Adsorption of cobalt on graphene: Electron correlation effects from a quantum chemical perspective

In this work, we investigate the adsorption of a single cobalt atom (Co) on graphene by means of the complete active space self-consistent field approach, additionally corrected by the second-order perturbation theory. The local structure of graphene is modeled by a planar hydrocarbon cluster (C$_{24}$H$_{12}$). Systematic treatment of the electron correlations and the possibility to study excited states allow us to reproduce the potential energy curves for different electronic configurations of Co. We find that upon approaching the surface, the ground-state configuration of Co undergoes several transitions, giving rise to two stable states. The first corresponds to the physisorption of the adatom in the high-spin $3d^74s^2$ ($S=3/2$) configuration, while the second results from the chemical bonding formed by strong orbital hybridization, leading to the low-spin $3d^9$ ($S=1/2$) state. Due to the instability of the $3d^9$ configuration, the adsorption energy of Co is small in both cases and does not exceed 0.35 eV. We analyze the obtained results in terms of a simple model Hamiltonian that involves Coulomb repulsion ($U$) and exchange coupling ($J$) parameters for the 3$d$ shell of Co, which we estimate from first-principles calculations. We show that while the exchange interaction remains constant upon adsorption ($\simeq1.1$ eV), the Coulomb repulsion significantly reduces for decreasing distances (from 5.3 to 2.6$\pm$0.2 eV). The screening of $U$ favors higher occupations of the 3$d$ shell and thus is largely responsible for the interconfigurational transitions of Co. Finally, we discuss the limitations of the approaches that are based on density functional theory with respect to transition metal atoms on graphene, and we conclude that a proper account of the electron correlations is crucial for the description of adsorption in such systems.Comment: 12 pages, 6 figures, 2 table

Exchange Coupling Interactions from the Density Matrix Renormalization Group and N-Electron Valence Perturbation Theory: Application to a Biomimetic Mixed Valence Manganese Complex

The accurate description of magnetic level energetics in oligonuclear exchange-coupled transition-metal complexes remains a formidable challenge for quantum chemistry. The density matrix renormalization group (DMRG) brings such systems for the first time easily within reach of multireference wave function methods by enabling the use of unprecedentedly large active spaces. But does this guarantee systematic improvement in predictive ability and, if so, under which conditions? We identify operational parameters in the use of DMRG using as a test system an experimentally characterized mixed-valence bis-μ-oxo/μ-acetato Mn(III,IV) dimer, a model for the oxygen-evolving complex of photosystem II. A complete active space of all metal 3d and bridge 2p orbitals proved to be the smallest meaningful starting point; this is readily accessible with DMRG and greatly improves on the unrealistic metal-only configuration interaction or complete active space self-consistent field (CASSCF) values. Orbital optimization is critical for stabilizing the antiferromagnetic state, while a state-averaged approach over all spin states involved is required to avoid artificial deviations from isotropic behavior that are associated with state-specific calculations. Selective inclusion of localized orbital subspaces enables probing the relative contributions of different ligands and distinct superexchange pathways. Overall, however, full-valence DMRG-CASSCF calculations fall short of providing a quantitative description of the exchange coupling owing to insufficient recovery of dynamic correlation. Quantitatively accurate results can be achieved through a DMRG implementation of second order N-electron valence perturbation theory (NEVPT2) in conjunction with a full-valence metal and ligand active space. Perspectives for future applications of DMRG-CASSCF/NEVPT2 to exchange coupling in oligonuclear clusters are discussed

Beyond Density Functional Theory: the Multiconfigurational Approach to Model Heterogeneous Catalysis

Catalytic processes are crucially important for many practical chemical applications. Heterogeneous catalysts are especially appealing because of their high stability and the relative ease with which they may be recovered and reused. Computational modeling can play an important role in the design of more catalytically active materials through the identification of reaction mechanisms and the opportunity to assess hypothetical catalysts in silico prior to experimental verification. Kohn-Sham density functional theory (KS-DFT) is the most used method in computational catalysis because it is affordable and it gives results of reasonable accuracy in many instances. Furthermore, it can be employed in a “black-box” mode that does not require significant a priori knowledge of the system. However, KS-DFT has some limitations: it suffers from self-interaction error (sometime referred to as delocalization error), but a greater concern is that it provides an intrinsically single-reference description of the electronic structure, and this can be especially problematic for modeling catalysis when transition metals are involved. In this perspective, we highlight some noteworthy applications of KS-DFT to heterogeneous computational catalysis, as well as cases where KS-DFT fails accurately to describe electronic structures and intermediate spin states in open-shell transition metal systems. We next provide an introduction to state-of-the-art multiconfigurational (MC; also referred to as multireference (MR)) methods and their advantages and limitations for modeling heterogeneous catalysis. We focus on specific examples to which MC methods have 2 been applied and discuss the challenges associated with these calculations. We conclude by offering our vision for how the community can make further progress in the development of MC methods for application to heterogeneous catalysis

The OpenMolcas Web: A Community-Driven Approach to Advancing Computational Chemistry

The developments of the open-source OpenMolcas chemistry software environment since spring 2020 are described, with a focus on novel functionalities accessible in the stable branch of the package or via interfaces with other packages. These developments span a wide range of topics in computational chemistry and are presented in thematic sections: electronic structure theory, electronic spectroscopy simulations, analytic gradients and molecular structure optimizations, ab initio molecular dynamics, and other new features. This report offers an overview of the chemical phenomena and processes OpenMolcas can address, while showing that OpenMolcas is an attractive platform for state-of-the-art atomistic computer simulations

Magnetic Interactions in a [Co(II)3Er(III)(OR)4] Model Cubane through Forefront Multiconfigurational Methods

Strong electron correlation effects are one of the major challenges in modern quantum chemistry. Polynuclear transition metal clusters are peculiar examples of systems featuring such forms of electron correlation. Multireference strategies, often based on but not limited to the concept of complete active space, are adopted to accurately account for strong electron correlation and to resolve their complex electronic structures. However, transition metal clusters already containing four magnetic centers with multiple unpaired electrons make conventional active space based strategies prohibitively expensive, due to their unfavorable scaling with the size of the active space. In this work, forefront techniques, such as density matrix renormalization group (DMRG), full configuration interaction quantum Monte Carlo (FCIQMC), and multiconfiguration pair-density functional theory (MCPDFT), are employed to overcome the computational limitation of conventional multireference approaches and to accurately investigate the magnetic interactions taking place in a [Co(II)3Er(III)(OR)4] (chemical formula [Co(II)3Er(III)(hmp)4(μ2-OAc)2(OH)3(H2O)], hmp = 2-(hydroxymethyl)-pyridine) model cubane water oxidation catalyst. Complete active spaces with up to 56 electrons in 56 orbitals have been constructed for the seven energetically lowest different spin states. Relative energies, local spin, and spin–spin correlation values are reported and provide crucial insights on the spin interactions for this model system, pivotal in the rationalization of the catalytic activity of this system in the water-splitting reaction. A ferromagnetic ground state is found with a very small, ∼50 cm–1, highest-to-lowest spin gap. Moreover, for the energetically lowest states, S = 3–6, the three Co(II) sites exhibit parallel aligned spins, and for the lower states, S = 0–2, two Co(II) sites retain strong parallel spin alignment

Poster Session

Posters presented by: P01: Adam S. Abbott, University of Georgia P02: Yasmeen Abdo, University of Mississippi P03: Vibin Abraham, Virginia Tech P04: Asim Alenaizan, Georgia Institute of Technology P05: Isuru R. Ariyanthna, Auburn University P06: Brandon W. Bakr, Georgia Institute of Technology P07: [Matthew Bassett, Georgia Southern University] P08: Alexandre P. Bazanté, University of Florida P09: Andrea N. Becker, University of Tennessee P10: Randi Beil, University of Tennessee P11: Andrea N. Bootsma, University of Georgia/Texas A&M University P12: Adam Bruner, Louisiana State University P13: Lori A. Burns, Georgia Institute of Technology P14: Chanxi Cai, Emory University P15: Katherine A. Charbonnet, University of Memphis P16: Marjory C. Clement, Virginia Tech P17: Wallace D. Derricotte, Emory University P18: Harkiran Dhah, University of Tennessee P19: Manuel Díaz-Tinoco, Auburn University P20: Vivek Dixit: Mississippi State University P21: Eric Van Dornshuld, Mississippi State University P22: Katelyn M. Dreux, University of Mississippi P23: Narendra Nath Dutta, Auburn University P24: William Earwood, University of Mississippi P25: Thomas L. Ellington, University of Mississippi P26: Marissa L. Estep, University of Georgia P27: Yanfei Guan, Texas A&M University P28: Andrew M. James, Virginia Tech P29: Yifan Jin, University of Florida P30: Dwayne John, Middle Tennessee State University P31: Sarah N. Johnson, University of Mississippi P32: Noor Md Shahriar Khan, Auburn University P33: Monika Kodrycka, Auburn University P34: Ashutosh Kumar, Virginia Tech P35: Elliot Lakner, University of Alabama P36: Robert W. Lamb, Mississippi State University P37: S. Paul Lee, University of Mississippi P38: Zachary Lee, University of Alabama P39: Conrad D. Lewis, Middle Tennessee State University P40: Guangchao Liang, Mississippi State University P41: Chenyang Li, Emory University P42: Hannah C. Lozano, University of Memphis P43: SharathChandra Mallojjala, University of Georgia/Texas A&M University P44: Zheng Ma, Duke University P45: Elvis Maradzike, Florida State University P46: Ashley S. McNeill, University of Alabama P47: Stephen R. Miller, University of Georgia P48: W. J. Morgan, University of Georgia P49: Apurba Nandi, Emory University P50: Daniel R. Nascimento, Florida State University P51: Brooke N. Nash, Mississippi College P52: Carlie M. Novak, Georgia Southern University P53: Young Choon Park, University of Florida P54: Kirk C. Pearce, Virginia Tech P55: Rudradatt (Randy) Persaud, University of Alabama P56: Karl Pierce, Virginia Tech P57: Kimberley N. Poland, University of Mississippi P58: Chen Qu, Emory University P59: Duminda S. Ranasinghe, University of Florida P60: Hailey B. Reed, University of Mississippi P61: Matthew Schieber, Georgia Institute of Technology P62: Jeffrey B. Schriber, Emory University P63: Thomas Sexton, University of Mississippi P64: Holden T. Smith, Louisiana State University P65: Aubrey Smyly, Mississippi College P66: B. T. Soto, University of Georgia P67: Trent H. Stein, University of Alabama P68: Cody J. Stephan, Georgia Southern University P69: Thomas Summers, University of Memphis P70: Zhi Sun, University of Georgia P71: Monica Vasiliu, University of Alabama P72: Jonathan M. Waldrop, Auburn University P73: Tommy Walls, Southern Louisiana University P74: Qingfeng (Kee) Wang, Emory University P75: Constance E. Warden, Georgia Institute of Technology P76: Jared D. Weidman, University of Georgia P77: Melody Williams, University of Memphis P78: Donna Xia, University of Alabama P79: Qi Yu, Emory University P80: Boyi Zhang, University of Georgia P81: Tianyuan Zhang, Emory University P82: Michael Zott, Georgia Institute of Technolog

Multireference equation of motion coupled cluster benchmark study of magnetic model systems

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.comptc.2018.03.009 © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/The multireference equation of motion coupled cluster (MREOM-CC) approach including spin-orbit coupling is applied to magnetic model systems FArO, FArOF and FArFOH, and compared to benchmark multireference Configuration Interaction (MRCI) approaches in the case of FArO. In the MREOM calculations, convenient high-spin states provide reference reduced density matrices, that are used to obtain a sequence of similarity transformations of the Hamiltonian. All low-lying magnetic states are obtained subsequently from a compact diagonalization of the transformed Hamiltonian. The accuracy of MREOM is shown to be comparable to MRCI+Q, but the approach is significantly more efficient for systems with a large number of electronic states. Moreover, MREOM is nearly size-consistent and this allows one to perform meaningful calculations of the strength of the magnetic coupling in the weak coupling limit.Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery gran

Multireference approaches for excited states of molecules

Understanding the properties of electronically excited states is a challenging task that becomes increasingly important for numerous applications in chemistry, molecular physics, molecular biology, and materials science. A substantial impact is exerted by the fascinating progress in time-resolved spectroscopy, which leads to a strongly growing demand for theoretical methods to describe the characteristic features of excited states accurately. Whereas for electronic ground state problems of stable molecules the quantum chemical methodology is now so well developed that informed nonexperts can use it efficiently, the situation is entirely different concerning the investigation of excited states. This review is devoted to a specific class of approaches, usually denoted as multireference (MR) methods, the generality of which is needed for solving many spectroscopic or photodynamical problems. However, the understanding and proper application of these MR methods is often found to be difficult due to their complexity and their computational cost. The purpose of this review is to provide an overview of the most important facts about the different theoretical approaches available and to present by means of a collection of characteristic examples useful information, which can guide the reader in performing their own applications