Abstract
In view of interpreting images obtained by scanning tunnelling microscopes with perturbation methods the electronic properties of the STM tip are of utmost importance. We have calculated with a first-principles method the spin-resolved electronic structure of a tungsten (100) surface with different apex atoms. The results presented show that the electronic states in the vacuum region are in each case considered a mixture of states with s-, p- and d-type symmetries. The relevance of this result for STM imaging and a calculation of the tunnelling current by numerical methods is discussed briefly.