Example: ab initio electron affinity calculations for aluminium chlorides

Reliable estimates of electron affinities (EA) for aluminum mono-, di- and trichloride molecules are required as input data for describing both thermodynamics and kinetics of numerous processes in aluminum chloride-containing plasma. Neither reasonable experimental nor theoretical data on these properties could be found in literature; the AlCl3 EA value was believed to lie between 1.2 and 1.8 eV. Kintech Lab experts determined these values using high-level ab initio electronic structure calculations.

We used approximate quadratic coupled cluster (AQCC) method, correlating all valence electrons and employing flexible atomic natural orbital (ANO) bases augmented by several sets of diffuse orbitals centered on Al. The convergence of the results with respect to basis set extension was carefully checked. The cases of small (few tenths of eV) EA values are particularly difficult for ab initio EA evaluation since the conventional SCF technique cannot provide more or less reasonable approximation for the anionic wavefunction. This is just the situation with AlCl. The problem was solved by invoking the multireference version of AQCC (MR AQCC), generating the reference set by putting the extra electron on one low-lying virtual orbital of AlCl in all possible ways.

The computational results are summarized in the table below.

System AlCln -> AlCln- geometry changes Electron affinity, eV
AlCl / AlCl- bond lengths increase by 0.14 A 1.3
AlCl2 / AlCl2- valence angle decrease by 20 deg, 2.2
AlCl3 / AlCl3- D3h -> C3v, bond lengths increase by 0.12 A 0.11

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