The debate over whether Ti 3p electrons should be treated as core or valence electrons when using pseudopotential-based density functional theory is addressed. Two pseudopotentials that treat the 3p electrons either as core or valence are compared by modeling bulk TiO2 and Ti, TiO2 surface adsorption and reduction, as well as TiClx reactions. The present study shows that the 3p electrons can often be treated as core states, but frozen 3p electrons may be unsuitable for processes involving large electronic changes (for example charge transfer or chemical bonding). (C) 2009 Elsevier B.V. All rights reserved.