63 research outputs found
Microscopic origins of the ferromagnetic exchange coupling in oxoverdazyl-based Cu(II) complex
The exchange channels governing the experimentally reported coupling constant Jexpt=6 cm−1
value in the verdazyl-ligand based Cu II complex Cu hfac 2 imvdz are inspected using wave
function-based difference dedicated configuration interaction calculations. The interaction between
the two spin 1/2 holders is summed up in a unique coupling constant J. Nevertheless, by gradually
increasing the level of calculation, different mechanisms of interaction are turned on step by step.
In the present system, the calculated exchange interaction then appears alternatively ferromagnetic/
antiferromagnetic/ferromagnetic. Our analysis demonstrates the tremendously importance of some
specific exchange mechanisms. It is actually shown that both parts of the imvdz ligand
simultaneously influence the ferromagnetic behavior which ultimately reaches Jcalc=6.3 cm−1, in
very good agreement with the experimental value. In accordance with the alternation of J, it is
shown that the nature of the magnetic behavior results from competing channels. First, an
antiferromagnetic contribution can be essentially attributed to single excitations involving the
network localized on the verdazyl part. In contrast, the ligand-to-metal charge transfer LMCT
involving the imidazole moiety affords a ferromagnetic contribution. The distinct nature / of the
mechanisms is responsible for the net ferromagnetic behavior. The intuitively innocent part of the
verdazyl-based ligands is deeply reconsidered and opens new routes into the rational design of
magnetic object
Theoretical study of the photoconduction and photomagnetism of the BPY[Ni(dmit)2]2 molecular crystal
The BPY[Ni(dmit)2]2 molecular crystal synthesized by Naito and coworkers (J. Am. Chem. Soc., 2012, 134, 18656) was characterized as a photo-magnetic-conductor. This crystal is a nonmagnetic semiconductor in the dark and becomes a magnetic conductor after UV irradiation. This work analyzes the key ingredients of the observed photomagnetism and photoconduction by means of wavefunction-based calculations on selected fragments and periodic calculations on the whole crystal. Our results demonstrate that UV-Vis light induces charge transfer processes between the closest [Ni(dmit)2]− and BPY2+ units, that introduce unpaired electrons on the unoccupied orbitals of the BPY cations. Since the conduction bands present a strong mixing of BPY and Ni(dmit)2, the optically activated anion–cation charge transfer enhances the conductivity. The photoinduced (BPY2+)* radicals can indeed interact with the close Ni(dmit)2 units, with non-negligible spin–spin magnetic couplings, which are responsible for the changes induced by the irradiation on the temperature dependence of the magnetic susceptibility.Ministerio de EconomÃa y Competitividad de España y fondos FEDER. CTQ-2015-69019-
A convenient decontraction procedure of internally contracted state-specific multireference algorithms
Internally contracted state-specific multireference MR algorithms, either perturbative such as
CASPT2 or NEVPT2, or nonperturbative such as contracted MR configuration interaction or MR
coupled cluster, are computationally efficient but they may suffer from the internal contraction of
the wave function in the reference space. The use of a low dimensional multistate model space only
offers limited flexibility and is not always practicable. The present paper suggests a convenient
state-specific procedure to decontract the reference part of the wave function from a series of
state-specific calculations using slightly perturbed zero-order wave functions. The method provides
an orthogonal valence bond reading of the ground state and an effective valence Hamiltonian, the
excited roots of which are shown to be relevant. The orthogonal valence bond functions can be
considered quasidiabatic states and the effective valence Hamiltonian gives therefore the
quasidiabatic energies and the electronic coupling among the quasidiabatic states. The efficiency of
the method is illustrated in two case problems where the dynamical correlation plays a crucial role,
namely, the LiF neutral/ionic avoided crossing and the F2 ground state wave functio
Computational demonstration of isomer- and spin-state-dependent charge transport in molecular junctions composed of charge-neutral iron( ii ) spin-crossover complexes
Analysis of the magnetic coupling in binuclear complexes. I. Physics of the coupling
Accurate estimates of the magnetic coupling in binuclear complexes can be obtained from ab initio
configuration interaction ~CI! calculations using the difference dedicated CI technique. The present
paper shows that the same technique also provides a way to analyze the various physical
contributions to the coupling and performs numerical analysis of their respective roles on four
binuclear complexes of Cu (d9) ions. The bare valence-only description ~including direct and
kinetic exchange! does not result in meaningful values. The spin-polarization phenomenon cannot
be neglected, its sign and amplitude depend on the system. The two leading dynamical correlation
effects have an antiferromagnetic character. The first one goes through the dynamical polarization of
the environment in the ionic valence bond forms ~i.e., the M1¯M2 structures!. The second one is
due to the double excitations involving simultaneously single excitations between the bridging
ligand and the magnetic orbitals and single excitations of the environment. This dispersive effect
results in an increase of the effective hopping integral between the magnetic orbitals. Moreover, it
is demonstrated to be responsible for the previously observed larger metal-ligand delocalization
occurring in natural orbitals with respect to the Hartree–Fock one
Direct generation of local orbitals for multireference treatment and subsequent uses for the calculation of the correlation energy
We present a method that uses the one-particle density matrix to generate directly localized orbitals
dedicated to multireference wave functions. On one hand, it is shown that the definition of local
orbitals making possible physically justified truncations of the CAS ~complete active space! is
particularly adequate for the treatment of multireference problems. On the other hand, as it will be
shown in the case of bond breaking, the control of the spatial location of the active orbitals may
permit description of the desired physics with a smaller number of active orbitals than when starting
from canonical molecular orbitals. The subsequent calculation of the dynamical correlation energy
can be achieved with a lower computational effort either due to this reduction of the active space,
or by truncation of the CAS to a shorter set of references. The ground- and excited-state energies are
very close to the current complete active space self-consistent field ones and several examples of
multireference singles and doubles calculations illustrate the interest of the procedur
Proposal of an extended t-J Hamiltonian for high-Tc cuprates from ab initio calculations on embedded clusters
A series of accurate ab initio calculations on Cu_pO-q finite clusters,
properly embedded on the Madelung potential of the infinite lattice, have been
performed in order to determine the local effective interactions in the CuO_2
planes of La_{2-x}Sr_xCuO_4 compounds. The values of the first-neighbor
interactions, magnetic coupling (J_{NN}=125 meV) and hopping integral
(t_{NN}=-555 meV), have been confirmed. Important additional effects are
evidenced, concerning essentially the second-neighbor hopping integral
t_{NNN}=+110meV, the displacement of a singlet toward an adjacent colinear
hole, h_{SD}^{abc}=-80 meV, a non-negligible hole-hole repulsion
V_{NN}-V_{NNN}=0.8 eV and a strong anisotropic effect of the presence of an
adjacent hole on the values of the first-neighbor interactions. The dependence
of J_{NN} and t_{NN} on the position of neighbor hole(s) has been rationalized
from the two-band model and checked from a series of additional ab initio
calculations. An extended t-J model Hamiltonian has been proposed on the basis
of these results. It is argued that the here-proposed three-body effects may
play a role in the charge/spin separation observed in these compounds, that is,
in the formation and dynamic of stripes.Comment: 24 pages, 4 figures, submitted to Phys. Rev.
Strategies to reengage patients lost to follow up in HIV care in high income countries, a scoping review
Background: Despite remarkable achievements in antiretroviral therapy (ART), losses to follow-up (LTFU) might prevent the long-term success of HIV treatment and might delay the achievement of the 90-90-90 objectives. This scoping review is aimed at the description and analysis of the strategies used in high-income countries to reengage LTFU in HIV care, their implementation and impact. Methods: A scoping review was done following Arksey & O'Malley's methodological framework and recommendations from Joanna Briggs Institute. Peer reviewed articles were searched for in Pubmed, Scopus and Web of Science; and grey literature was searched for in Google and other sources of information. Documents were charted according to the information presented on LTFU, the reengagement procedures used in HIV units in high-income countries, published during the last 15 years. In addition, bibliographies of chosen articles were reviewed for additional articles. Results: Twenty-eight documents were finally included, over 80% of them published in the United States later than 2015. Database searches, phone calls and/or mail contacts were the most common strategies used to locate and track LTFU, while motivational interviews and strengths-based techniques were used most often during reengagement visits. Outcomes like tracing activities efficacy, rates of reengagement and viral load reduction were reported as outcome measures. Conclusions: This review shows a recent and growing trend in developing and implementing patient reengagement strategies in HIV care. However, most of these strategies have been implemented in the United States and little information is available for other high-income countries. The procedures used to trace and contact LTFU are similar across reviewed studies, but their impact and sustainability are widely different depending on the country studied
First-principles periodic calculation of four-body spin terms in high-Tc cuprate superconductors
A general mapping between the energy of pertinent magnetic solutions and the diagonal terms of the spin Hamiltonian in a local representation provides the first general framework to extract accurate values for the many body terms of extended spin Hamiltonians from periodic first-principle calculations. Estimates of these terms for La2CuO4, the paradigm of high-Tc superconductor parent compounds, and for the SrCu2O3 ladder compound are reported. For La2CuO4, present results support experimental evidence by Toader et al. [Phys. Rev. Lett. 94, 197202 (2005)]. For SrCu2O3 even larger four-body spin amplitudes are found together with Jl/Jr=1 and non-negligible ferromagnetic interladder exchange
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