In this work CASPT2 calculations of polyacenes (from naphthalene to heptacene) were performed to find a methodology suitable for calculations of the absorption spectra, in particular of the L (B state) and L (B state) bands, of more extended systems. The effect of the extension of the active space and of freezing σ orbitals was investigated. The MCSCF excitation energy of the B state is not sensitive to the size of the active space used. However, the CASPT2 results depend strongly on the amount of σ orbitals frozen reflecting the ionic character of the B state. On the other hand, the excitation energies of the B state are much more sensitive to the size of the active space used in the calculations reflecting its multiconfigurational character. We found a good agreement with experimental data for both bands by including 14 electrons in 14 π orbitals in the active space followed by the CASPT2(14,14) perturbation scheme in which both σ and π orbitals are included.

Acenes are fascinating polyaromatic compounds that combine impressive semiconductor properties with an open-shell character by varying their molecular sizes. However, the increasing chemical instabilities related to their biradicaloid structures pose a great challenge for synthetic chemistry. Modifying the p-bond topology through chemical doping allows modulation of the electronic properties of graphene-related materials. In spite of the practical importance of these techniques, remarkably little is known about the basic question -the extent of the radical character created or quenched thereby. In this work, we report a high-level computational study on two acene oligomers doubly-doped with boron and nitrogen, respectively. These calculations demonstrate precisely which the chemical route is in order to either quench or enhance the radical character. Moving the dopants from the terminal rings to the central ones leads to a remarkable variation in the biradicaloid character (and thereby also in the chemical stability). This effect is related to a p-charge transfer involving the dopants and the radical carbon centers at the zigzag edges. This study also provides specific guidelines for a rational design of large polyaromatic compounds with enhanced chemical stability.

The present work focuses on establishing a function to rank the stability of small silicon clusters to characterize their magic numbers. This function is composed by a thermodynamic descriptor, the atomization Gibbs free energy, and indirect kinetic descriptors, the highest occupied molecular orbital energy and the lowest excitation energy of each system. The silicon clusters geometries were optimized using density functional theory within a hybrid meta-GGA approximation (M06), while the electronic energy was corrected by single-point calculation using CASPT2 level of theory to obtain the molecular properties. Both methodologies were combined with polarized diffused triple zeta, 6-311++G(3df,3pd), basis set for all atoms. Some molecular properties and their combinations were considered to create the aforementioned function to represent the clusters chemical stability and their magic numbers. The chosen stability ranking function, called ε, presents results in agreement with the previous mass spectrometry experimental data identifying 4, 6, 7 and 10 as magic numbers for small silicon clusters. We believe this stability ranking function can be useful to study other intramolecular atomic and molecular clusters. Graphical abstract Stability ranking function, ε, applied on Si (n = 2 - 11) clusters showing Fukui's functions for the Si (n = 2 - 11) obtained by the electronic density difference through CASPT2//M06/6-311++G(3df,3pd) with an isosurface value equal to 0.003.

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