Transferable basis sets of numerical atomic orbitals

We show that numerical atomic orbital basis sets that are variationally optimized for specific compounds are biased and not fully transferable to other compounds. The situation improves when the basis sets are optimized for several compounds and a compromise is made. We demonstrate this for basis sets for compounds containing H, O, N, Al, Cl, and Cu atoms, using the SIESTA code. Reaction energies are compared to benchmark calculations with the ADF code. Our double-zeta basis sets give a root mean square error of 0.2 eV. With triple-zeta basis sets, we obtain a root mean square error of only 0.1 eV. Finally, for the case of mixed phase systems, we propose that optimizing basis sets for gaseous molecules and then using them also for bulk compounds, is better than the other way around.