Polarized–unpolarized ground state of small polycyclic aromatic hydrocarbons

Abstract

Do polyacenes, circumacenes, periacenes, nanographenes and graphene nanoribbons show a spin polarized ground state? In this work, we present mono-determinantal (Hartree-Fock and Density Functional Theory types), and multi-determinantal calculations ( Møller-Plesset and Coupled Cluster), for several families of unsaturated organic molecules (n-Acenes, n-Periacenes and n-Circumacenes).All HF calculations and many DFT show a spin-polarized (antiferromagnetic) ground state, in agreement with previous calculations. Nevertheless, the multi-determinantal calculations, carried out with perturbative and variational wavefunctions, show that the more stable state is obtained starting from the unpolarized Hartree-Fock wavefunction.The trend of the stabilisation of wavefunctions (polarized or unpolarized) with respect to exchange and correlation potentials, and to the number of benzene rings, has been analysed. A study of the spin (<Ŝ 2 >) and the spin density on the carbon atoms has also been carried out.

Sub-Nanometer Width Armchair Graphene Nanoribbon Energy Gap Atlas

“…Although spin contamination is usually a minor issue in density functional methods, it may affect the length dependent onset of antiferromagnetism with respect to multideterminant methods. 54 Additional firstprinciples DFT calculations were performed with the ADF computational package for selected compounds in order to validate the DFTB results. This comparison is presented in the Supporting Information together with more details on the computational protocol.…”

“…DFTB incurs in additional stabilization of delocalized states, , potentially underestimating emergence of edge states, therefore serving as a higher bound for predicting magnetic states. Although spin contamination is usually a minor issue in density functional methods, it may affect the length dependent onset of antiferromagnetism with respect to multideterminant methods . Additional first-principles DFT calculations were performed with the ADF computational package for selected compounds in order to validate the DFTB results.…”

Cyclobutadiene automerization and rotation of ethylene: Energetics of the barriers by using Spin‐polarized wave functions

“…In this article, we will use the AEB of cyclobutadiene and the torsional barrier of ethylene to test a calculation strategy suggested previously . It basically consists in the use and subsequent comparison of spin‐projected perturbative theories and coupled cluster calculations, all made from a monodeterminantal broken symmetry spin‐polarized wave function, with the aim to show how the use of a reference spin‐polarized wave function can lead to the right solution for any of the two procedure.…”

“…[43] In this article, we will use the AEB of cyclobutadiene and the torsional barrier of ethylene to test a calculation strategy suggested previously. [44][45][46] It basically consists in the use and subsequent comparison of spin-projected perturbative theories and coupled cluster calculations, all made from a monodeterminantal broken symmetry spin-polarized wave function, with the aim to show how the use of a reference spin-polarized wave function can lead to the right solution for any of the two procedure. The former seems to be a good strategy as its simplicity allows arriving at very good perturbative levels by high quality basis sets, and making a complete optimization of the molecular geometry, which is a level of calculation forbidden when applying other methods such as the CCSDTQ.…”

From cyclic nanorings to single-walled carbon nanotubes: disclosing the evolution of their electronic structure with the help of theoretical methods

“…Multi-Configurational (MC) methods have been used in the past for open-shell systems with great success, 28 but their pronounced computational scaling with the system size precludes its applica-tion to linear, planar, or cyclic PAHs composed of more than a few tens of carbon atoms. On the other hand, the cost-effective Density-Functional Theory (DFT) has been traditionally discarded for these (partly) open-shell systems because of the spin-contamination problem and/or the historical difficulties to deal with (near-)degeneracy effects, 29,30 noting some recent efforts to model the electronic structure of open-shell singlet biradicals by time-dependent versions. 31 Hence, another way to afford these challenging effects is by relying on a fractional occupation formalism, mimicking thus the occupancy of the open-shell configurations and preventing spin contamination issues at a reasonable computational cost.…”