Koopmans' theorem and its density-functional-theory (DFT) analog with the statistical average of orbital (model) potentials (SAOP) are assessed in evaluation of the red shift of the first vertical ionization potential (VIP) of the fluoric acid in the prototype hydrogen-bonded complexes HF center dot center dot center dot HF and the H2O center dot center dot center dot HF. The calculations point to the appreciable VIP red shift, which is stronger for the H2O center dot center dot center dot HF. Comparison with the reference data shows that SAOP provides a good quality of both estimated VIP and its red shift upon complexation. (C) 2017 Elsevier B.V. All rights reserved.