zo[b,d]thiophene-5,5-dioxide (DBTS) and the closely related, but more rigid carbazole (Cz). Since lattice deformations belong to the softest intramolecular degrees of freedom, DBTS planarizes readily upon pressure application, which leads to a spectroscopic red shift. Cz, on the contrary, is more rigid and rather insensitive to the sterical influence of its surrounding, hence, its absorption and emission spectra are almost unaffected by pressure. In another study, it was observed that conformational changes can turn an expected red shift to a blue shift upon compression. An important class of piezochromic materials that shall briefly be mentioned here are transition metal complexes. In these cases, the complicated electronic structure poses a formidable challenge to the applied level of theory (cf. Section 3.1.2). DFT has been used to investigate piezochromism in Ir(III), Ni(II) and Cu(I) complexes. In the latter case, Energy Decomposition Analysis (EDA) al-lowed the quantification of important energetic factors, e.g., Pauli repulsion, electrostatic interactions and dispersion, each of which shows complex and system-specific pressure-dependence. In a very recent study, DFT was combined with multireference methods to quantify the effects of energy level changes of electronic states and structural changes on spectroscopic properties of a Cr(III) complex. Turning to vibrational spectra, high-pressure Raman spectroscopy of chemical compounds is particularly widespread, since Raman spectroscopy on a reference material is typically used anyway to gauge the pressure in a DAC setup. Hence, vibrational properties of molecules under pressure have been calculated