It is worth noting that not all softened modes disappeared with SOC. Many of softened optical modes survived and their large phonon linewidth demonstrates their importance for EPC and thus for superconductivity. The influence of SOC on ABi2 family is similar to the case of other family of Bi-rich superconductors, ABi3, which crystallize in a simple cubic AuCu3-type structure, within which the all A-Bi and Bi-Bi distances are equal. In case of lighter SrBi3 the SOC only removes softened modes with large phonon linewidth, while in case of heavier BaBi3 it also stabilizes the structure. In addition, both Ba-and Bi-derived modes of BaBi3 are lower than in the case of SrBi3, leading to nearly 30% larger value of EPC constant, which is equal to 1.43 and 1.11 respectively . In the ABi2 family the substitution of lighter Rb with heavier Cs leads to only about 10% larger EPC constant. Modes associated with Cs atoms have lower frequency than those associated with Rb (average frequency of Cs is 9% lower than that of Rb, see Table ). This is expected since Cs is over 55% heavier than Rb. The average frequency of Bi is of 5% lower in CsBi2 than in RbBi2. Surprisingly, not all modes of Bi are lowered leading to a completely different shape of phonon DOS, where the peak around 1.3 THz is spread out.