Lattice expansion and anharmonic thermal nuclear motion strongly affect the collective vibrations of the low-frequency region. This effect is much less pronounced at higher frequencies. We obtain excellent agreement with experimental lineshapes at frequencies below 200 cm−1, highlighting the necessity of going beyond the harmonic approximation and providing unambiguous polymorph assignment. In order to bypass the cost of DFPT evaluations of the polarisability tensor, we employ different forms of Kernel Ridge Regression (KRR) and discuss their efficiency. Training our models on several hundreds of points, we reproduce Raman spectra that would otherwise require the calculation of tens of thousands of points. [1] H. Shang, N. Raimbault et al., New J. Phys. 20, 073040 (2018)