Saad Yalouz

In a recent work [S. Yalouz and V. Robert, J. Chem. Theory Comput. 19, 1381 (2023)], we introduced the foundations of an orthogonally constrained complete active space self-consistent field (OC-CASSCF) framework that produces state-specific molecular orbitals for mutually orthogonal multiconfigurational electronic states. In the present study, we extend this approach by incorporating a Newton–Raphson orbital-optimization scheme, for which we derive the analytical expressions of the orbital gradient and Hessian. Furthermore, we outline a practical route toward the evaluation of analytical nuclear gradients, enabling geometry optimizations within the OC-CASSCF formalism. Benchmark calculations on the three lowest singlet states of LiH and H2O molecules demonstrate a systematic improvement as compared to conventional state-averaged CASSCF, even when using modestly sized active spaces.

Cette thèse théorique s'inscrit dans l'univers de l'Informatique quantique et celui du transfert d'énergie. Nous étudions le transport quantique d'un exciton utilisé dans le but de véhiculer une information quantique, ou de l'énergie, sur des graphes moléculaires complexes. Dans ce contexte, nous nous intéressons aux effets de différents environnements quantiques pouvant moduler le transport excitonique. Une première partie du manuscrit porte sur le transport d'information quantique en pré­sence d'un environnement de phonons locaux. Dans ce contexte, nous introduisons une ap­proche théorique appelée PT* permettant de traiter sur un pied d'égalité exciton et pho­nons. Cette théorie est tout d'abord appliquée au cas particulier du graphe en étoile. Par la suite, PT* est comparée à des calculs exacts menés sur une collection de graphes variés. Nous montrons ainsi que la théorie PT* possède une très grande force de prédictibilité et de multiples avantages théoriques et numériques ( durée de simulation, interprétations liées à l'intrication ... ) . Dans une deuxième partie du manuscrit, nous étudions le transport quantique d'énergie sur un graphe complexe en contact avec un système externe absorbant. Nous nous intéressons tout particulièrement à la caractérisation du phénomène d'absorption énergétique et son opti­misation (transition de superradiance). Nous mettons en évidence l'impact de la topologie du réseau sur l'évolution du processus d'absorption. Pour étendre cette étude, nous considérons ensuite la présence d'un désordre local brisant la symétrie du réseau de base. Nous montrons alors que le désordre peut influencer positivement l'évolution du processus d'absorption.

In this work, we introduce a selective and scalable extension of the multi-step Rayleigh–Schrödinger (RS) and Brillouin–Wigner (BW) perturbative scheme (see Bindech et al (2024) Electron. Struct. 6 045009), designed to efficiently access the low-energy spectrum of molecular systems. The method proceeds by combining successive effective Hamiltonian diagonalizations inspired by second-order RS perturbation theory, with a BW correction applied to individually optimized states using an updated partitioning of the Hamiltonian. At each step, a candidate zeroth-order state is identified and progressively decoupled from the remaining higher-lying states, thereby enabling a well-conditioned BW expansion for the energy correction. In contrast to previous approaches, the method selectively targets a small number of low-lying states, significantly reducing the numerical overhead while maintaining chemical accuracy. The robustness of the method is demonstrated on the LiH and H ₄ molecules, where accurate excitation energies are obtained for the lowest singlet states using compact model spaces, confirming its potential for realistic applications.

In a recent work, we introduced the foundations of an orthogonally constrained complete active space self-consistent field (OC-CASSCF) framework that produces state-specific molecular orbitals for mutually orthogonal multiconfigurational electronic states. In the present study, we extend this approach by incorporating a Newton-Raphson orbital-optimization scheme, for which we derive analytical expressions of the orbital gradient and Hessian. Furthermore, we outline a practical route toward the evaluation of analytical nuclear gradients, enabling geometry optimizations within the OC-CASSCF formalism. Benchmark calculations on the three lowest singlet states of LiH and H$_2$O molecules demonstrate a systematic improvement as compared to conventional state-averaged CASSCF, even when using modestly sized active spaces.

1,5-Dimethyl-3-(2,2'-bipyridine-6-yl)-6-oxoverda-zyl (bipyvd) was used as a tridentate radical ligand toward iron(II) perchlorate to yield bis(1,5-dimethyl-3-(2,2'-bipyridine-6-yl)-6-oxoverdazyl)iron(II) perchlorate, [Fe(bipyvd)₂] (ClO₄)₂. The single-crystal X-ray Fe-N distances are compatible with a low spin (S_Fe = 0) iron(II) center surrounded by two bipyvd radical ligands. A similar conclusion is reached from difference dedicated configuration interaction (DDCI) wave function calculations, which support the coupling of radicals through a closed-shell metal ion bridge. The singlet-triplet splitting is ferromagnetic, with a calculated exchange coupling constant J_LL^S_Fe = 0 = +120 cm^-1. Higher in energy, the calculated spin states split into two subsets successively characterized by pure local spin states S_Fe = 2 and S_Fe = 1. In contrast, the coordination sphere exhibits either a pure local spin state or a superposition of S_L = 1 and S_L = 0 local spin states, a phenomenon previously coined into spinmerism. A marked modification of the ligand-ligand ferromagnetic coupling is evaluated, with J_LL^S_Fe = 2 = +89 cm^-1 and J_LL^S_Fe = 1 = +140 cm^-1, whereas the metal-ligand interactions remain almost constant at ca. +290 cm^-1. The control of the magnetic interactions between organic radicals through a spin-crossover ion revisits the traditional picture dominated by the metal ions and stresses the manifestation of spin entanglement between the coordination sphere and the metal center environment.

In the present work, we examine how the recent quantum-computing algorithm known as the state-average orbital-optimized variational quantum eigensolver (SA-OO-VQE), viewed within the context of quantum chemistry as a type of multiconfiguration self-consistent field (MCSCF) electronic-structure approach, exhibits a propensity to produce an ab initio quasi-diabatic representation "for free" if considered as a least-transformed block-diagonalization procedure, as alluded to in our previous work (Yalouz, S. et al. J. Chem. Theory Comput. 2022, 18, 776-794) and thoroughly assessed herein. To this end, we introduce intrinsic and residual descriptors of diabaticity and re-explore the definition and linear-algebra properties, as well as their consequences on the vibronic nonadiabatic couplings, of an optimal diabatic representation within this context and how much one may deviate from it. Such considerations are illustrated numerically in the prototypical case of formaldimine, which presents a well-known conical intersection between its ground and first-excited singlet electronic states.