A quantum chemical model for electrostatic effects is developed starting from orthogonalized atomic orbitals which are combined to localized hybrids for lone pairs and sigma bonds and to partially delocalized hybrids for pi bonds. The procedure allows one to subdivide the density matrix and the Fock matrix in block form for the corresponding subsystems. The parametrization uses only atomic and bond parameters. The parameters are chosen in such a way that they correlate atomic charges and atomic dipole moments of this model with the results from ab initio calculations. A representative number of 160 molecules important as components of biomolecules is chosen as a test set. Finally it is demonstrated that the electrostatic potentials obtained with this model compare very well with those of ab initio calculations.