Periodic, self-consistent density functional theory calculations are used to explain the observed decrease in the vibrational frequency of CO on methanol synthesis catalysts under severe reducing conditions (N.-Y. Topsoe, H. Topsoe, J. Mol. Catal. A Chem. 141 (1999) 95-105). Vibrational frequencies for CO on eight different models of the methanol synthesis catalyst surface have been determined. The calculated vibrational frequency of CO on Cu(111) (1/9 ML CO coverage) with 1/9 ML of Zn adatoms shows a decrease of between 15 and 38 cm(-1) from the corresponding calculated CO frequency on clean Cu(111) (2073 cm(-1)). The calculated vibrational frequency of CO on Cu(111) with 1/9 ML of ZnO species shows a decrease of up to 72 cm(-1) from the CO stretch frequency on clean Cu(111). These calculated CO vibrational frequency decreases agree with the experimentally measured decrease (ca. 50 cm(-1)), suggesting that Zn and/or ZnO species may be present in the vicinity of active Cu sites of methanol synthesis catalysts under highly reducing conditions. In addition, CO vibrational frequencies on partially oxidized Cu(111) surfaces are shown to increase from the corresponding frequencies on clean Cu(111), in agreement with experimental results. (C) 2003 Elsevier Science (USA). All rights reserved.