## 2 Computational Method  
According to the computational time necessary for the calculation of the considered properties, three different methodologies, molecular mechanics (mm), quantum/molecular mechanics (qm/mm)  
Figure 2: Synthetic scheme for acryloxylyltriethylammonium bromides (AATEABs) [19]. and a pure quantum-based approach (DFT), were used in this work. These methodologies were used in sequence, as follows: (i) a conformational analysis was first carried out through an MM approach; (ii) the most stable conformers for each of the surfactants were then used as initial geometries in successive QM/MM optimizations, to consider the interactions with explicit water molecules; and (iii) pure quantum optimizations were performed and the calculation of the target properties was achieved. The investigated surfactants, at room temperature, show a huge number of conformers that exponentially increase with the aliphatic chain length, thus a huge number of structures should be considered in the quantum calculations. Moreover, for the systems with different local minima, the choice of the initial geometry for the quantum optimizations is important, as it controls the search for the nearest minimum. Thus, a MM based conformational analysis was first performed using the universal force field and Avogadro code [22], an application of the Open Babel platform [23]. For each surfactant, the most stable structures were found analyzing a sample of \(30\times 10^{4}\) conformers through a weighted rotor search. Then, the thirty most stable conformers were optimized at an MM level, as all the analyzed conformers, using the conjugate gradient algorithm, \(10^{4}\) maximum steps, and a threshold of \(10^{-6}\) for the energy convergence. The most stable conformers were grouped as a function of their maximum molecular length (\(h\)), so that the structures belonging to a particular group showed a very similar length. The group with the maximum number of conformers was named \(pcg\) (i.e., the most populated conformers group), while the group containing the conformers with smaller \(h\) among the thirty most stable was called \(sc\). A structure from the \(pcg\) was chosen as the initial geometry for the successive QM/MM optimizations (\(pcg\) initial geometry) and, for each surfactant, another structure was chosen from the \(sc\) group (\(sc\) initial geometry). The ratios between the \(pcg\) and \(sc\) populations, R\({}_{\text{popul}}\), were showed in Table 1. Although the \(pcg\) group is markedly more abundant than \(sc\), one structure from the latter group was chosen, because the maximum length is an important property for the antibacterial activity of these compounds; thus the shortest conformers, chosen as the initial geometry for the quantum optimizations, are important as lower bounds. The quantum calculations were performed using NWChem code [24] in the framework of Density Functional Theory using the [PERSON]'s DFT-D3 dispersion correction [25; 26]. The hybrid energy functional Becke-Lee-Yang-Parr (B3 LYP) [27] was employed in addition to linear combinations of Gaussian-type orbitals. Double-\(\zeta\) orbital bases were used for the carbons, and double-\(\zeta\) bases with a polarization function were used for the O, N, and H atoms. The coulomb and exchange-correlation potentials were numerically integrated on an adaptive grid with accuracy equal to \(10^{-5}\). The threshold for the energy convergence in the self-consistent field procedure was set equal to \(10^{-6}\) (a.u. ), while the root mean square of the electron density was equal to \(2\times 10^{-5}\) (a.u.). The QAS electronic proprieties were calculated with good results using B3 LYP and s similar or smaller basis set. Several works, such as [28; 29; 30; 31; 32], used B3 LYP to calculate the partial charges of cations and neutral molecules. In reference [28], atomic charges were calculated using the hybrid B3 LYP, obtaining values of the same quality as the correlated \(ab\)\(initio\) methods.