Polycyclic
aromatic hydrocarbons (PAHs) are a family of organic
compounds comprising two or more fused aromatic rings which feature
manifold applications in modern technology. Among these species, those
presenting an open-shell magnetic ground state are of particular interest
for organic electronic, spintronic, and non-linear optics and energy
storage devices. Within PAHs, special attention has been devoted in
recent years to the synthesis and study of the acene and fused acene
(periacene) families, steered by their decreasing HOMO–LUMO
gap with length and predicted open-shell character above some size.
However, an experimental fingerprint of such magnetic ground state
has remained elusive. Here, we report on the in-depth electronic characterization
of isolated peripentacene molecules on a Au(111) surface. Scanning
tunnelling spectroscopy, complemented by computational investigations,
reveals an antiferromagnetic singlet ground state, characterized by
singlet–triplet inelastic excitations with an experimental
effective exchange coupling (J
eff) of
40.5 meV. Our results deepen the fundamental understanding of organic
compounds with magnetic ground states, featuring perspectives in carbon-based
spintronic devices.
The quantum chemical version of the density matrix renormalization group (DMRG) method has established itself as one of the methods of choice for calculations of strongly correlated molecular systems. Despite its great ability to capture strong electronic correlation in large active spaces, it is not suitable for computations of dynamical electron correlation. In this work, we present a new approach to the electronic structure problem of strongly correlated molecules, in which DMRG is responsible for a proper description of the strong correlation, whereas dynamical correlation is computed via the recently developed adiabatic connection (AC) technique which requires only up to two-body active space reduced density matrices. We report the encouraging results of this approach on typical candidates for DMRG computations, namely, n-acenes (n = 2 → 7), Fe(II)−porphyrin, and the Fe 3 S 4 cluster.
Strong electron correlation
can be captured with multireference
wave function methods, but an accurate description of the electronic
structure requires accounting for the dynamic correlation, which they
miss. In this work, a new approach for the correlation energy based
on the adiabatic connection (AC) is proposed. The AC
n
method accounts for terms up to order
n
in the coupling constant, and it is size-consistent and free from
instabilities. It employs the multireference random phase approximation
and the Cholesky decomposition technique, leading to a computational
cost growing with the fifth power of the system size. Because of the
dependence on only one- and two-electron reduced density matrices,
AC
n
is more efficient than existing
ab initio
multireference dynamic correlation methods. AC
n
affords excellent results for singlet–triplet
gaps of challenging organic biradicals. The development presented
in this work opens new perspectives for accurate calculations of systems
with dozens of strongly correlated electrons.
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