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Pathak S.,Indian Association for The Cultivation of Science | Bast R.,CNRS Laboratory for Quantum Chemistry and Physics | Ruud K.,University of Tromso
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. Source


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. Source


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. Source


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. Source


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. Source

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