**2. Static Investigation Approach**

#### *2.1. Objective of the Static Calculations*

The most straightforward approach to the theoretical characterization of a new photochemical system relies on a "static" quantum-chemical investigation, which itself might be a target research strategy or an initial step of a more complex protocol. The static ESIPT investigation is primarily oriented toward providing high-quality absorption and emission optical energies, as well as the topographical description of the investigated system's PE landscape. It might also be considered a lower-cost computational option for large polyatomic molecules as this protocol, compared to the dynamic ones, usually involves a relatively limited number of demanding energy-gradient calculations and facilitates further savings by allowing calculations under fixed system symmetry, if such is present. Typical outcomes of the static approach are absorption and emission vertical electronic energies [33–36], upper-bound estimations for possible energy barriers in the ground and electronically excited states [36–38], and detailed characterization of these states in terms of symmetry and orbital configuration [38,39]. Moreover, the number of other molecular features complementing the experimental ESIPT characterization can be determined, including, e.g., tautomers relative energies [35,40], atomic charges [41–43], and vibrational modes attribution [44,45].
