63 research outputs found
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Strain to alter the covalency and superconductivity in transition metal diborides
Among layered metal diborides, MB2, only MgB2 is a superconductor. However, a strategically applied mechanical stress that removes metal–boron covalency can turn ScB2 into a superconductor with the same mechanism
Systematic electronic structure in the cuprate parent state from quantum many-body simulations
The quantitative description of correlated electron materials remains a
modern computational challenge. We demonstrate a numerical strategy to simulate
correlated materials at the fully ab initio level beyond the solution of
effective low-energy models, and apply it to gain a detailed microscopic
understanding across a family of cuprate superconducting materials in their
parent undoped states. We uncover microscopic trends in the electron
correlations and reveal the link between the material composition and magnetic
energy scales via a many-body picture of excitation processes involving the
buffer layers. Our work illustrates a path towards a quantitative and reliable
understanding of more complex states of correlated materials at the ab initio
many-body level.Comment: 21 pages, 5 figures, with Supplementary Material
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Oxidative Dehydrogenation of Cyclohexane by Cu vs Pd Clusters: Selectivity Control by Specific Cluster Dynamics
Externally Corrected CCSD with Renormalized Perturbative Triples (R-ecCCSD(T)) and the Density Matrix Renormalization Group and Selected Configuration Interaction External Sources
We investigate the renormalized perturbative triples correction together with the externally corrected coupled-cluster singles and doubles (ecCCSD) method. We use the density matrix renormalization group (DMRG) and heat-bath CI (HCI) as external sources for the ecCCSD equations. The accuracy is assessed for the potential energy surfaces of Hâ‚‚O, Nâ‚‚, and Fâ‚‚. We find that the triples correction significantly improves upon ecCCSD, and we do not see any instability of the renormalized triples with respect to dissociation. We explore how to balance the cost of computing the external source amplitudes against the accuracy of the subsequent CC calculation. In this context, we find that very approximate wave functions (and their large amplitudes) serve as an efficient and accurate external source. Finally, we characterize the domain of correlation treatable using the ecCCSD and renormalized triples combination studied in this work via a well-known wave function diagnostic
Multireference protonation energetics of a dimeric model of nitrogenase iron-sulfur clusters
Characterizing the electronic structure of the iron--sulfur clusters in
nitrogenase is necessary to understand their role in the nitrogen fixation
process. One challenging task is to determine the protonation state of the
intermediates in the nitrogen fixing cycle. Here, we use a dimeric iron--sulfur
model to study relative energies of protonation at C, S or Fe. Using a
composite method based on coupled cluster and density matrix renormalization
group energetics, we converge the relative energies of four protonated
configurations with respect to basis set and correlation level. We find that
accurate relative energies require large basis sets, as well as a proper
treatment of multireference and relativistic effects. We have also tested ten
density functional approximations for these systems. Most of them give large
errors in the relative energies. The best performing functional in this system
is B3LYP, which gives mean absolute and maximum errors of only 10 and 13 kJ/mol
with respect to our correlated wavefunction estimates, respectively. Our work
provides benchmark results for the calibration of new approximate electronic
structure methods and density functionals for these problems.Comment: 13 pages, 8 figure
Hierarchical Clifford transformations to reduce entanglement in quantum chemistry wavefunctions
The performance of computational methods for many-body physics and chemistry
is strongly dependent on the choice of basis used to cast the problem; hence,
the search for better bases and similarity transformations is important for
progress in the field. So far, tools from theoretical quantum information have
been not thoroughly explored for this task. Here we take a step in this
direction by presenting efficiently computable Clifford similarity
transformations for quantum chemistry Hamiltonians, which expose bases with
reduced entanglement in the corresponding molecular ground states. These
transformations are constructed via block diagonalization of a hierarchy of
truncated molecular Hamiltonians, preserving the full spectrum of the original
problem. We show that the bases introduced here allow for more efficient
classical and quantum computation of ground state properties. First, we find a
systematic reduction of bipartite entanglement in molecular ground states as
compared to standard problem representations. This entanglement reduction has
implications in classical numerical methods such as ones based on the density
matrix renormalization group. Then, we develop variational quantum algorithms
that exploit the structure in the new bases, showing again improved results
when the hierarchical Clifford transformations are used.Comment: 14 pages, 11 figure
Interacting models for twisted bilayer graphene: a quantum chemistry approach
The nature of correlated states in twisted bilayer graphene (TBG) at the
magic angle has received intense attention in recent years. We present a
numerical study of an interacting Bistritzer-MacDonald (IBM) model of TBG using
a suite of methods in quantum chemistry, including Hartree-Fock, coupled
cluster singles, doubles (CCSD), and perturbative triples (CCSD(T)), as well as
a quantum chemistry formulation of the density matrix renormalization group
method (DMRG). Our treatment of TBG is agnostic to gauge choices, and hence we
present a new gauge-invariant formulation to detect the spontaneous symmetry
breaking in interacting models. To benchmark our approach, we focus on a
simplified spinless, valleyless IBM model. At integer filling (), all
numerical methods agree in terms of energy and symmetry
breaking. Additionally, as part of our benchmarking, we explore the impact of
different schemes for removing ``double-counting'' in the IBM model. Our
results at integer filling suggest that cross-validation of different IBM
models may be needed for future studies of the TBG system. After benchmarking
our approach at integer filling, we perform the first systematic study of the
IBM model near integer filling (for ). In this regime, we find that
the ground state can be in a metallic and symmetry
breaking phase. The ground state appears to have low entropy, and therefore can
be relatively well approximated by a single Slater determinant. Furthermore, we
observe many low entropy states with energies very close to the ground state
energy in the near integer filling regime
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