The electronic state of poly(vinyl alcohol) (PVA)-I-3(-) ion complex has been successfully described for the first time on the basis of the density functional theoretical (DFT) calculations. (1) As the first trial, complex structure models between a short PVA chain (5-mer) and I-3(-) ion were constructed, which were geometrically optimized and the energy change during the complexation was analyzed. Mainly the interaction between OH groups of PVA and iodine has been found to determine the stability of the complex. The interaction strength depends on the stereoregularity of the chain: the syndiotactic PVA sequence gave the strongest interactions between I-3(-) and four or five OH sequence along the chain axis, while the atactic and isotactic chain models are less stable than the former case. The calculated interaction energy between PVA and I-3(-) species was 36.0, 31.2 and 25.6 kcal/mol for syndiotactic, atactic and isotactic PVA, respectively. The calculation of orbital interactions revealed that the highest occupied molecular orbital (HOMO) of I-3(-) is relatively close to the lowest unoccupied MO (LUMO) of PVA, causing an electron transfer from I-3(-) to PVA. (2) In the second-stage calculation, the crystal structure model of PVA-iodine complex including counter ions (K+) was built up by referring to the X-ray analyzed structure. The K+ ions were found to be localized in the highly-negatively-charged space surrounded by OH groups and iodine atoms, corresponding well to the electrostatic potential minimum created by the PVA chains and iodine ions. The local molecular orbitals (LMO) calculated for these two models [(1) and (2)] revealed the existence of H-bonding interaction between the OH group and the iodine ion. (C) 2016 Elsevier Ltd. All rights reserved.