An important chemical property emerging from density-functional theory is the hardness, which can be evaluated as half of the difference between the vertical ionisation energy and electron affinity of the system. For many gas phase molecules, however, the electron affinity is negative and standard ways of evaluating this property are troublesome. In this contribution, we investigate an unconventional approximation for the electron affinity, based on the Kohn-Sham orbital energies of the frontier orbitals and the ionisation potential. It is shown that, for a large series of molecules possessing negative electron affinities, this methodology yields reasonable values for this quantity and that the correlation of the computed values with the experimental affinities from electron transmission spectroscopy is superior to other theoretical approaches. In a second part of this contribution, the hardness of a series of stable negative ions is evaluated in aqueous solution.
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