In plane wave based electronic structure calcns. the interaction of core and valence electrons is usually represented by at. effective core potentials. They are constructed in such a way that the shape of the at. valence orbitals outside a certain core radius is reproduced correctly with respect to the corresponding all-electron calcns. Here we present a method which, in conjunction with d. functional perturbation theory, allows to optimize effective core potentials in order to reproduce ground-state mol. properties from arbitrarily accurate ref. calcns. within std. d. functional calcns. We demonstrate the wide range of possible applications in theor. chem. of such optimized effective core potentials (OECPs) by means of two examples. We first use OECPs to tackle the link atom problem in quantum mechanics/mol. mechanics (QM/MM) schemes proposing a fully automatized procedure for the design of link OECPs, which are designed in such a way that they minimally perturb the electronic structure in the QM region. In the second application, we use OECPs in two sample mols. (water and acetic acid) such as to reproduce electronic densities and derived mol. properties of hybrid (B3LYP) quality within general gradient approximated (BLYP) d. functional calcns. [on SciFinder (R)]