8 research outputs found
Ground- and Excited-State Dipole Moments and Oscillator Strengths of Full Configuration Interaction Quality
We report ground- and excited-state dipole moments and oscillator strengths
(computed in different ``gauges'' or representations) of full configuration
interaction (FCI) quality using the selected configuration interaction method
known as \textit{Configuration Interaction using a Perturbative Selection made
Iteratively} (CIPSI). Thanks to a set encompassing 35 ground- and excited-state
properties computed in 11 small molecules, the present near-FCI estimates allow
us to assess the accuracy of high-order coupled-cluster (CC) calculations
including up to quadruple excitations. In particular, we show that incrementing
the excitation degree of the CC expansion (from CCSD to CCSDT or from CCSDT to
CCSDTQ) reduces the average error with respect to the near-FCI reference values
by approximately one order of magnitude.Comment: 14 pages, 8 figures (supporting information available
SnCNâ: A Carbodiimide with an Innovative Approach for Energy Storage Systems and Phosphors in Modern LED Technology
The carbodiimide SnCN was prepared at low temperatures (400â°Câ550â°C) by using a patented urea precursor route. The crystal structure of SnCN was determined from singleâcrystal data in space group C2/c (no. 15) with a=9.1547(5), b=5.0209(3), c=6.0903(3) Ă
, ÎČ=117.672(3), V=247.92 Ă
and Z=4. As carbodiimide compounds display remarkably high thermal and chemical resistivity, SnCN has been doped with Eu and Tb to test it for its application in future phosphorâconverted LEDs. This doping of SnCN proved that a color tuning of the carbodiimide host with different activator ions and the combination of the latter ones is possible. Additionally, as the search for novel highâperforming electrode materials is essential for current battery technologies, this carbodiimide has been investigated concerning its use in lithiumâion batteries. To further elucidate its application possibilities in materials science, several characterization steps and physical measurements (XRD, in situ XANES, Sn Mössbauer spectroscopy, thermal expansion, IR spectroscopy, MottâSchottky analysis) were carried out. The electronic structure of the nâtype semiconductor SnCN has been probed using Xâray absorption spectroscopy and density functional theory (DFT) computations
Hierarchy Configuration Interaction: Combining Seniority Number and Excitation Degree
We propose a novel partitioning of the Hilbert space, hierarchy configuration
interaction (hCI), where the excitation degree (with respect to a given
reference determinant) and the seniority number (i.e., the number of unpaired
electrons) are combined in a single hierarchy parameter. The key appealing
feature of hCI is that each hierarchy level accounts for all classes of
determinants whose number share the same scaling with system size. By surveying
the dissociation of multiple molecular systems, we found that the overall
performance of hCI usually exceeds or, at least, parallels that of
excitation-based CI. For higher orders of hCI and excitation-based CI, the
additional computational burden related to orbital optimization usually do not
compensate the marginal improvements compared with results obtained with
Hartree-Fock orbitals. The exception is orbital-optimized CI with single
excitations, a minimally correlated model displaying the qualitatively correct
description of single bond breaking, at a very modest computational cost.Comment: 7 pages, 3 figures (supporting information available
State-Specific Coupled-Cluster Methods for Excited States
15 pages, 4 figuresWe reexamine âCCSD, a state-specific coupled-cluster (CC) with single and double excitations (CCSD) approach that targets excited states through the utilization of non-Aufbau determinants. This methodology is particularly efficient when dealing with doubly excited states, a domain where the standard equation-of-motion CCSD (EOM-CCSD) formalism falls short. Our goal here is to evaluate the effectiveness of âCCSD when applied to other types of excited states, comparing its consistency and accuracy with EOM-CCSD. To this end, we report a benchmark on excitation energies computed with the âCCSD and EOM-CCSD methods, for a set of molecular excited-state energies that encompasses not only doubly excited states but also doublet-doublet transitions and (singlet and triplet) singly-excited states of closed-shell systems. In the latter case, we rely on a minimalist version of multireference CC known as the two-determinant CCSD method to compute the excited states. Our dataset, consisting of 276 excited states stemming from the quest database [VĂ©ril et al., WIREs Comput. Mol. Sci. 2021, 11, e1517], provides a significant base to draw general conclusions concerning the accuracy of âCCSD. Except for the doubly-excited states, we found that âCCSD underperforms EOM-CCSD. For doublet-doublet transitions, the difference between the mean absolute errors (MAEs) of the two methodologies (of 0.10 eV and 0.07 eV) is less pronounced than that obtained for singly-excited states of closed-shell systems (MAEs of 0.15 eV and 0.08 eV). This discrepancy is largely attributed to a greater number of excited states in the latter set exhibiting multiconfigurational characters, which are more challenging for âCCSD. We also found typically small improvements by employing state-specific optimized orbitals
Accurate full configuration interaction correlation energy estimates for five- and six-membered rings
13 pages, 5 figuresFollowing our recent work on the benzene molecule [\href{https://doi.org/10.1063/5.0027617}{J.~Chem.~Phys.~\textbf{153}, 176101 (2020)}], itself motivated by the blind challenge of Eriksen \textit{et al.} [\href{https://doi.org/10.1021/acs.jpclett.0c02621}{J.~Phys.~Chem.~Lett.~\textbf{11}, 8922 (2020)}] on the same system, we report accurate full configuration interaction (FCI) frozen-core correlation energy estimates for twelve five- and six-membered ring molecules in the standard correlation-consistent double- Dunning basis set (cc-pVDZ). Our FCI correlation energy estimates, with estimated error smaller than 1 millihartree, are based on energetically optimized-orbital selected configuration interaction (SCI) calculations performed with the \textit{Configuration Interaction using a Perturbative Selection made Iteratively} (CIPSI) algorithm. Having at our disposal these accurate reference energies, the respective performance and convergence properties of several popular and widely-used families of single-reference quantum chemistry methods are investigated. In particular, we study the convergence properties of i) the M{\o}ller-Plesset perturbation series up to fifth-order (MP2, MP3, MP4, and MP5), ii) the iterative approximate coupled-cluster series CC2, CC3, and CC4, and iii) the coupled-cluster series CCSD, CCSDT, and CCSDTQ. The performance of the ground-state gold standard CCSD(T) as well as the completely renormalized CC model, CR-CC(2,3), are also investigated
Ground-and Excited-State Dipole Moments and Oscillator Strengths of Full Configuration Interaction Quality
14 pages, 8 figures (supporting information available)We report ground- and excited-state dipole moments and oscillator strengths (computed in different ``gauges'' or representations) of full configuration interaction (FCI) quality using the selected configuration interaction method known as \textit{Configuration Interaction using a Perturbative Selection made Iteratively} (CIPSI). Thanks to a set encompassing 35 ground- and excited-state properties computed in 11 small molecules, the present near-FCI estimates allow us to assess the accuracy of high-order coupled-cluster (CC) calculations including up to quadruple excitations. In particular, we show that incrementing the excitation degree of the CC expansion (from CCSD to CCSDT or from CCSDT to CCSDTQ) reduces the average error with respect to the near-FCI reference values by approximately one order of magnitude