Time dependent density functional theory calculations are completed for five Ni(II) complexes formed by polydentate peptides to predict the electronic absorption spectrum. The ligands examined were glycyl-glycyl-glycine (GGG), glycyl-glycyl-glycyl-glycine (GGGG), glycyl-glycyl-histidine (GGH), glycyl-glycyl-cysteine (GGC), and triethylenetetramine (trien). Fifteen functionals and two basis sets were tested. On the basis of the mean absolute percent deviation (MAPD), the ranking among the functionals is:ing the basis sets, the triple-f def2-TZVP performs better than the double-f LANL2DZ. With the functional HSE06 and basis set def2-TZVP the MAPD with respect to the experimental k max is 1.65% with a standard deviation of 1.26%. The absorption electronic spectra were interpreted in terms of vertical excitations between occupied and virtual MOs based on Ni-d atomic orbitals. The electronic structure of the Ni(II) species is also discussed. K E Y W O R D S electronic structure, nickel, peptides, time dependent density functional theory, UV-Vis spectroscopy 1 | I N TR ODU C TI ONThe prediction of the structure, reactivity, and stability of a chemical compound has always been a big challenge not only in chemistry, but also in physics and biology. During the past decades, computational methods allowed the chemists to calculate the structure, molecular properties, and energetics of many chemical species. Among these methods, density functional theory (DFT) [1] has reached an enormous popularity and many reviews have been published. [2][3][4][5][6][7][8][9][10] The main advantage of DFT is that many packages are available, commercially or free of charge, which allow one calculations on large molecules in an user friendly manner; nowadays, molecules with more than one hundred atoms can be treated routinely. At the Int J Quantum Chem. 2018;e25655. https://doi.org/10.1002/qua.25655