QCB9 – Poster Contribution
26-January-2010
In Pursuit of the Electronic Ground State, Ionization Energies,
Electron Affinities and Singlet-Triplet Energy Gaps of
Benzene and Linear Acenes at the Confines of Non-Relativistic
Quantum Mechanics
Balázs Hajgató, M.S. Deleuze
Universiteit Hasselt, Departement SBG, Agoralaan Gebouw D,
B-3590 Diepenbeek, Belgium
A benchmark theoretical study of the electronic ground state and of the vertical and adiabatic
ionization energies, electron affinities, and singlet-triplet (S0-T1) excitation energies of benzene
(n=1) and n-acenes (C4n+2H2n+4) ranging from naphthalene (n=2) to hexacene (n=6) or heptacene
(n=7) is presented, on the ground of single- and multi-reference calculations based on restricted
or unrestricted zero-order wave functions. High-level and large scale treatments of electronic
correlation in the ground state are found to be necessary for compensating giant but unphysical
symmetry-breaking effects in unrestricted single-reference treatments. The composition of
multi-configurational wave functions, the topologies of natural orbitals in symmetryunrestricted CASSCF calculations, the T1 diagnostics of Coupled Cluster theory and further
energy-based criteria demonstrate that all investigated systems exhibit a 1Ag singlet closed-shell
electronic ground state. The above electronic transition energies can be therefore determined
within chemical accuracy (1 kcal/mol, i.e. ~0.04 eV) by applying the principles of a Focal Point
Analysis onto the results of a series of single-point and symmetry-restricted calculations
employing correlation consistent cc-pVXZ or aug-cc-pVXZ basis sets (X = D, T, Q, 5) and
single-reference methods [HF, MP2, MP3, MP4SDQ, CCSD, CCSD(T)] of improving quality,
in order to extrapolate CCSD(T) results to asymptotically complete basis sets (cc-pVZ, augcc-pVZ). Molecular relaxation energies and the corresponding changes in zero-point
vibrational energies can be accurately determined on the ground of DFT calculations employing
the B3LYP functional. Highly quantitative insights into experiments employing electron
transmission spectroscopy on systems characterised by negative electron affinities,
corresponding to so-called metastable anions, are in particular amenable with such an approach,
provided diffuse atomic functions are deliberately removed from the basis set, in order to
enforce confinement in the molecular region and enable a determination of pseudo-adiabatic
electron affinities (with respect to the timescale of nuclear motions). Comparison is made with
calculations of electron affinities employing Density Functional Theory and especially designed
models that exploit the integer discontinuity in the potential or incorporate a potential wall in
the unrestricted Kohn-Sham orbital equation for the anion. In line with the absence of Peierls
distortions, extrapolations of results indicate a vanishingly small S 0-T1 energy gap of 0 to ~4
kcal/mol (~0.17 eV) in the limit of an infinitely large polyacene.
References [1] M. S. Deleuze, L. Claes, E. S. Kryachko and J. -P. François, J. Chem. Phys., 119,
3106 (2003); [2] B. Hajgató, M. S. Deleuze, D. J. Tozer, F. De Proft, J. Chem. Phys., 129, 084308
(2008); [3] B. Hajgató, M. S. Deleuze, D. Szieberth, F. De Proft, and P. Geerlings, J. Chem. Phys.,
accepted.