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Title: A benchmark theoretical study of the electronic ground state and of the singlet-triplet split of benzene and linear acenes
Authors: HAJGATO, Balazs 
Szieberth, D.
Geerlings, P.
De Proft, F.
DELEUZE, Michael 
Issue Date: 2009
Abstract: A benchmark theoretical study of the electronic ground state and of the vertical and adiabatic singlet-triplet (ST) excitation energies of benzene (n=1) and n-acenes (C4n+2H2n+4) ranging from naphthalene (n=2) to heptacene (n=7) is presented, on the ground of single- and multireference 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 multiconfigurational wave functions, the topologies of natural orbitals in symmetry-unrestricted CASSCF calculations, the T1 diagnostics of coupled cluster theory, and further energy-based criteria demonstrate that all investigated systems exhibit a (1)A(g) singlet closed-shell electronic ground state. Singlet-triplet (S-0-T-1) energy gaps can therefore be very accurately determined 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 basis sets (X=D, T, Q, 5) and single-reference methods [HF, MP2, MP3, MP4SDQ, CCSD, CCSD(T)] of improving quality. According to our best estimates, which amount to a dual extrapolation of energy differences to the level of coupled cluster theory including single, double, and perturbative estimates of connected triple excitations [CCSD(T)] in the limit of an asymptotically complete basis set (cc-pV infinity Z), the S-0-T-1 vertical excitation energies of benzene (n=1) and n-acenes (n=2-7) amount to 100.79, 76.28, 56.97, 40.69, 31.51, 22.96, and 18.16 kcal/mol, respectively. Values of 87.02, 62.87, 46.22, 32.23, 24.19, 16.79, and 12.56 kcal/mol are correspondingly obtained at the CCSD(T)/cc-pV infinity Z level for the S-0-T-1 adiabatic excitation energies, upon including B3LYP/cc-PVTZ corrections for zero-point vibrational energies. In line with the absence of Peierls distortions, extrapolations of results indicate a vanishingly small S-0-T-1 energy gap of 0 to similar to 4 kcal/mol (similar to 0.17 eV) in the limit of an infinitely large polyacene.
Notes: [Hajgato, B.; Deleuze, M. S.] Hasselt Univ, Res Grp Theoret Chem & Mol Modeling, B-3590 Diepenbeek, Belgium. [Hajgato, B.; Geerlings, P.; De Proft, F.] Vrije Univ Brussels, B-1050 Brussels, Belgium. [Szieberth, D.] Budapest Univ Technol & Econ, Fac Chem Engn, Dept Inorgan & Analyt Chem, H-1111 Budapest, Hungary.
Keywords: coupled cluster calculations; electron correlations; extrapolation; ground states; HF calculations; molecular configurations; molecular electronic states; orbital calculations; organic compounds; perturbation theory; potential energy functions; SCF calculations; triplet state; vibrational states; wave functions
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ISSN: 0021-9606
e-ISSN: 1089-7690
DOI: 10.1063/1.3270190
ISI #: 000272803000038
Rights: Copyright (2009) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.
Category: A1
Type: Journal Contribution
Validations: ecoom 2011
Appears in Collections:Research publications

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