In contrast, the PO molecule on the NaCl surface remained intact. Structure optimiza- tion calculations for PO and the ring-opening product on Na and the NaCl surfaces yield that the ring-opening reaction is exothermic on sodium metal, while it is endothermic on the NaCl surface. Hence, the NaCl surface stabilizes the PO molecule. Apart from the thermodynamic stability, PO must not bind strongly to the NaCl surface to be detectable by GC measurements. We compared the adsorption energies of PO on the NaCl surfaces with the adsorption energy on the sodium metal surfaces and found PO to bind with energies below 1eV on all tested surfaces consistently; the optimized structures are depicted in Fig. .