| 1. |
- Apriliyanto, Yusuf Bramastya, et al.
(author)
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Toward a Generalized Hückel Rule : The Electronic Structure of Carbon Nanocones
- 2021
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In: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 125:45, s. 9819-9825
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Journal article (peer-reviewed)abstract
- In this work, we investigate a particular class of carbon nanocones, which we name graphannulenes, and present a generalized Hückel rule (GHR) that predicts the character of their ground state based on simply the three topological indices that uniquely define them. Importantly, this rule applies to both flat and curved systems, encompassing a wide variety of known structures that do not satisfy the “classic” 4n + 2 rule such as coronene, corannulene, and Kekulene. We test this rule at the Hückel level of theory for a large number of systems, including structures that are convex and flat, with a saddle-like geometry, and at the CASSCF level of theory for a selected representative subset. All the performed calculations support the GHR that we propose in this work.
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| 2. |
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| 3. |
- Aquilante, Francesco, et al.
(author)
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Modern quantum chemistry with [Open]Molcas
- 2020
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 152:21
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Journal article (peer-reviewed)abstract
- MOLCAS/OpenMolcas is an ab initio electronic structure program providing a large set of computational methods from Hartree-Fock and density functional theory to various implementations of multiconfigurational theory. This article provides a comprehensive overview of the main features of the code, specifically reviewing the use of the code in previously reported chemical applications as well as more recent applications including the calculation of magnetic properties from optimized density matrix renormalization group wave functions.
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| 4. |
- Battaglia, Stefano, et al.
(author)
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Extended Dynamically Weighted CASPT2 : The Best of Two Worlds
- 2020
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In: Journal of Chemical Theory and Computation. - : AMER CHEMICAL SOC. - 1549-9618 .- 1549-9626. ; 16:3, s. 1555-1567
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Journal article (peer-reviewed)abstract
- We introduce a new variant of the complete active A space second-order perturbation theory (CASPT2) method that performs similarly to multistate CASPT2 (MS-CASPT2) in regions of the potential energy surface where the electronic states are energetically well separated and is akin to extended MS-CASPT2 (XMS-CASPT2) in case the underlying zeroth-order references are near-degenerate. Our approach follows a recipe analogous to that of XMS-CASPT2 to ensure approximate invariance under unitary transformations of the model states and a dynamic weighting scheme to smoothly interpolate the Fock operator between state-specific and state-average regimes. The resulting extended dynamically weighted CASPT2 (XDW-CASPT2) methodology possesses the most desirable features of both MS-CASPT2 and XMS-CASPT2, that is, the ability to provide accurate transition energies and correctly describe avoided crossings and conical intersections. The reliability of XDW-CASPT2 is assessed on a number of molecular systems. First, we consider the dissociation of lithium fluoride, highlighting the distinctive characteristics of the new approach. Second, the invariance of the theory is investigated by studying the conical intersection of the distorted allene molecule. Finally, the relative accuracy in the calculation of vertical excitation energies is benchmarked on a set of 26 organic compounds. We found that XDW-CASPT2, albeit being only approximately invariant, produces smooth potential energy surfaces around conical intersections and avoided crossings, performing equally well to the strictly invariant XMS-CASPT2 method. The accuracy of vertical transition energies is almost identical to MS-CASPT2, with a mean absolute deviation of 0.01-0.02 eV, in contrast to 0.12 eV for XMS-CASPT2.
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| 5. |
- Battaglia, Stefano, et al.
(author)
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Multiconfigurational quantum chemistry: The CASPT2 method
- 2023
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In: Theoretical and Computational Photochemistry. - Amsterdam : Elsevier. ; , s. 135-162
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Book chapter (peer-reviewed)abstract
- This chapter presents the theory behind the CASPT2 method and its adaptation to a multistate formalism. The chapter starts with an introduction to the theory of the CASPT2 method—an application of the Rayleigh-Schrödinger perturbation theory applied to the multiconfigurational reference function—as it was originally presented. In particular, we discuss the nature of the reference Hamiltonian and the first-order interacting space. This is followed by some detailed discussion with respect to the intruder state problem and various shift techniques to address this problem. Afterward, a longer review on alternative reference Hamiltonians, which to some degree or completely remove the intruder state problem, is put forward. Subsequently, the presently proposed multistate versions of the CASPT2 method are discussed in some detail. The chapter is concluded with a review of different benchmark assessments of the accuracy of the method and a qualified suggestion on the future development potentials of the approach.
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| 6. |
- Battaglia, Stefano, et al.
(author)
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On the role of symmetry in XDW-CASPT2
- 2021
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In: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 154:3
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Journal article (peer-reviewed)abstract
- Herewith, we propose two new exponents for the recently introduced XDW-CASPT2 method [S. Battaglia and R. Lindh, J. Chem. Theory Comput. 16, 1555-1567 (2020)], which fix one of the largest issues hindering this approach. By using the first-order effective Hamiltonian coupling elements, the weighting scheme implicitly takes into account the symmetry of the states, thereby averaging Fock operators only if the zeroth-order wave functions interact with each other. The use of Hamiltonian couplings also provides a physically sounder approach to quantitate the relative weights; however, it introduces new difficulties when these rapidly die off to zero. The improved XDW-CASPT2 method is critically tested on several systems of photochemical relevance, and it is shown that it succeeds in its original intent of maintaining MS-CASPT2 accuracy for the evaluation of transition energies and at the same time providing smooth potential energy surfaces around near-degenerate points akin to XMS-CASPT2.
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| 7. |
- Battaglia, Stefano, et al.
(author)
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Regularized CASPT2 : an Intruder-State-Free Approach
- 2022
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In: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 18:8, s. 4814-4825
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Journal article (peer-reviewed)abstract
- In this work we present a new approach to fix the intruder-state problem (ISP) in CASPT2 based on sigma p regularization. The resulting sigma(p)-CASPT2 method is compared to previous techniques, namely, the real and imaginary level shifts, on a theoretical basis and by performing a series of systematic calculations. The analysis is focused on two aspects, the effectiveness of sigma(p)-CASPT2 in removing the ISP and the sensitivity of the approach with respect to the input parameter. We found that sigma p- CASPT2 compares favorably with respect to previous approaches and that different versions, sigma(1)-CASPT2 and sigma(2)-CASPT2, have different potential application domains. This analysis also reveals the unsuitability of the real level shift technique as a general way to avoid the intruder-state problem.
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| 8. |
- Manni, Giovanni Li, et al.
(author)
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The OpenMolcas Web : A Community-Driven Approach to Advancing Computational Chemistry
- 2023
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In: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 19:20, s. 6933-6991
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Journal article (peer-reviewed)abstract
- 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.
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| 9. |
- Pennisi, G., et al.
(author)
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A Genetic and Metabolic Staging System for Predicting the Outcome of Nonalcoholic Fatty Liver Disease
- 2022
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In: Hepatology Communications. - : Ovid Technologies (Wolters Kluwer Health). - 2471-254X. ; 6:5, s. 1032-1044
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Journal article (peer-reviewed)abstract
- Nonalcoholic fatty liver disease (NAFLD) is an emerging cause of liver-related events (LREs). Here, we have assessed the ability of a composite score based on clinical features, metabolic comorbidities, and genetic variants to predict LREs. A total of 546 consecutive patients with NAFLD were recruited and stratified according to the fibrosis-4 (FIB-4) index. LREs were defined as occurrence of hepatocellular carcinoma or hepatic decompensation. Cox regression multivariate analysis was used to identify baseline variables associated with LREs. The UK Biobank was used as the validation cohort, and severe liver disease (incidence of cirrhosis, decompensated liver disease, hepatocellular carcinoma, and/or liver transplantation) was used as the outcome. LREs were experienced by 58 patients, only one of whom was in the cohort of patients with a FIB-4 score < 1.3. Multivariate Cox regression analysis of 229 patients with a FIB-4 score >= 1.3 highlighted clinical variables independently associated with the development of LREs, including older age, low platelet count, low albumin, low high-density lipoprotein cholesterol, certain genetic factors, and interactions between genetic factors and sex or diabetes. The area under the curve (AUC) for the model was 0.87 at 1, 3, and 5 years. Our novel Genetic and Metabolic Staging (GEMS) scoring system was derived from the Cox model linear predictor, ranked from 0 to 10, and categorized into five classes (0-5, 5-6, 6-7, 7-8, and 8-10). The risk of LREs increased from 4% in patients in the best class (GEMS score 0-5) to 91% in the worst (GEMS score 8-10). GEMS score was associated with incident severe liver disease in the study population (hazard ratio, 1.56; 95% confidence interval, 1.48-1.65; P < 0.001) as well as in the UK Biobank cohort where AUCs for prediction of severe liver disease at 1, 3, and 5 years were 0.70, 0.69, and 0.67, respectively. Conclusion: The novel GEMS scoring system has an adequate ability to predict the outcome of patients with NAFLD.
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| 10. |
- Reiersolmoen, Ann Christin, et al.
(author)
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Symmetry of three-center, four-electron bonds
- 2020
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In: Chemical Science. - : Royal Society of Chemistry (RSC). - 2041-6520 .- 2041-6539. ; 11:30, s. 7979-7990
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Journal article (peer-reviewed)abstract
- Three-center, four-electron bonds provide unusually strong interactions; however, their nature remains ununderstood. Investigations of the strength, symmetry and the covalentversuselectrostatic character of three-center hydrogen bonds have vastly contributed to the understanding of chemical bonding, whereas the assessments of the analogous three-center halogen, chalcogen, tetrel and metallic & xe111;-type long bonding are still lagging behind. Herein, we disclose the X-ray crystallographic, NMR spectroscopic and computational investigation of three-center, four-electron [D-X-D](+)bonding for a variety of cations (X+= H+, Li+, Na+, F+, Cl+, Br+, I+, Ag(+)and Au+) using a benchmark bidentate model system. Formation of a three-center bond, [D-X-D](+)is accompanied by an at least 30% shortening of the D-X bonds. We introduce a numerical index that correlates symmetry to the ionic size and the electron affinity of the central cation, X+. Providing an improved understanding of the fundamental factors determining bond symmetry on a comprehensive level is expected to facilitate future developments and applications of secondary bonding and hypervalent chemistry.
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