المؤلفون: Dunning, Thom H., Cooper, David L., Xu, Lu T., Karadakov, Peter B.

المصدر: Comprehensive Computational Chemistry ; page 354-402 ; ISBN 9780128232569

الإتاحة: https://doi.org/10.1016/b978-0-12-821978-2.00017-9Test
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المؤلفون: Dunning, Thom H.¹ (AUTHOR) thdjr@uw.edu, Xu, Lu T.¹ (AUTHOR)

المصدر: Journal of Computational Chemistry. 3/15/2024, Vol. 45 Issue 7, p405-418. 14p.

مصطلحات موضوعية: *ELECTRONIC structure, *ELECTRON configuration, *ELECTRON affinity, *SMALL molecules, *LITHIUM ions, *IONIZATION energy

مستخلص: The current study of the small lithium molecules Li2+,0,− and Li3+,0,− focuses on the nature of the bonding in these molecules as well as their structures and energetics (bond energies, ionization energies, and electron affinities). Valence CASSCF (2s,2p) calculations incorporate nondynamical electron correlation in the calculations, while the corresponding multireference configuration interaction and coupled cluster calculations incorporate dynamical electron correlation. Treatment of nondynamical correlation is critical for properly describing the Li2,3+,0,− molecules as well as the Li− anion with dynamical correlation, in general, only fine‐tuning the predictions. All lithium molecules and ions are bound, with the Li3+ and Li2+ ions being the most strongly bound, followed by Li3−, Li2, Li2− and Li3. The minimum energy structures of Li3+,0,− are, respectively, an equilateral triangle, an isosceles triangle, and a linear structure. The results of SCGVB calculations are analyzed to obtain insights into the nature of the bonding in these molecules. An important finding of this work is that interstitial orbitals, a concept first put forward by McAdon and Goddard in 1985, play an essential role in the bonding of all lithium molecules considered here except for Li2. The interstitial orbitals found in the Li3+,0 molecules likely give rise to the non‐nuclear attractors/maxima observed in these molecules. [ABSTRACT FROM AUTHOR]

المؤلفون: Dunning, Thom H., Xu, Lu T.

المصدر: Journal of Computational Chemistry; 3/15/2024, Vol. 45 Issue 7, p405-418, 14p

مصطلحات موضوعية: ELECTRONIC structure, ELECTRON configuration, ELECTRON affinity, SMALL molecules, LITHIUM ions, IONIZATION energy

مستخلص: The current study of the small lithium molecules Li2+,0,− and Li3+,0,− focuses on the nature of the bonding in these molecules as well as their structures and energetics (bond energies, ionization energies, and electron affinities). Valence CASSCF (2s,2p) calculations incorporate nondynamical electron correlation in the calculations, while the corresponding multireference configuration interaction and coupled cluster calculations incorporate dynamical electron correlation. Treatment of nondynamical correlation is critical for properly describing the Li2,3+,0,− molecules as well as the Li− anion with dynamical correlation, in general, only fine‐tuning the predictions. All lithium molecules and ions are bound, with the Li3+ and Li2+ ions being the most strongly bound, followed by Li3−, Li2, Li2− and Li3. The minimum energy structures of Li3+,0,− are, respectively, an equilateral triangle, an isosceles triangle, and a linear structure. The results of SCGVB calculations are analyzed to obtain insights into the nature of the bonding in these molecules. An important finding of this work is that interstitial orbitals, a concept first put forward by McAdon and Goddard in 1985, play an essential role in the bonding of all lithium molecules considered here except for Li2. The interstitial orbitals found in the Li3+,0 molecules likely give rise to the non‐nuclear attractors/maxima observed in these molecules. [ABSTRACT FROM AUTHOR]

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المؤلفون: Dunning Jr., Thom H.¹ (AUTHOR) thdjr@uw.edu, Xu, Lu T.¹ (AUTHOR) luxu.qc@gmail.com

المصدر: Journal of Chemical Physics. 8/28/2022, Vol. 157 Issue 8, p1-9. 9p.

مصطلحات موضوعية: *ELECTRON configuration, *CHEMICAL bonds, *STATE bonds, *CHEMICAL bond lengths, *ELECTRON pairs, *COVALENT bonds

مستخلص: We extended our studies of the effect of dynamical electron correlation on the covalent bonds in the AH and AF series (A = B–F) to the recoupled pair bonds in the excited a4Σ− states of the CH and CF molecules. Dynamical correlation is energetically less important in the a4Σ− states than in the corresponding X2Π states for both molecules, which is reflected in smaller changes in bond energies (De). Changes in the equilibrium bond distance (Re) and vibrational frequency (ωe), on the other hand, are influenced by the changes in the slope and curvature of the dynamical electron correlation energy as a function of the internuclear distance R, EDEC(R). In the CH(a4Σ−) state, these changes are much smaller than in the CH(X2Π) state, but in the CF(a4Σ−) state, they are larger, reflecting a significant difference in the shapes of EDEC(R) curves. At large R, the shape of EDEC(R) curves for covalent and recoupled pair bonds is similar although different in magnitude. For the CH(a4Σ−) state, EDEC(R) has a minimum at R = Re + 0.72 Å as the orbitals associated with the formation of the recoupled pair bond switch places. EDEC(R) for the CF(a4Σ−) state decreases continuously throughout the bound region of the potential energy curve because the dynamical correlation energy associated with the electrons in the lone pair orbitals is increasing. These results support our earlier conclusion that we still have much to learn about the nature of dynamical electron correlation in molecules. [ABSTRACT FROM AUTHOR]

المؤلفون: Xu, Lu T.¹ (AUTHOR) luxu.qc@gmail.com, Dunning Jr., Thom H.¹ (AUTHOR) thdjr@uw.edu

المصدر: Journal of Chemical Physics. 7/7/2022, Vol. 157 Issue 1, p1-19. 19p.

مصطلحات موضوعية: *ELECTRON configuration, *CHEMICAL bonds, *COVALENT bonds, *POLAR molecules, *VALENCE bonds, *CHEMICAL bond lengths

مستخلص: Dynamical electron correlation has a major impact on the computed values of molecular properties and the energetics of molecular processes. This study focused on the effect of dynamical electron correlation on the spectroscopic constants (Re, ωe, De), and potential energy curves, ΔE(R), of the covalently bound AH and AF molecules, A = B–F. The changes in the spectroscopic constants (ΔRe, Δωe, ΔDe) caused by dynamical correlation are erratic and, at times, even surprising. These changes can be understood based on the dependence of the dynamical electron correlation energies of the AH and AF molecules as a function of the bond distance, i.e., ΔEDEC(R). At large R, the magnitude of ΔEDEC(R) increases nearly exponentially with decreasing R, but this increase slows as R continues to decrease and, in many cases, even reverses at very short R. The changes in ΔEDEC(R) in the region around Re were as unexpected as they were surprising, e.g., distinct minima and maxima were found in the curves of ΔEDEC(R) for the most polar molecules. The variations in ΔEDEC(R) for R ≲ Re are directly correlated with major changes in the electronic structure of the molecules as revealed by a detailed analysis of the spin-coupled generalized valence bond wave function. The results reported here indicate that we have much to learn about the nature of dynamical electron correlation and its effect on chemical bonds and molecular properties and processes. [ABSTRACT FROM AUTHOR]

المؤلفون: Xu, Lu T., Dunning, Thom H.

المصدر: The Journal of Physical Chemistry A ; volume 125, issue 40, page 9026-9026 ; ISSN 1089-5639 1520-5215

مصطلحات موضوعية: Physical and Theoretical Chemistry

الإتاحة: https://doi.org/10.1021/acs.jpca.1c08386Test

المؤلفون: Dunning, Thom H., Xu, Lu T.

المساهمون: U.S. Department of Energy

المصدر: Journal of Computational Chemistry ; volume 45, issue 7, page 405-418 ; ISSN 0192-8651 1096-987X

الوصف: The current study of the small lithium molecules Li 2 +,0,− and Li 3 +,0,− focuses on the nature of the bonding in these molecules as well as their structures and energetics (bond energies, ionization energies, and electron affinities). Valence CASSCF (2s,2p) calculations incorporate nondynamical electron correlation in the calculations, while the corresponding multireference configuration interaction and coupled cluster calculations incorporate dynamical electron correlation. Treatment of nondynamical correlation is critical for properly describing the Li 2,3 +,0,− molecules as well as the Li − anion with dynamical correlation, in general, only fine‐tuning the predictions. All lithium molecules and ions are bound, with the Li 3 + and Li 2 + ions being the most strongly bound, followed by Li 3 − , Li 2 , Li 2 − and Li 3 . The minimum energy structures of Li 3 +,0,− are, respectively, an equilateral triangle, an isosceles triangle, and a linear structure. The results of SCGVB calculations are analyzed to obtain insights into the nature of the bonding in these molecules. An important finding of this work is that interstitial orbitals, a concept first put forward by McAdon and Goddard in 1985, play an essential role in the bonding of all lithium molecules considered here except for Li 2 . The interstitial orbitals found in the Li 3 +,0 molecules likely give rise to the non‐nuclear attractors/maxima observed in these molecules.

الإتاحة: https://doi.org/10.1002/jcc.27246Test

المؤلفون: Dunning Jr, Thom H, Cooper, David L, Xu, Lu T, Karadakov, Peter B

المساهمون: Boyd, Russell J, Yáñez, Manuel

الوصف: This chapter outlines the basic elements of Spin-Coupled Generalized Valence Bond (SCGVB) theory, including optimization and analysis of SCGVB wavefunctions. It is shown how such wavefunctions, which account for non-dynamical electron correlation, provide compelling descriptions of the electronic structures of the ground and excited states of molecules and of the electronic mechanisms of molecular reactions, using as representative examples: the “traditional” HCN molecule, and its fragmentation into CH and N; the formation of “hypervalent” SF₄; resonance/antiresonance in benzene and C₇H₇⁺; excited states of CH and of O₃/SO₂; and the reaction of triplet methylene with H₂. Starting from a natural orbital representation of the SCGVB wavefunction, traditional multireference configuration interaction calculations are used to account for dynamical correlation effects, which are required for quantitative predictions of molecular properties.

العلاقة: Dunning Jr, Thom H, Cooper, David L orcid:0000-0003-0639-0794 , Xu, Lu T and Karadakov, Peter B (2022) Spin-Coupled Generalized Valence Bond Theory: An Appealing Orbital Theory of the Electronic Structure of Atoms and Molecules. In: Comprehensive Computational Chemistry. Elsevier.

الإتاحة: http://livrepository.liverpool.ac.uk/3151843Test/

المؤلفون: Xu, Lu T.¹ (AUTHOR) ltxu@uw.edu, Dunning, Thom H.¹ (AUTHOR) thdjr@uw.edu

المصدر: Journal of Chemical Physics. 9/21/2020, Vol. 153 Issue 11, p1-9. 9p.

مصطلحات موضوعية: *WAVE functions, *ELECTRON configuration, *ELECTRON pairs, *ELECTRONIC structure, *POTENTIAL energy, *VALENCE bonds

مستخلص: In the full optimized reaction space and valence-complete active space self-consistent field (vCAS) methods, a set of active orbitals is defined as the union of the valence orbitals on the atoms, all possible configurations involving the active orbitals are generated, and the orbitals and configuration coefficients are self-consistently optimized. Such wave functions have tremendous flexibility, which makes these methods incredibly powerful but can also lead to inconsistencies in the description of the electronic structure of molecules. In this paper, the problems that can arise in vCAS calculations are illustrated by calculations on the BH and BF molecules. BH is well described by the full vCAS wave function, which accounts for molecular dissociation and 2s–2p near-degeneracy in the boron atom. The same is not true for the full vCAS wave function for BF. There is mixing of core and active orbitals at short internuclear distances and swapping of core and active orbitals at large internuclear distances. In addition, the virtual 2π orbitals, which were included in the active space to account for the 2s–2p near degeneracy effect, are used instead to describe radial correlation of the electrons in the F2pπ-like pairs. Although the above changes lead to lower vCAS energies, they lead to higher vCAS+1+2 energies as well as irregularities and/or discontinuities in the potential energy curves. All of the above problems can be addressed by using the spin-coupled generalized valence bond-inspired vCAS wave function for BF, which includes only a subset of the atomic valence orbitals in the active space. [ABSTRACT FROM AUTHOR]

المؤلفون: Xu, Lu. T.¹ (AUTHOR) ltxu@uw.edu, Dunning, Thom H.¹ (AUTHOR) thdjr@uw.edu

المصدر: Journal of Chemical Physics. 6/7/2020, Vol. 152 Issue 21, p1-12. 12p. 1 Diagram, 2 Charts, 8 Graphs.

مصطلحات موضوعية: *WAVE functions, *ELECTRON pairs, *VALENCE bonds, *POTENTIAL energy, *CHEMICAL bonds

مستخلص: In the spin-coupled generalized valence bond (SCGVB) description of Be2, there is a pair of electrons in highly overlapping "inner" orbitals corresponding to a traditional σ bond, but this bond is compromised by Pauli repulsion arising from its overlap with a second "outer" pair. The presence of this outer pair of electrons leads to a repulsive potential energy curve at long range and a bound, but metastable molecule at short range. To obtain further insights into the nature of the bond in Be2, we determined the non-dynamical and dynamical correlation contributions to the potential energy curve of Be2 using four different choices for the zero-order wave function: Restricted Hartree–Fock (RHF), SCGVB, valence-CASSCF(4,4), and valence-CASSCF(4,8). The SCGVB and valence-CASSCF(4,4) wave functions yield similar breakdowns of the total correlation energy, with non-dynamical correlation being the more important contribution. For the RHF and valence-CASSCF(4,8) wave functions, dynamical correlation is critical, without which the potential energy curve is purely repulsive. High accuracy calculations on the HBen−1Be–BeBen−1H molecule as a function of n (n = 1–6) suggest that the intrinsic strength of a Be–Be σ bond uncompromised by Pauli repulsion is on the order of 62–63 kcal/mol, and its length is 2.13–2.14 Å, ∼60 kcal/mol stronger and ∼0.35 Å shorter than in Be2. [ABSTRACT FROM AUTHOR]