In many experiments, a spectroscopic red shift of absorption and fluorescence signals is observed upon pressure application. DFT using PBCs or QM/MM approaches are often used to identify the intermolecular interactions responsible for this effect, e.g., π-π and CH-π interactions (Figure ). In a mixed experimental/computational study it was found that intermolecular interactions may even become so strong under pressure that an electron can be transferred, which has tremendous influence on the piezochromic behavior of the system. At the molecular level, minute changes in dihedral angles, e.g., those modulating the overlap of neighboring π systems, or the planarity of individual π systems play an important role in piezochromic behavior. A QM/MM study by Zhao et al., e.g., compared the conformationally flexible diben-Figure : Comparison of intra-and intermolecular effects in a piezochromic paracyclophane-containing organoboron crystal reported by Irii et al. a) Superimposed view of the compound (H atoms omitted) at P = 1 atm (thin) and P = 3.3 GPa (solid). b) Analogous superimposed side view of the π stacking dimer. c) Hirshfeld surfaces at P = 1 atm (left) and P = 3.3 GPa (right), signifying the most important intermolecular contacts. Reproduced with permission from Irii et al., Copyright 2022, Elsevier.