We present an efficient implementation of the electronic susceptibility tensor within density functional theory. The susceptibility is represented by means of its eigensystem, which is computed using an iterative Lanczos diagonalization technique for the susceptibility tensor within density functional perturbation theory. We show that a representation in a finite basis of eigenstates is sufficiently accurate to compute the linear response of the electronic density to external potentials. Once the eigensystem representation is computed, the actual response computation can be done at very low computational cost. The method is applied to the water molecule in a dipole field as a benchmark system. The results illustrate the potential of the approach for the first-principles calculation of supramolecular interactions in complex disordered systems in the condensed phase.