  "Alchemical" interpolation paths, i.e.~coupling systems along fictitious
paths that without realistic correspondence, are frequently used within
materials and molecular modeling and simulation protocols for the estimation of
relative changes in state functions such as free energies. We discuss
alchemical changes in the context of quantum chemistry, and present
illustrative numerical results for the changes of HOMO eigenvalues of the He
atom due to a linear alchemical teleportation---the simultaneous annihilation
and creation of nuclear charges at different locations. To demonstrate the
predictive power of alchemical first order derivatives (Hellmann-Feynman) the
covalent bond potential of hydrogen fluoride and hydrogen chloride is
investigated, as well as the van-der-Waals binding in the water-water and
water-hydrogen fluoride dimer, respectively. Based on converged electron
densities for one configuration, the versatility of alchemical derivatives is
exemplified for the screening of entire binding potentials with reasonable
accuracy. Finally, we discuss constraints for the identification of non-linear
coupling potentials for which the energy's Hellmann-Feynman derivative will
yield accurate predictions.
