Nadia Ben Amor

Size-consistent self-consistent configuration interaction from a complete active space

Abstract The size-consistent self-consistent (SC) 2 method is based on intermediate Hamiltonians and ensures size-extensivity of any configuration interaction (CI) by correcting its diagonal elements. In this work, an (SC) 2 dressing is proposed on a complete active space SDCI. This approach yields a more efficient code which can treat larger multireference problems. Tests are proposed on the potential energy curve of F 2 , the bond stretching of water and the inclusion of an Be atom in the H 2 molecule. Comparisons with approximate methods such as average quadratic coupled cluster (AQCC) are presented. AQCC appears as a good approximation to (SC) 2 .

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. J. Am. Chem. Soc. 2002, 124, 5197) has driven this state-of-the-art ab initio inspection. The wave function expansion based upon localized orbitals allows us to selectively turn on specific mechanisms and quantitatively evaluate their roles in the exchange interactions. Our singlet-triplet splitting calculations demonstrate the enhancement of the magnetic coupling through a concerted oxygen-to-metal charge transfer and electronic redistribution within the OH bond of the OH···O magnetic linker. This mechanism accounts for ∼35% of the total experimentally measured singlet-triplet energy difference. This analysis strongly suggests that H bonds might be particularly useful not only in the establishment of intermolecular contacts but also within the basic units of magnetic materials.

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., the possibility to neglect long range interaction is enhanced. Indeed, while in the zone of the molecule where the important phenomena occur, the interaction cut off may be as small as necessary to get relevant results, in the most part of the system it can be taken rather large, so that results of good quality may be obtained at a lower cost. The method is tested on several systems. In one of them, the definition of the various regions is not based on topological considerations, but on the nature, σ or π, of the localised orbitals, which puts in evidence the generality of the approach.

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. Chem. 100, 6288 (1996)]. The method assumes a complete active space reference space and separates the nonreference determinants in several classes according to their number of inactive holes and particles. The correction is formulated as a dressing of the diagonal energies of these determinants, which depends on their class, as originally proposed by Ruttink et al. [J. Chem. Phys. 94, 7212 (1991)]. The exclusion principle violating corrections are evaluated through a simple counting of the various excitation processes which remain possible on each class. The efficiency of the method has been tested on a series of multireference problems for which full configuration interaction results are available (OH(2) bond breaking, Be insertion in H(2), excited states of CH(2)). The dressing of a given state not only provides excellent results for this state but also provides accurate excited roots. The efficiency of state-specific dressings is dramatic. The adaptation of this proposal to difference-dedicated configuration interactions can be extremely fruitful, as illustrated in the calculation of the 1 (1)A(g)-1(1)B(u)(pi->pi(*)) transition energy of the trans-butadiene molecule.

Approximate size‐consistent treatments of Heisenberg Hamiltonians for large systems

A truncation of the Heisenberg Hamiltonian CI matrix is proposed in order to treat large molecules or clusters. The criterion for the selection of the model space is energetic. To correct the size‐inconsistency error due to the truncation of the CI matrix, the diagonal part is dressed by the spin configurations of the outer space. This correction remains simple despite its self‐consistent character and gives accurates energies. Using a geometry‐dependent Heisenberg Hamiltonian for conjugated hydrocarbons, it provides reliable geometries at low computational cost. Test calculations on linear polyenes are compared with the results of the full matrix diagonalization, and the large linear polyene C32H34 and the cyclic C32H32 polyene are studied.

Ab initio study of the ammonia ion–ammonia reaction paths

The three reactions NH+3+NH3→NH2+NH+4 (proton transfer), NH+3+NH3→NH+4+NH2 (atom transfer) and NH+3+NH3→NH3+NH+3 (charge transfer) are studied in an ab initio framework. All geometry optimizations are carried out at the MP2 level, and a SDCI(TQ) calculation is performed at the optimized geometry. For the charge transfer reaction, the energy is calculated as a function of the N–N internuclear distance. The intermediate complex is found to have D3d symmetry. The geometry of the NH3+NH+3 system is optimized for each value of the N–N distance. For the proton transfer reaction, the energy is calculated as a function of two variables which are the two N–H internuclear distances of the central part N–H–N of the complex. For each N–H–N configuration, other coordinates of the system are completely optimized. This approach shows that the atom transfer reaction can be interpreted as a charge transfer process followed by a proton transfer. The influence of the vibrational excitation of the NH+3 reagent on the reactio...

In Search of Organic Compounds Presenting a Double Exchange Phenomenon

The objective of this paper is to design a consistent series of organic molecules that may present a double exchange mechanism and study their low energy spectrum using spin unrestricted Density Functional Theory. For this purpose, organic tetra-methylene methane units having an S = 1 spin ground state and diamagnetic organic bridges are taken as building blocks for constructing molecules having two or more magnetic units. When biunit systems are ionized, the ground state of the resulting molecular ions may be either a quartet, if the spectrum is ruled by a double exchange mechanism, or a doublet, if it obeys the logic of a monoelectronic picture. A strategy based on the physical analysis of the leading interactions is followed in order to energetically favor a high-spin ground state. It is shown that the most promising compounds involve bridges that have both a large gap between the highest occupied and the lowest unoccupied molecular orbitals and small coefficients on the atoms to which the magnetic units are connected. While the followed strategy enables one to conceive organic compounds exhibiting a double exchange phenomenon, it is shown that the electronic mechanism ruling the spectrum of such organic double exchange compounds is different from that of their inorganic homologues. A new method to reconstruct the spectrum of low energy from various spin unrestricted DFT solutions is proposed and applied. Finally monodimensional and bidimensional periodic lattices based on the most promising organic architecture are suggested.

Methodological CASPT2 study of the valence excited states of an iron-porphyrin complex

The singlet valence excited states of an iron-porphyrin-pyrazine-carbonyl complex are investigated up to the Soret band (about 3 eV) using multi-state complete active space with perturbation at the second order (MS-CASPT2). This complex is a model for the active site of carboxy-hemoglobin/myoglobin. The spectrum of the excited states is rather dense, comprising states of different nature: d→π* transitions, d→d states, π→π* excitations of the porphyrin, and doubly excited states involving simultaneous intra-porphyrin π→π* and d→d transitions. Specific features of the MS-CASPT2 method are investigated. The effect of varying the number of roots in the state average calculation is quantified as well as the consequence of targeted modifications of the active space. The effect of inclusion of standard ionization potential-electron affinity (IPEA) shift in the perturbation treatment is also investigated.