  We carried out density functional theory calculations to model the electronic
structure and the magnetic interactions in copper doped anatase and rutile
titanium dioxide in order to shed light on the potential of these systems as
magnetic oxides using different density functional schemes. In both polymorphs
copper dopant was found to be most stable in substitutional lattice positions.
Ferromagnetism is predicted to be stable well above room temperature with long
range interactions prevailing in the anatase phase while the rutile phase
exhibits only short range superexchange interaction among nearest neighbouring
Cu ions. Additionally, energetic evaluation of dopants in scattered and compact
configurations reveals a dopant clustering tendency in anatase TiO2.
