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Malrieu J.P.,CNRS Laboratory for Quantum Chemistry and Physics | Caballol R.,Rovira i Virgili University | Calzado C.J.,University of Seville | De Graaf C.,Rovira i Virgili University | And 2 more authors.
Chemical Reviews | Year: 2014

The use of accurate model Hamiltonians is crucial to extract information on the magnetic interactions from the raw data obtained from magnetic susceptibility measurements, EPR, or NMR spectroscopy. Combining information from different experimental techniques can be used to propose topologies and magnitudes of the magnetic couplings in the system. It provides a rigorous and rational way to study the magnetic interactions in molecular complexes and extended systems without the need of fitting a set of parameters of an a priori defined model Hamiltonian. Theoretical treatments, able to discriminate between different physical effects, permit one to understand the various mechanisms involved in the magnetic couplings and to establish magneto-structural correlations. It has enabled theoreticians to discriminate between through space and through ligand interactions and to establish the balance between direct exchange, kinetic exchange, and spin polarization contributions to the magnetic coupling.

Fleig T.,CNRS Laboratory for Quantum Chemistry and Physics | Nayak M.K.,Bhabha Atomic Research Center
Journal of Molecular Spectroscopy | Year: 2014

A recently implemented relativistic four-component configuration interaction approach to study ℘- and T-odd interaction constants in atoms and molecules is employed to determine the electron electric dipole moment effective electric field in the Ω=1 first excited state of the ThO molecule. We obtain a value of Eeff=75.2 [GV/cm] with an estimated error bar of 3% and 10% smaller than a previously reported result (Skripnikov et al., 2013). Using the same wavefunction model we obtain an excitation energy of TvΩ=1 =5410 (cm-1), in accord with the experimental value within 2%. In addition, we report the implementation of the magnetic hyperfine interaction constant A|∥ as an expectation value, resulting in A∥=-1339 (MHz) for the Ω=1 state in ThO. The smaller effective electric field increases the previously determined upper bound (Baron et al., 2014) on the electron electric dipole moment to |de|<9.7×10-29e cm and thus mildly mitigates constraints to possible extensions of the Standard Model of particle physics. © 2014 Elsevier Inc. All rights reserved.

Fleig T.,CNRS Laboratory for Quantum Chemistry and Physics | Nayak M.K.,Bhabha Atomic Research Center
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2013

We present a rigorous method for accurate ab initio calculations of the electron electric-dipole-moment P,T-odd interaction constant Wd. The approach uses configuration interaction wave functions and Dirac four-component spinors as one-particle basis functions, and the interaction constant W d is obtained as an expectation value over these correlated wave functions. We apply the method to the HfF+ molecular ion and determine spectroscopic constants for four low-lying electronic states. For one of these states (Ω=1) we determine the effective electric field (E eff=WdΩ), which amounts to 23.3 GV/cm, correlating 34 valence and outer atomic core electrons and using wave-function expansions with nearly 5×108 coefficients. © 2013 American Physical Society.

Pathak S.,Indian Association for The Cultivation of Science | Bast R.,CNRS Laboratory for Quantum Chemistry and Physics | Ruud K.,University of Tromsø
Journal of Chemical Theory and Computation | Year: 2013

Nondynamical electron correlation based on a genuine multiconfigurational theory is of considerable importance for a balanced ab initio calculation of aromatic and antiaromatic molecules either with open-shell character or quasi-degeneracy in the electronic states. Among the various aromaticity indices, the calculation of magnetically induced ring current densities (MICD) has emerged as a strong contender, providing both a qualitative and a quantitative description of the effect. We report here the first implementation of MICD at the multiconfigurational self-consistent field (MCSCF) level of theory together with example calculations. This extension makes the method applicable to systems that cannot be appropriately handled with earlier implementations based on a single-reference starting function. We present the formulation of the MCSCF MICD theory along with applications to a prototypical antiaromatic (cyclobutadiene) and an aromatic (benzyne) system, both systems that require a multiconfigurational description. We compare the MCSCF results to those obtained using Hartree-Fock and Kohn-Sham density functional theory and discuss the effects of static correlation on the aromaticity. © 2013 American Chemical Society.

Kullie O.,CNRS Strasbourg Institute of Chemistry | Saue T.,CNRS Laboratory for Quantum Chemistry and Physics
Chemical Physics | Year: 2012

We report the implementation of long-range second-order Møller-Plesset perturbation theory coupled with short-range density functional theory (MP2-srDFT) based on the 4-component relativistic Dirac-Coulomb Hamiltonian. The range separation of the two-electron interaction is based on the error function, such that the long-range interaction, to be handled by wave function theory, corresponds to the potential of finite electrons with a Gaussian charge distribution. We argue that the interelectronic distance associated with the range-separation parameter should accordingly be determined from a Gaussian rather than a hard-sphere model. As a first application of our relativistic MP2-srDFT implementation we calculate spectroscopic constants of the complete series of homoatomic rare gas dimers, from helium to the superheavy element 118 and with bonding dominated by dispersion forces. We find that the MP2-srDFT method is less sensitive to the basis set quality than pure MP2, but for the heavier rare gas dimers the computational cost is approximately the same as for pure MP2 if one seeks convergence with respect to both basis set and number of correlated electrons. The inclusion of a short-range DFT contribution allows to dampen the tendency of pure MP2 to overbind the heavier dimers, but it is difficult to find an optimal range-separation parameter for the whole series of diatomics. Interestingly, MP2-srLDA shows better performance than MP2-srPBE for the selected molecules. © 2011 Elsevier B.V. All rights reserved.

Boggio-Pasqua M.,CNRS Laboratory for Quantum Chemistry and Physics | Groenhof G.,Max Planck Institute for Chemistry
Journal of Physical Chemistry B | Year: 2011

We have performed ab initio CASSCF, CASPT2, and EOM-CCSD calculations on doubly deprotonated p-coumaric acid (pCA2-), the chromophore precursor of the photoactive yellow protein. The results of the calculations demonstrate that pCA2- can undergo only photoisomerization of the double bond. In contrast, the chromophore derivative with the acid replaced by a ketone (p-hydroxybenzylidene acetone, pCK-) undergoes both single- and double-bond photoisomerization, with the single-bond relaxation channel more favorable than the double-bond channel. The substitution alters the nature of the first excited states and the associated potential energy landscape. The calculations show that the electronic nature of the first two (π,π*) excited states are interchanged in vacuo due to the substitution. In pCK -, the first excited state is a charge-transfer (CT π,π*) state, in which the negative charge has migrated from the phenolate ring onto the alkene tail of the chromophore, whereas the locally excited (LE π,π*) state, in which the excitation involves the orbitals on the phenol ring, lies higher in energy and is the fourth excited state. In pCA 2-, the CT state is higher in energy due the presence of a negative charge on the tail of the chromophore, and the first excited state is the LE state. In isolated pCA2-, there is a 68 kJ/mol barrier for double-bond photoisomerization on the potential energy surface of this LE state. In water, however, hydrogen bonding with water molecules reduces this barrier to 9 kJ/mol. The barrier separates the local trans minimum near the Franck-Condon region from the global minimum on the excited-state potential energy surface. The lowest energy conical intersection was located near this minimum. In contrast to pCK-, single-bond isomerization is highly unfavorable both in the LE and CT states of pCA2-. These results demonstrate that pCA2- can only decay efficiently in water and exclusively by double-bond photoisomerization. These findings provide a rationale for the experimental observations that pCA2- has both a longer excited-state lifetime and a higher isomerization quantum yield than pCK-. © 2011 American Chemical Society.

Berger J.A.,CNRS Laboratory for Quantum Chemistry and Physics
Physical Review Letters | Year: 2015

We present a fully parameter-free density-functional approach for the accurate description of optical absorption spectra of insulators, semiconductors, and metals. We show that this can be achieved within time-dependent current-density-functional theory using a simple dynamical polarization functional. We derive this functional from physical principles that govern optical spectra. Our method is truly predictive because not a single parameter is used. In particular, we do not use an ad hoc material-dependent broadening parameter to compare theory to experiment as is usually done. Our approach is numerically efficient; the cost equals that of a calculation within the random-phase approximation. © 2015 American Physical Society.

Malrieu J.-P.,CNRS Laboratory for Quantum Chemistry and Physics
Molecular Physics | Year: 2013

This work first returns on the intrinsic difficulties of multi-reference coupled cluster (MR-CC) formalisms. They may be formulated either in an effective Hamiltonian frame or in an intermediate effective Hamiltonian (IEH) one. In the former case complete model space approach being intractable, the incomplete model space approach is re-examined, and is formulated in terms of an IEH, despite the fact that the model space dimension is equal to the number of desired roots. Some of its drawbacks are illustrated on the magnetic systems problem. Then one proposes a multi-root complete active space (CAS)-based CC-SD, which only handles single and double excitation operators, generalising a previously proposed State Specific MR-CC formalism. The method proceeds through an iterative dressing of the matrix elements between the singles and doubles and the CAS determinants. © 2013 Taylor and Francis Group, LLC.

Saue T.,CNRS Laboratory for Quantum Chemistry and Physics
ChemPhysChem | Year: 2011

Entering Dirac territory: This Minireview provides a guide to two- and four-component relativistic Hamiltonians for use in quantum chemistry with particular emphasis on the recently developed eXact two-component (X2C) Hamiltonian. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fleig T.,CNRS Laboratory for Quantum Chemistry and Physics
Chemical Physics | Year: 2012

Recent developments in molecular relativistic electronic-structure theory are reviewed, with a particular emphasis on post-Hartree-Fock electron correlation methodology. The approaches discussed encompass methods devised for the treatment of small molecules, such as four-component coupled cluster of general excitation rank, ranging to two-component methods based on perturbation theory which are applicable to larger molecules. A critique of the merits and shortcomings of the available methodology is put forward, including a comparison where appropriate. © 2011 Elsevier B.V. All rights reserved.

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