Water and ammonia adsorption mechanisms on anatase TiO2 (001) slab surface are investigated by means of periodic DFT approach. Molecular and dissociative adsorption energies for water are calculated to be -15 kcal/mol and -32 kcal/mol, respectively. Similarly, molecular and dissociative adsorption energies of ammonia on the same surface are found as 25 kcal/mol and 20 kcal/mol. A reverse result in this order is reached for the previous case of ONIOM cluster study (23 kcal/mol and 37 kcal/mol, respectively). The vibration frequency values are computed for the optimized geometries of adsorbed water and ammonia molecules on anatase TiO2 (001) slab surface and compared with the values reported in the literature. (C) 2010 Elsevier B.V. All rights reserved.