All quantum-chemical calculations have been performed using DFT with the Amsterdam Density-Functional (ADF) engine of the Amsterdam Modeling Suite (AMS) with the PBE0 hybrid functional and a triple-zeta plus polarization (TZP) basis set of Slater-type orbitals . A Becke integration grid of "good" accuracy and the numerical accuracy setting "good" have been used throughout. All total energies have been obtained with ADF's total energy implementation .