Studies of the negative ions of polar molecules LiH-, NaH-, NaCl-

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Abstract
Electrons may attach to highly polar molecules, giving rise to stable negative ions. In this paper we present a detailed study of the type of diffuse functions that must be included in the basis sets in order to obtain a proper description of such anions. In the case of ionic diatomic molecules the ‘extra’ electron occupies a non-bonding orbital locating most of its charge density behind the electropositive atom. We demonstrate, with the specific examples of LiH and NaH, that such an orbital may be formed either by hybridizing functions centred on the electropositive atom or by employing functions centred behind the electropositive atom. Our new studies on LiH and NaH demonstrate that the electron affinity is relatively insensitive to the basis set employed, provided that it yields a good estimate of the dipole moment of the neutral molecule and that the ‘proper’ diffuse functions are included on the electropositive atom. Ab initio calculations are also performed on NaCl and its ground-state negative ion. The anion is predicted to be stable, the calculated electron affinity being 0·65 eV.