The input geometries were drawn and preoptimized by molecular mechanics in Avogadro 1.2.0 software. Further calculations were performed using the GAMESS 2019(R2) suite. The structures were optimized in delocalized internal coordinates by the B3LYP and PBE0 exchange-correlation functionals (DFT grid: 96 radial / 302 angular points) with the correlation-consistent valence triple-ζ cc-pVTZ (default) and the augmented small-core pseudopotential-based aug-cc-pVTZ-PP (Sb, Sn) basis sets in vacuum. Grimme's empirical D3 dispersion correction with Becke-Johnson damping was used in all calculations. Following convergence criteria were used: 10 -5 for the largest absolute change in the density matrix and 5•10 -5 Hartree Bohr -1 for the largest component of the geometry gradient.