The native defects in the compound semiconductor GaP have been studied using a pseudopotential Density Functional Theory method in order to

determine their relative concentrations and the most stable charge states. The electronic and atomic structures are presented and the defe

ct concentrations are estimated using calculated formation energies. Relaxation effects are taken into account fully and produce negative-U

charge transfer levels for V\sS{P}{} and P\sS{Ga}{}. The concentration of V\sS{Ga}{} is in good agreement with the results of positron ann

ihilation experiments. The charge transfer levels presented compare qualitatively well with experiments where available. The effect of stoi

chiometry on the defect concentrations is also described and is shown to be considerable.

The lowest formation energies are found for P\sS{Ga}{+2} in p-type and V\sS{Ga}{-3} in n-type GaP under P-rich conditions, and for Ga\sS{P}

{-2} in n-type GaP under Ga-rich conditions.

Finally, the finite size errors arising from the use of supercells with periodic boundary conditions are examined.