## 3 Results
### Density Functional Theory (DFT) Calculations
Figure 10 presents a comprehensive depiction of the ELF and LOL maps for both the **4d** molecule and Aspirin, shedding light on their electronic localization and bonding features. The ELF map ranges from 0 to 1, with regions exhibiting values below 0.5 signifying delocalized electronic zones, reflecting reduced localization of electrons in those areas. In contrast, when the electron density is more tightly concentrated, the LOL phase rises above 0.5, highlighting areas where electron localization is more pronounced. The increased LOL value signifies regions with robust covalent bonding or nuclear layers, marked by a notable concentration of electron density, which indicates a higher degree of electron localization in those areas. In the study of the **4d** and Aspirin molecules, the electron cloud was extensively delocalized across certain carbon, oxygen, nitrogen and sulfur atoms, with blue regions marking the areas of electron dispersion. Alternatively, the red and orange regions around the hydrogen atoms exhibit higher ELF values, pointing to a strong covalent bond character with a significant density of electrons concentrated in these areas. In addition, the central areas of some hydrogen atoms in the molecules appeared white, signifying that their electron density exceeded the upper limit of the color scale (0.80), as illustrated in Figure 10.  
The stability of the compounds in this study was mainly ascribed to weak inter/intramolecular interactions. These interactions were investigated via the RDG analysis, which focuses on non-covalent interactions. The \(\lambda\)2 sign was utilized to distinguish between bonded, \(\lambda\)2 \(<\) 0 and non-bonded, \(\lambda\)2 \(>\) 0 interactions. In addition, the \(\lambda\)2 sign the \(\rho\) function varied between \(-\)0.05 and 0.05 a.u., as shown on the RDG plot (Figure 11). Figure 10: **(A1)** ELF (**4d** in the **left** and Aspirin in the **right**) and **(A2)** LOL (**4d** in the **right** and Aspirin in the **right**). Furthermore, red peaks (\(\lambda\)2 \(>\) 0; \(\rho\) \(>\) 0) signify steric repulsion effects within the ring structure. Green peaks (\(\lambda\)2 \(=\) 0; \(\rho\) \(=\) 0) are associated with Van der Waals interactions, whereas blue peaks (\(\rho\) \(>\) 0; \(\lambda\)2 \(<\) 0) indicate electrostatic interactions, such as hydrogen or halogen bonds [54,55]. The RDG plots (Figure 11) reveal that the blue surface in the **4d** molecule emphasizes strong intermolecular H-bonding for N-H...N. The RDG scatter plot further supports this, showing blue contours for \(-\)0.05 a.u. \(<\)\(\lambda\)2\(\rho\)\(<\)\(-\)0.01 a.u., indicating robust H-bonding. Van der Waals (VDW) interactions are identified by red-green mixed spikes, ranging from \(-\)0.02 a.u. \(<\)\(\lambda\)2\(\rho\)\(<\) 0.02 a.u. These spikes are observed between the hydrogen atoms. The range of \(\lambda\)2\(\rho\) values confirms the existence of these interactions. These interactions further demonstrate the proximity and interaction of hydrogen atoms in the molecule. Steric effects, highlighted by red coloration, were detected in the range of 0.005 a.u. \(<\)\(\lambda\)2\(\rho\)\(<\) 0.045 a.u. These effects were especially prominent at the core of the thiazole group. The red regions correspond to areas of significant steric repulsion.