At the Kick-off Meeting of the Division Computational & Theoretical Chemistry (CTC) of the KNCV, Paul Geerlings presented his work on conceptual DFT. Partial derivatives of the total energy with respect to the numer of electrons or the external potential give respectively the electronegativity and electron density. Second derivatives include chemical hardness and the Fukui and linear response functions. He showed that with ‘alchemical’ derivatives properties of chemical compounds can be explored.