Low-lying excited states of model proteins: Performances of the CC2 method versus multireference methods.

A benchmark set of relevant geometries of a model protein, the N-acetylphenylalanylamide, is presented to assess the validity of the approximate second-order coupled cluster (CC2) method in studying low-lying excited states of such bio-relevant systems. The studies comprise investigations of basis-set dependence as well as comparison with two multireference methods, the multistate complete active space 2nd order perturbation theory (MS-CASPT2) and the multireference difference dedicated configuration interaction (DDCI) methods. First of all, the applicability and the accuracy of the quasi-linear multireference difference dedicated configuration interaction method have been demonstrated on bio-relevant systems by comparison with the results obtained by the standard MS-CASPT2.

Highly efficient perturbative + variational strategy based on orthogonal valence bond theory for the evaluation of magnetic coupling constants. Application to the trinuclear Cu(ii) site of multicopper oxidases.

A new strategy based on orthogonal valence-bond analysis of the wave function combined with intermediate Hamiltonian theory has been applied to the evaluation of the magnetic coupling constants in two AF systems. This approach provides both a quantitative estimate of the J value and a detailed analysis of the main physical mechanisms controlling the coupling, using a combined perturbative + variational scheme. The procedure requires a selection of the dominant excitations to be treated variationally.

Magnetic coupling constants of self-assembled Cu(II) [3×3] grids: alternative spin model from theoretical calculations.

This paper reports a theoretical analysis of the electronic structure and magnetic properties of a ferromagnetic Cu(II) [3×3] grid. A two-step strategy, combining calculations on the whole grid and on binuclear fragments, has been employed to evaluate all the magnetic interactions in the grid. The calculations confirm an S = 7/2 ground state, which is in accordance with the magnetisation versus field curve and the thermal dependence of the magnetic moment data.

A rational reduction of CI expansions: combining localized molecular orbitals and selected charge excitations.

Based on localized molecular orbitals, the proposed method reduces large configuration interaction (CI) spaces while maintaining agreement with reference values. Our strategy concentrates the numerical effort on physically pertinent CI-contributions and is to be considered as a tool to tackle large systems including numerous open-shells. To show the efficiency of our method we consider two 4-electron parent systems.

Do π-π stacking interactions really play a role in the magnetic coupling mechanisms of [Cu2(μ2-CH3COO)2L2(H2O)2]n+ (L = heterocyclic base, n = 0, 2) complexes? An ab initio inspection.

The magnetic properties of two bis-acetate binuclear copper(II) complexes, namely [Cu2(μ2-CH3COO)2(bpydiol-H)2(H2O)2] (bpydiol-H = mono deprotonated 2,2'-bipyridine-3,3'-diol) and [Cu2(μ2-CH3COO)2(phen)2(H2O)2](2+) (phen = 1,10-phenantroline), is revisited using ab initio wave function-based calculations (CASSCF, DDCI). Thanks to an analysis of the magnetic exchange coupling based on localized orbitals, it is shown that, unlike stated in the original work [C. Hou et al.

Multi-scale multireference configuration interaction calculations for large systems using localized orbitals: partition in zones.

A new multireference configuration interaction method using localised orbitals is proposed, in which a molecular system is divided into regions of unequal importance. The advantage of dealing with local orbitals, i.e.

Evaluation of magnetic terms in Cu4O4 cubane-like systems from selected configuration interaction calculations: a case study of polynuclear transition-metal systems.

We present the evaluation of magnetic terms in a Cu(4)O(4) cubane-like system from truncated CI calculations, as a case study of polynuclear transition-metal complexes. We employ a new excitation selected configuration interaction (EXSCI) method based on the use of local orbitals. Taking advantage of the locality and then of the fact that the interactions vanish when the distance is large, the dimension of the CI is largely reduced.

Direct selected multireference configuration interaction calculations for large systems using localized orbitals.

A selected multireference configuration interaction (CI) method and the corresponding code are presented. It is based on a procedure of localization that permits to obtain well localized occupied and virtual orbitals. Due to the local character of the electron correlation, using local orbitals allows one to neglect long range interactions.

A regionally contracted multireference configuration interaction method: general theory and results of an incremental version.

This work proposes to take benefit of the localizability of both occupied and virtual inactive molecular orbitals (MOs) in the context of complete active space singles and doubles configuration interaction (CAS-SDCI). The doubly occupied MOs are partitioned into blocks, or regions, corresponding to a subset of adjacent bonds and lone pairs. The localized virtual MOs are attributed to these regions from a spatial criterion.

Addressing Through-H Magnetic Interactions: A Comprehensive ab Initio Analysis of This Efficient Coupler.

The exchange coupling in a structuraly characterized Cu(II)2 complex is analyzed to highlight the role of H bonds in the generation of efficient magnetic interactions. The interest for complementary insights which are not accessible through DFT calculations (Desplanches, C. et al.

A study of the fixed-node error in quantum Monte Carlo calculations of electronic transitions: the case of the singlet n-->pi* (CO) transition of the acrolein.

We report fixed-node diffusion Monte Carlo (FN-DMC) calculations of the singlet n-->pi( *) (CO) vertical transition of acrolein. The impact of the fixed-node approximation on the excitation energy is investigated. To do that, trial wave functions corresponding to various nodal patterns are used.

Restoring the size consistency of multireference configuration interactions through class dressings: applications to ground and excited states.

The present paper presents a revised version of a size-consistency correction to the multireference configuration interaction techniques previously proposed by Szalay et al. [J. Phys.

Inspection of the duality of a verdazyl-based radical in transition metal complexes: a pi* donor ligand and a magnetic partner.

The behavior of a verdazyl-based radical bound to open-shell transition metal ions in the structurally and magnetically characterized [M(hfac)2imvd(o)] (M = Mn, Ni; hfac = (1,1,1,5,5,5)hexafluoroacetylacetonate; imvd(o) = 3-(2'-imidazolyl)-1,5-dimethyl-6-oxoverdazyl) complexes is rationalized using ab initio wave-function-based calculations analysis. The calculated exchange coupling constants J (H = -J(s(M) x s(imvd(o)); J(Mn)(calcd) = -63 cm(-1), J(Ni)(calcd) = 205 cm(-1)) are in excellent agreement with the experimental ones (J(Mn)(exp) = -63 cm(-1), J(Ni)(exp) = 193 cm(-1)). Even though both rings are involved through the binding mode of the imvd(o) radical, the spin density remains essentially localized on the nitrogen-rich ring.

Ab-initio multireference study of an organic mixed-valence Spiro molecular system.

The electronic structure and some electron transfer properties of a model mixed-valence Spiro molecular cation have been investigated at CAS-SCF, CAS+S, and CAS+SD levels starting from canonical and localized orbitals, using SZ, DZ, and TZP basis sets. The potential energy surfaces of the adiabatic ground and the lowest three excited electronic states have been computed, within a two-state model, and a double-well potential has been obtained for the ground electronic state. We have demonstrated the low coupling interaction between the two redox moieties of this molecular cation by following the charge localization/delocalization in the valence pi system through the reaction coordinate of the intramolecular charge transfer.

Can the second order multireference perturbation theory be considered a reliable tool to study mixed-valence compounds?

In this paper, the problem of the calculation of the electronic structure of mixed-valence compounds is addressed in the frame of multireference perturbation theory (MRPT). Using a simple mixed-valence compound (the 5,5(') (4H,4H('))-spirobi[ciclopenta[c]pyrrole] 2,2('),6,6(') tetrahydro cation), and the n-electron valence state perturbation theory (NEVPT2) and CASPT2 approaches, it is shown that the ground state (GS) energy curve presents an unphysical "well" for nuclear coordinates close to the symmetric case, where a maximum is expected. For NEVPT, the correct shape of the energy curve is retrieved by applying the MPRT at the (computationally expensive) third order.

The Effect of the Basis-Set Superposition Error on the Calculation of Dispersion Interactions: A Test Study on the Neon Dimer.

The dispersion interactions of the Ne2 dimer were studied using both the long-range perturbative and supramolecular approaches:  for the long-range approach, full CI or string-truncated CI methods were used, while for the supramolecular treatments, the energy curves were computed by using configuration interaction with single and double excitation (CISD), coupled cluster with single and double excitation, and coupled-cluster with single and double (and perturbative) triple excitations. From the interatomic potential-energy curves obtained by the supramolecular approach, the C6 and C8 dispersion coefficients were computed via an interpolation scheme, and they were compared with the corresponding values obtained within the long-range perturbative treatment. We found that the lack of size consistency of the CISD approach makes this method completely useless to compute dispersion coefficients even when the effect of the basis-set superposition error on the dimer curves is considered.

Selected excitation for CAS-SDCI calculations.

A new selected-configuration interaction method is proposed, based on the use of local orbitals. A corresponding code has been written, which is devoted to CI calculations of rather large systems (about 50-100 carbon-like atoms). Taking advantage of the locality, and then of the fact that interactions vanish when the distance is large, the dimension of the CI space is largely reduced.

Correlated ab initio study of the excited state of the iron-coordinated-mode noninnocent glyoxalbis(mercaptoanil) ligand.

The intriguing and theoretically unresolved magnetic coupling in the Fe(gma)CN (1) compound [gma = glyoxalbis(mercaptoanil)] has been investigated by means of first-principle correlated ab initio calculations. The low-energy spectrum of the complex has been studied using the difference dedicated configuration interaction method, which is a dynamically correlated multiconfigurational method. In agreement with available spectroscopic information, we found that the ground-state doublet is dominated by the coupling between an iron-centered quartet and the first excited triplet on the gma ligand.

Does a Sodium Atom Bind to C60?

A Multi-Reference Configuration-Interaction study of the NaC60 system is presented. It is shown that the experimentally measured dipole moment of this system can be explained by the existence of a charge-transfer state of Na(+)C60(-) nature. Moreover, the present work shows that Configuration-Interaction techniques based on local orbitals permit a Multi-Reference treatment of systems containing several tens of atoms.

Large systems at ab initio multireference level: a cheap treatment thanks to a division into fragments.

Thanks to the use of localized orbitals and the subsequent possibility of neglecting long-range interactions, the linear-scaling methods have allowed to treat large systems at ab initio level. However, the limitation of the number of active orbitals in a complete active space self consistent-field (CASSCF) calculation remains unchanged. The method presented in this paper suggests to divide the system into fragments containing only a small number of active orbitals.

A combined freeze-and-cut strategy for the description of large molecular systems using a localized orbitals approach.

A technique to reduce the computational effort in calculating ab initio energies using a localized orbitals approach is presented. By exploiting freeze strategy at the self-consistent field (SCF) level and a cut of the unneeded atomic orbitals, it is possible to perform a localized complete active space (CAS-SCF) calculation on a reduced system. This will open the possibility to perform ab initio treatments on very large molecular systems, provided that the chemically important phenomena happen in a localized zone of the molecule.

Can density functional methods be used for open-shell actinide molecules? Comparison with multiconfigurational spin-orbit studies.

The geometries, electronic structures, and vibrational frequencies of two isoelectronic compounds PuO(2)(2+) and PuN(2) have been studied in detail at the density functional theory (DFT) and multiconfigurational ab initio levels of theory. Dynamic correlation was taken into account using second-order perturbation theory (CASPT2) and the variational difference-dedicated configuration interaction method for comparison with the results of the DFT study. Spin-orbit effects were included within the framework of an effective uncontracted spin-orbit configuration-interaction method which considers electron correlation effects and spin-orbit coupling on equal footing.

Vertical spectrum of the C2H2+ system. An open shell (SC)2-CAS-SDCI study.

The open shell (SC)(2)-CAS-SDCI method along with a basis set of atomic natural orbitals (ANO) has been applied for calculating the main ionization potentials of acetylene, as well as the manifold of excited states of the different symmetries up to 32 eV. In this method, the single and double excitations of a CAS space are generated and the corresponding CI matrix is corrected by means of the (SC)(2) procedure that cancels the size-extensivity error and adds some high order contributions. The mean absolute error for the outer-valence X (2)Pi(u)(1pi(u) (-1)), A (2)Sigma(g) (+)(3sigma(g) (-1)), and B (2)Sigma(u) (+)(2sigma(u) (-1)) states, and the inner-valence C (2)Sigma(g) (+)(2sigma(g) (-1)) state is 0.

Reducing CAS-SDCI space. Using selected spaces in configuration interaction calculations in an efficient way.

A new method is presented, which allows an important reduction of the size of some Configuration Interaction (CI) matrices. Starting from a Complete Active Space (CAS), the numerous configurations that have a small weight in the CAS wave function are eliminated. When excited configurations (e.