Benchmark theoretical study of the ionization threshold of benzene and oligoacenes

Abstract

In straightforward continuation of Green’s function studies of the ultraviolet photoelectron spectra of polycyclic aromatic compounds [Deleuze et al., J. Chem. Phys. 115, 5859 (2001); M. S. Deleuze, ibid. 116, 7012 (2002)], we present a benchmark theoretical determination of the ionization thresholds of benzene, naphthalene, anthracene, naphthacene (tetracene), pentacene, and hexacene, within chemical accuracy [0.02–0.07 eV]. The vertical ionization potentials of these compounds have been obtained from series of single-point calculations at the Hartree–Fock, second-, third-, and partial fourth-order Møller–Plesset (MP2, MP3, MP4SDQ) levels, and from coupled cluster calculations including single and double excitations (CCSD) as well as a perturbative estimate of connected triple excitations [CCSD(T)], using basis sets of improving quality, introducing up to 510, 790, 1070, 1350, 1630, and 1910 basis functions in the computations, respectively. A focal point analysis of the convergence of the calculated ionization potentials has been performed in order to extrapolate the CCSD(T) results to an asymptotically (cc-pV∞Z) complete basis set. The present results confirm the adequacy of the outer-valence Green’s function scheme for strongly correlated systems. Adiabatic ionization energies have been further determined by incorporating Beck-three-parameter Lee–Yang–Parr functional corrections for zero-point vibrational energies and for geometrical relaxations. Extension of the analysis to the CCSD(T)/cc-pV∞Z level shows that the energy minimum form of the benzene radical cation is an obtuse structure related to the 2B2g state. Isotopic shifts of the adiabatic ionization potentials, due to deuterium substitution of hydrogens, have also been discussed.

Theoretical evaluation of PAH dication properties

“…Although basis set convergence is not yet expected at this level, in view of the large systems under study, some compromise between accuracy and computational costs had to be made. Thus, while the B3LYP/6−31+G level of theory shows good agreement with experiments for the electron affinities of PAHs (Dessent 2000;Malloci et al 2005), from previous works (Schröder et al 2001;Deleuze et al 2003;Woon & Park 2004;Kadantsev et al 2006) it is clear that this same level is certainly unable to predict absolute ionisation energies with chemical accuracy (±0.1 eV). For example, using the 6−311g basis set, Schröder et al (2001) obtained mean deviations of −0.3 and −0.8 eV by comparing their computed values for the single and double AIEs of corannulene (C 20 H 10 ) and coronene (C 24 H 12 ) with their measured photoionisation thresholds.…”

“…These bases have been used in a benchmark-quality study of the ionisation energies of benzene and oligoacenes, to obtain the impressive accuracy of 0.02−0.07 eV (Deleuze et al 2003). Even if the cc-pVDZ set is less accurate than the 6−31+G* set and the cc-pVTZ is far from complete, this comparison provides us some insight into basis set requirements.…”

“…Hybrid DFT functionals are a little more expensive than other exchange-correlation functionals but yield better results. The B3LYP functional, in particular, is widely used in the study of PAHs and related species (Martin et al 1996;Langhoff 1996;Wiberg 1997;Bauschlicher & Langhoff 1997;Kato et al 1999;Kato & Yamabe 2002;Dessent 2000;Bauschlicher & Bakes 2000Schröder et al 2001;Rienstra-Kiracofe et al 2001;Bauschlicher 2002;Hirata et al 2003;Deleuze et al 2003;Rosi et al 2004;Woon & Park 2004;Jolibois et al 2005;Witt et al 2006;Kadantsev et al 2006). To assess the impact of this choice we also considered some more recently developed functionals, such as the so-called B−97 (Becke 1997) and HCTH (Hamprecht et al 1998), but no significant improvements, if any, were obtained with respect to B3LYP for calculating the ionisation energies of PAHs.…”

Role of exact exchange in thermally-assisted-occupation density functional theory: A proposal of new hybrid schemes

“…Consequently, the experimental singlet-triplet energy gaps (ST gaps) of n-acenes are only available up to pentacene [109][110][111][112]. On the theoretical side, since n-acenes belong to conjugated π-orbital systems, high-level ab initio multi-reference methods, such as the density matrix renormalization group (DMRG) algorithm [115,122], the variational twoelectron reduced density matrix (2-RDM) method [121,124], and other high-level methods [114,[118][119][120], are typically required to capture the essential strong static correlation effects.…”

“…Recently, linear n-acenes (C 4n+2 H 2n+4 ), containing n linearly fused benzene rings (see Figure 10), have attracted considerable interest in the research community owing to their promising electronic properties [20,21,[45][46][47][48][109][110][111][112][113][114][115][116][117][118][119][120][121][122][123][124]. The electronic properties of nacenes have been found to be highly dependent on the chain lengths.…”