Applications of DFT with the help of ASE: Non-resonant Raman spectra and molecular forces

Many properties and methods even at quite deep levels are independent of the calculator used and can therefore be transferred to ASE [1]. We present two examples for this. Both are calculated using GPAW [2], but could be used by other DFT calculators without much effort. The literature about the calculation of non-resonant and resonant Raman spectra is quite large due to the ease and importance of this method in experiments. Nevertheless, the connection between different approaches is often unclear and not made explicit. We analyze how to obtain non-resonant and resonant Raman spectra within the Placzek as well as the Albrecht approximation. It is shown that the Placzek expression results from a semi-classical approximation of the combined electronic and vibrational transition energies. Molecular hydrogen, water and butadiene are studied as test cases [3]. Mechanical breaking of covalent bonds is induced by thermal fluctuations that lead to temperature and loading rate dependent rupture forces. These forces are much smaller than the maximum slope of the bond potential at practically accessible loading rates. The situation is analyzed in a two-dimensional picture taking the broken bond explicitly into account. A single force-independent universal path appears that contains full information about the force dependence of the minima and transition states. Strong variations of rupture forces with loading rate and temperature are found for mechanochromophores, where the rupture force is more than one order of magnitude smaller than the maximum slope in agreement with experiment [4].

References:

1. A. H. Larsen et al., J. Phys. Condens. Matter 29, 263002 (2017).
2. J. J. Enkovaara et al., J. Phys: Condens. Matter 22, 253202 (2010).
3. M. Walter and M. Moseler, arXiv:1806.03840
4. O. Brügner and M. Walter, Phys. Rev. Materials 2, 113603 (2018)