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C J Umrigar - One of the best experts on this subject based on the ideXlab platform.
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orbital optimization in selected Configuration Interaction Methods
Journal of Chemical Theory and Computation, 2021Co-Authors: Yuan Yao, C J UmrigarAbstract:We study several approaches to orbital optimization in selected Configuration Interaction (SCI) plus perturbation theory Methods and test them on the ground and excited states of three molecules using the semistochastic heat-bath Configuration Interaction method. We discuss the ways in which the orbital optimization problem in SCI resembles and differs from that in complete active space self-consistent field. Starting from natural orbitals, these approaches divide into three classes of optimization Methods according to how they treat coupling between Configuration Interaction coefficients and orbital parameters, namely uncoupled, fully coupled, and quasi-fully coupled Methods. We demonstrate that taking the coupling into account is crucial for fast convergence and recommend two quasi-fully coupled Methods for such applications: accelerated diagonal Newton and Broyden-Fletcher-Goldfarb-Shanno.
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fast semistochastic heat bath Configuration Interaction
Journal of Chemical Physics, 2018Co-Authors: Matthew Otten, Adam A Holmes, Sandeep Sharma, C J UmrigarAbstract:This paper presents in detail our fast semistochastic heat-bath Configuration Interaction (SHCI) method for solving the many-body Schrodinger equation. We identify and eliminate computational bottlenecks in both the variational and perturbative steps of the SHCI algorithm. We also describe the parallelization and the key data structures in our implementation, such as the distributed hash table. The improved SHCI algorithm enables us to include in our variational wavefunction two orders of magnitude more determinants than has been reported previously with other selected Configuration Interaction Methods. We use our algorithm to calculate an accurate benchmark energy for the chromium dimer with the X2C relativistic Hamiltonian in the cc-pVDZ-DK basis, correlating 28 electrons in 76 spatial orbitals. Our largest calculation uses two billion Slater determinants in the variational space and semistochastically includes perturbative contributions from at least trillions of additional determinants with better than 10-5 Ha statistical uncertainty.
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fast semistochastic heat bath Configuration Interaction
arXiv: Chemical Physics, 2018Co-Authors: Matthew Otten, Adam A Holmes, Sandeep Sharma, C J UmrigarAbstract:This paper presents in detail our fast semistochastic heat-bath Configuration Interaction (SHCI) method for solving the many-body Schrodinger equation. We identify and eliminate computational bottlenecks in both the variational and perturbative steps of the SHCI algorithm. We also describe the parallelization and the key data structures in our implementation, such as the distributed hash table. The improved SHCI algorithm enables us to include in our variational wavefunction two orders of magnitude more determinants than has been reported previously with other selected Configuration Interaction Methods. We use our algorithm to calculate an accurate benchmark energy for the chromium dimer with the X2C relativistic Hamiltonian in the cc-pVDZ-DK basis, correlating 28 electrons in 76 spatial orbitals. Our largest calculation uses two billion Slater determinants in the variational space, and semistochastically includes perturbative contributions from at least trillions of additional determinants with better than 10 microhartree statistical uncertainty.
Michael H. Palmer - One of the best experts on this subject based on the ideXlab platform.
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the electronically excited states of cyclooctatetraene an analysis of the vacuum ultraviolet absorption spectrum by ab initio Configuration Interaction Methods
Journal of Chemical Physics, 2019Co-Authors: Michael H. Palmer, Nykola C Jones, S V Hoffmann, M Coreno, Monica De Simone, C GrazioliAbstract:A new synchrotron-based study of the vacuum ultraviolet (VUV) absorption spectrum for cyclooctatetraene (COT) shows a series of broad peaks. A significant sharp structure was extracted from the strongest band between 5.9 and 6.3 eV by fitting this range of the spectrum to a polynomial; the regular residuals show a set of sharp peaks. Comparison of this region of the VUV with the photoelectron spectrum demonstrates the presence of several Rydberg states, all based on the lowest observed ionization energy ionic state. The UV onset contains a broad band in the range 4.0 eV-5.3 eV. Theoretical vertical excitation energies, determined by Configuration Interaction (CI) studies at the multireference multiroot singles and doubles CI level, enabled interpretation of the principal absorption bands of the VUV spectrum. Adiabatic excitation energies (AEEs) for several singlet and triplet valence states (V) were evaluated by multiConfiguration self-consistent field Methods. Theoretical Rydberg series AEEs were obtained by use of extremely diffuse Gaussian orbitals in highly correlated wave-functions. The second moments of the charge distribution identify which roots are valence or Rydberg states. A contrast was found between some density functional Methods and Hartree-Fock (HF) wave-functions during single-excitation CI, when degenerate orbitals were involved in the leading Configurations. The 7a16e* state contained the expected 8-membered ring in the density functional theory calculations. The HF wave-functions led to a 1,5-cross-ring Interaction which converged on a singlet excited state of a bicyclo[3,3,0]octatriene; this is reminiscent of the photochemical conversion of COT to semibullvalene.
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the electronically excited states of cyclooctatetraene an analysis of the vacuum ultraviolet absorption spectrum by ab initio Configuration Interaction Methods
Journal of Chemical Physics, 2019Co-Authors: Michael H. Palmer, Nykola C Jones, S V Hoffmann, M Coreno, Monica De Simone, C GrazioliAbstract:A new synchrotron-based study of the vacuum ultraviolet (VUV) absorption spectrum for cyclooctatetraene (COT) shows a series of broad peaks. A significant sharp structure was extracted from the strongest band between 5.9 and 6.3 eV by fitting this range of the spectrum to a polynomial; the regular residuals show a set of sharp peaks. Comparison of this region of the VUV with the photoelectron spectrum demonstrates the presence of several Rydberg states, all based on the lowest observed ionization energy ionic state. The UV onset contains a broad band in the range 4.0 eV–5.3 eV. Theoretical vertical excitation energies, determined by Configuration Interaction (CI) studies at the multireference multiroot singles and doubles CI level, enabled interpretation of the principal absorption bands of the VUV spectrum. Adiabatic excitation energies (AEEs) for several singlet and triplet valence states (V) were evaluated by multiConfiguration self-consistent field Methods. Theoretical Rydberg series AEEs were obtained by use of extremely diffuse Gaussian orbitals in highly correlated wave-functions. The second moments of the charge distribution identify which roots are valence or Rydberg states. A contrast was found between some density functional Methods and Hartree-Fock (HF) wave-functions during single-excitation CI, when degenerate orbitals were involved in the leading Configurations. The 7a16e* state contained the expected 8-membered ring in the density functional theory calculations. The HF wave-functions led to a 1,5-cross-ring Interaction which converged on a singlet excited state of a bicyclo[3,3,0]octatriene; this is reminiscent of the photochemical conversion of COT to semibullvalene.A new synchrotron-based study of the vacuum ultraviolet (VUV) absorption spectrum for cyclooctatetraene (COT) shows a series of broad peaks. A significant sharp structure was extracted from the strongest band between 5.9 and 6.3 eV by fitting this range of the spectrum to a polynomial; the regular residuals show a set of sharp peaks. Comparison of this region of the VUV with the photoelectron spectrum demonstrates the presence of several Rydberg states, all based on the lowest observed ionization energy ionic state. The UV onset contains a broad band in the range 4.0 eV–5.3 eV. Theoretical vertical excitation energies, determined by Configuration Interaction (CI) studies at the multireference multiroot singles and doubles CI level, enabled interpretation of the principal absorption bands of the VUV spectrum. Adiabatic excitation energies (AEEs) for several singlet and triplet valence states (V) were evaluated by multiConfiguration self-consistent field Methods. Theoretical Rydberg series AEEs were obtaine...
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interpretation of the vacuum ultraviolet photoabsorption spectrum of iodobenzene by ab initio computations
Journal of Chemical Physics, 2015Co-Authors: Michael H. Palmer, Nykola C Jones, S V Hoffmann, Trevor Ridley, M Coreno, Monica De Simone, C Grazioli, Malgorzata Biczysko, Alberto Baiardi, P LimaovieiraAbstract:Identification of many Rydberg states in iodobenzene, especially from the first and fourth ionization energies (IE1 and IE4, X2B1 and C2B1), has become possible using a new ultraviolet (UV) and vacuum-ultraviolet (VUV) absorption spectrum, in the region 29 000-87 000 cm−1 (3.60-10.79 eV), measured at room temperature with synchrotron radiation. A few Rydberg states based on IE2 (A2A2) were found, but those based on IE3 (B2B2) are undetectable. The almost complete absence of observable Rydberg states relating to IE2 and IE3 (A2A2 and B2B2, respectively) is attributed to them being coupled to the near-continuum, high-energy region of Rydberg series converging on IE1. Theoretical studies of the UV and VUV spectra used both time-dependent density functional (TDDFT) and multi-reference multi-root doubles and singles-Configuration Interaction Methods. The theoretical adiabatic excitation energies, and their corresponding vibrational profiles, gave a satisfactory interpretation of the experimental results. The c...
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the electronic states of 1 2 3 triazole studied by vacuum ultraviolet photoabsorption and ultraviolet photoelectron spectroscopy and a comparison with ab initio Configuration Interaction Methods
Journal of Chemical Physics, 2011Co-Authors: Michael H. Palmer, Ashley R Head, Nykola C Jones, S V Hoffmann, Dennis L. LichtenbergerAbstract:The Rydberg states in the vacuum ultraviolet photoabsorption spectrum of 1,2,3-triazole have been measured and analyzed with the aid of comparison to the UV valence photoelectron ionizations and the results of ab initio Configuration Interaction (CI) calculations. Calculated electronic ionization and excitation energies for singlet, triplet valence, and Rydberg states were obtained using multireference multiroot CI procedures with an aug-cc-pVTZ [5s3p3d1f] basis set and a set of Rydberg [4s3p3d3f] functions. Adiabatic excitation energies obtained for several electronic states using coupled-cluster (singles, doubles, and triples) and complete active space self-consistent field procedures agree well with experimental values. Variations in bond lengths with the electronic state are discussed. The lowest energy UV band (∼5.5–6.5 eV) is assigned to three electronically excited states and demonstrates the occurrence of a nonplanar upper state on the low energy side. A UV photoelectron spectrum with an improved ...
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the electronic states of chlorine oxide cl2o studied by ab initio Configuration Interaction Methods and spectroscopy
Chemical Physics, 2008Co-Authors: Michael H. Palmer, Alistair D NelsonAbstract:The singlet, triplet and cationic manifolds of Cl2O have been studied by multi-reference multi-root Configuration Interaction calculations, with large basis sets which include Rydberg functions. Numerous electronically excited and ionic states were obtained, which provide the most comprehensive interpretation of the known absorption (energy range to ∼10 eV) and the photoionization efficiency spectra (energy range 11–16 eV). There is a good correlation between the observed Rydberg state energies and the present calculated ones, for a series of bands involving the 1st and 4th ionization energies. Little is known concerning Rydberg states for the 2nd and 3rd ionization energies. The separation of Rydberg states with same upper state but varying lower state, show systematic trends, which independently confirm the order of cationic states in the UV-PES spectrum as: 2B1 < 2B2 < 2A1 < 2A2 < 2B1 < 9A1 < 2B2. The lowest electronic state found for this system is the unsymmetrical 3A″ (CS) form, where the asymmetry allows a lower energy channel towards dissociation. At this level of theoretical study, the dissociation energy of Cl2O → Cl + OCl is 1.382 eV. A series of low-lying singlet and triplet states with either C2V or D∞h symmetry were also determined; in the former, the bond angle in particularly variable with the electronic state.
Hiroshi Tatewaki - One of the best experts on this subject based on the ideXlab platform.
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Ground and low-lying excited states of DyCl studied by the four-component relativistic Configuration Interaction Methods
Theoretical Chemistry Accounts, 2018Co-Authors: Shigeyoshi Yamamoto, Hiroshi TatewakiAbstract:The ground and low-lying excited states of the DyCl molecule are investigated by the four-component relativistic CI Methods. Electronic states are classified into so-called families by applying the f -shell Omega decomposition method to the CI wavefunctions, and the characters of the states are clarified. The X 7.5 ground state may be described as Dy^+[(4 f )^9(6 s )^2]Cl^−, but at large nuclear distance ( R > 4.80 au), beyond the equilibrium nuclear distance (4.724 au), the dominant Configuration changes to Dy^+[(4 f )^10(6 s )^1]Cl^−. The dominant Configuration of Dy^+[(4 f )^9(6 s )^2]F^− is retained in DyF, but the dominant Configuration in DyCl changes drastically as R increases. The calculated value (231 cm^−1) of the vibrational frequency ( ω _e) agrees well with the value of 233 cm^−1 observed by Linton et al. (J Mol Spectrosc 232:30–39, 2005 ). This low frequency reflects the weakness of the Cl^− ligand compared to F^−. The Y [0.15]8.5, Z [0.85]7.5, and [0.97] states observed by Linton et al. are found to have a dominant Configuration of [(4 f )^10(6 s )^1], and the A [16.4]8.5 and B [15.4]Ω states observed both have (4 f )^10([6 p _1/2,1/2])^1, belonging to different families. (The number in square brackets denotes the excitation energy in unit of k cm^−1, and the number after a right square bracket denotes an Ω value.) The Ω value of the B [15.4] Ω state, which has not been determined experimentally, is calculated to be 6.5.
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electronic spectra of dyf studied by four component relativistic Configuration Interaction Methods
Journal of Chemical Physics, 2015Co-Authors: Shigeyoshi Yamamoto, Hiroshi TatewakiAbstract:The electronic states of the DyF molecule below 3.0 eV are studied using 4-component relativistic CI Methods. Spinors generated by the average-of-Configuration Hartree-Fock method with the Dirac-Coulomb Hamiltonian were used in CI calculations by the KRCI (Kramers-restricted Configuration Interaction) program. The CI reference space was generated by distributing 11 electrons among the 11 Kramers pairs composed mainly of Dy [4f], [6s], [6p] atomic spinors, and double excitations are allowed from this space to the virtual molecular spinors. The CI calculations indicate that the ground state has the dominant Configuration (4f9)(6s2)(Ω = 7.5). Above this ground state, 4 low-lying excited states (Ω = 8.5, 7.5, 7.5, 7.5) are found with dominant Configurations (4f10)(6s). These results are consistent with the experimental studies of McCarthy et al. Above these 5 states, 2 states were observed at T0 = 2.39 eV, 2.52 eV by McCarthy et al. and were named as [19.3]8.5 and [20.3]8.5. McCarthy et al. proposed that both...
C Grazioli - One of the best experts on this subject based on the ideXlab platform.
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the electronically excited states of cyclooctatetraene an analysis of the vacuum ultraviolet absorption spectrum by ab initio Configuration Interaction Methods
Journal of Chemical Physics, 2019Co-Authors: Michael H. Palmer, Nykola C Jones, S V Hoffmann, M Coreno, Monica De Simone, C GrazioliAbstract:A new synchrotron-based study of the vacuum ultraviolet (VUV) absorption spectrum for cyclooctatetraene (COT) shows a series of broad peaks. A significant sharp structure was extracted from the strongest band between 5.9 and 6.3 eV by fitting this range of the spectrum to a polynomial; the regular residuals show a set of sharp peaks. Comparison of this region of the VUV with the photoelectron spectrum demonstrates the presence of several Rydberg states, all based on the lowest observed ionization energy ionic state. The UV onset contains a broad band in the range 4.0 eV-5.3 eV. Theoretical vertical excitation energies, determined by Configuration Interaction (CI) studies at the multireference multiroot singles and doubles CI level, enabled interpretation of the principal absorption bands of the VUV spectrum. Adiabatic excitation energies (AEEs) for several singlet and triplet valence states (V) were evaluated by multiConfiguration self-consistent field Methods. Theoretical Rydberg series AEEs were obtained by use of extremely diffuse Gaussian orbitals in highly correlated wave-functions. The second moments of the charge distribution identify which roots are valence or Rydberg states. A contrast was found between some density functional Methods and Hartree-Fock (HF) wave-functions during single-excitation CI, when degenerate orbitals were involved in the leading Configurations. The 7a16e* state contained the expected 8-membered ring in the density functional theory calculations. The HF wave-functions led to a 1,5-cross-ring Interaction which converged on a singlet excited state of a bicyclo[3,3,0]octatriene; this is reminiscent of the photochemical conversion of COT to semibullvalene.
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the electronically excited states of cyclooctatetraene an analysis of the vacuum ultraviolet absorption spectrum by ab initio Configuration Interaction Methods
Journal of Chemical Physics, 2019Co-Authors: Michael H. Palmer, Nykola C Jones, S V Hoffmann, M Coreno, Monica De Simone, C GrazioliAbstract:A new synchrotron-based study of the vacuum ultraviolet (VUV) absorption spectrum for cyclooctatetraene (COT) shows a series of broad peaks. A significant sharp structure was extracted from the strongest band between 5.9 and 6.3 eV by fitting this range of the spectrum to a polynomial; the regular residuals show a set of sharp peaks. Comparison of this region of the VUV with the photoelectron spectrum demonstrates the presence of several Rydberg states, all based on the lowest observed ionization energy ionic state. The UV onset contains a broad band in the range 4.0 eV–5.3 eV. Theoretical vertical excitation energies, determined by Configuration Interaction (CI) studies at the multireference multiroot singles and doubles CI level, enabled interpretation of the principal absorption bands of the VUV spectrum. Adiabatic excitation energies (AEEs) for several singlet and triplet valence states (V) were evaluated by multiConfiguration self-consistent field Methods. Theoretical Rydberg series AEEs were obtained by use of extremely diffuse Gaussian orbitals in highly correlated wave-functions. The second moments of the charge distribution identify which roots are valence or Rydberg states. A contrast was found between some density functional Methods and Hartree-Fock (HF) wave-functions during single-excitation CI, when degenerate orbitals were involved in the leading Configurations. The 7a16e* state contained the expected 8-membered ring in the density functional theory calculations. The HF wave-functions led to a 1,5-cross-ring Interaction which converged on a singlet excited state of a bicyclo[3,3,0]octatriene; this is reminiscent of the photochemical conversion of COT to semibullvalene.A new synchrotron-based study of the vacuum ultraviolet (VUV) absorption spectrum for cyclooctatetraene (COT) shows a series of broad peaks. A significant sharp structure was extracted from the strongest band between 5.9 and 6.3 eV by fitting this range of the spectrum to a polynomial; the regular residuals show a set of sharp peaks. Comparison of this region of the VUV with the photoelectron spectrum demonstrates the presence of several Rydberg states, all based on the lowest observed ionization energy ionic state. The UV onset contains a broad band in the range 4.0 eV–5.3 eV. Theoretical vertical excitation energies, determined by Configuration Interaction (CI) studies at the multireference multiroot singles and doubles CI level, enabled interpretation of the principal absorption bands of the VUV spectrum. Adiabatic excitation energies (AEEs) for several singlet and triplet valence states (V) were evaluated by multiConfiguration self-consistent field Methods. Theoretical Rydberg series AEEs were obtaine...
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interpretation of the vacuum ultraviolet photoabsorption spectrum of iodobenzene by ab initio computations
Journal of Chemical Physics, 2015Co-Authors: Michael H. Palmer, Nykola C Jones, S V Hoffmann, Trevor Ridley, M Coreno, Monica De Simone, C Grazioli, Malgorzata Biczysko, Alberto Baiardi, P LimaovieiraAbstract:Identification of many Rydberg states in iodobenzene, especially from the first and fourth ionization energies (IE1 and IE4, X2B1 and C2B1), has become possible using a new ultraviolet (UV) and vacuum-ultraviolet (VUV) absorption spectrum, in the region 29 000-87 000 cm−1 (3.60-10.79 eV), measured at room temperature with synchrotron radiation. A few Rydberg states based on IE2 (A2A2) were found, but those based on IE3 (B2B2) are undetectable. The almost complete absence of observable Rydberg states relating to IE2 and IE3 (A2A2 and B2B2, respectively) is attributed to them being coupled to the near-continuum, high-energy region of Rydberg series converging on IE1. Theoretical studies of the UV and VUV spectra used both time-dependent density functional (TDDFT) and multi-reference multi-root doubles and singles-Configuration Interaction Methods. The theoretical adiabatic excitation energies, and their corresponding vibrational profiles, gave a satisfactory interpretation of the experimental results. The c...
Nykola C Jones - One of the best experts on this subject based on the ideXlab platform.
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the electronically excited states of cyclooctatetraene an analysis of the vacuum ultraviolet absorption spectrum by ab initio Configuration Interaction Methods
Journal of Chemical Physics, 2019Co-Authors: Michael H. Palmer, Nykola C Jones, S V Hoffmann, M Coreno, Monica De Simone, C GrazioliAbstract:A new synchrotron-based study of the vacuum ultraviolet (VUV) absorption spectrum for cyclooctatetraene (COT) shows a series of broad peaks. A significant sharp structure was extracted from the strongest band between 5.9 and 6.3 eV by fitting this range of the spectrum to a polynomial; the regular residuals show a set of sharp peaks. Comparison of this region of the VUV with the photoelectron spectrum demonstrates the presence of several Rydberg states, all based on the lowest observed ionization energy ionic state. The UV onset contains a broad band in the range 4.0 eV-5.3 eV. Theoretical vertical excitation energies, determined by Configuration Interaction (CI) studies at the multireference multiroot singles and doubles CI level, enabled interpretation of the principal absorption bands of the VUV spectrum. Adiabatic excitation energies (AEEs) for several singlet and triplet valence states (V) were evaluated by multiConfiguration self-consistent field Methods. Theoretical Rydberg series AEEs were obtained by use of extremely diffuse Gaussian orbitals in highly correlated wave-functions. The second moments of the charge distribution identify which roots are valence or Rydberg states. A contrast was found between some density functional Methods and Hartree-Fock (HF) wave-functions during single-excitation CI, when degenerate orbitals were involved in the leading Configurations. The 7a16e* state contained the expected 8-membered ring in the density functional theory calculations. The HF wave-functions led to a 1,5-cross-ring Interaction which converged on a singlet excited state of a bicyclo[3,3,0]octatriene; this is reminiscent of the photochemical conversion of COT to semibullvalene.
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the electronically excited states of cyclooctatetraene an analysis of the vacuum ultraviolet absorption spectrum by ab initio Configuration Interaction Methods
Journal of Chemical Physics, 2019Co-Authors: Michael H. Palmer, Nykola C Jones, S V Hoffmann, M Coreno, Monica De Simone, C GrazioliAbstract:A new synchrotron-based study of the vacuum ultraviolet (VUV) absorption spectrum for cyclooctatetraene (COT) shows a series of broad peaks. A significant sharp structure was extracted from the strongest band between 5.9 and 6.3 eV by fitting this range of the spectrum to a polynomial; the regular residuals show a set of sharp peaks. Comparison of this region of the VUV with the photoelectron spectrum demonstrates the presence of several Rydberg states, all based on the lowest observed ionization energy ionic state. The UV onset contains a broad band in the range 4.0 eV–5.3 eV. Theoretical vertical excitation energies, determined by Configuration Interaction (CI) studies at the multireference multiroot singles and doubles CI level, enabled interpretation of the principal absorption bands of the VUV spectrum. Adiabatic excitation energies (AEEs) for several singlet and triplet valence states (V) were evaluated by multiConfiguration self-consistent field Methods. Theoretical Rydberg series AEEs were obtained by use of extremely diffuse Gaussian orbitals in highly correlated wave-functions. The second moments of the charge distribution identify which roots are valence or Rydberg states. A contrast was found between some density functional Methods and Hartree-Fock (HF) wave-functions during single-excitation CI, when degenerate orbitals were involved in the leading Configurations. The 7a16e* state contained the expected 8-membered ring in the density functional theory calculations. The HF wave-functions led to a 1,5-cross-ring Interaction which converged on a singlet excited state of a bicyclo[3,3,0]octatriene; this is reminiscent of the photochemical conversion of COT to semibullvalene.A new synchrotron-based study of the vacuum ultraviolet (VUV) absorption spectrum for cyclooctatetraene (COT) shows a series of broad peaks. A significant sharp structure was extracted from the strongest band between 5.9 and 6.3 eV by fitting this range of the spectrum to a polynomial; the regular residuals show a set of sharp peaks. Comparison of this region of the VUV with the photoelectron spectrum demonstrates the presence of several Rydberg states, all based on the lowest observed ionization energy ionic state. The UV onset contains a broad band in the range 4.0 eV–5.3 eV. Theoretical vertical excitation energies, determined by Configuration Interaction (CI) studies at the multireference multiroot singles and doubles CI level, enabled interpretation of the principal absorption bands of the VUV spectrum. Adiabatic excitation energies (AEEs) for several singlet and triplet valence states (V) were evaluated by multiConfiguration self-consistent field Methods. Theoretical Rydberg series AEEs were obtaine...
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interpretation of the vacuum ultraviolet photoabsorption spectrum of iodobenzene by ab initio computations
Journal of Chemical Physics, 2015Co-Authors: Michael H. Palmer, Nykola C Jones, S V Hoffmann, Trevor Ridley, M Coreno, Monica De Simone, C Grazioli, Malgorzata Biczysko, Alberto Baiardi, P LimaovieiraAbstract:Identification of many Rydberg states in iodobenzene, especially from the first and fourth ionization energies (IE1 and IE4, X2B1 and C2B1), has become possible using a new ultraviolet (UV) and vacuum-ultraviolet (VUV) absorption spectrum, in the region 29 000-87 000 cm−1 (3.60-10.79 eV), measured at room temperature with synchrotron radiation. A few Rydberg states based on IE2 (A2A2) were found, but those based on IE3 (B2B2) are undetectable. The almost complete absence of observable Rydberg states relating to IE2 and IE3 (A2A2 and B2B2, respectively) is attributed to them being coupled to the near-continuum, high-energy region of Rydberg series converging on IE1. Theoretical studies of the UV and VUV spectra used both time-dependent density functional (TDDFT) and multi-reference multi-root doubles and singles-Configuration Interaction Methods. The theoretical adiabatic excitation energies, and their corresponding vibrational profiles, gave a satisfactory interpretation of the experimental results. The c...
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the electronic states of 1 2 3 triazole studied by vacuum ultraviolet photoabsorption and ultraviolet photoelectron spectroscopy and a comparison with ab initio Configuration Interaction Methods
Journal of Chemical Physics, 2011Co-Authors: Michael H. Palmer, Ashley R Head, Nykola C Jones, S V Hoffmann, Dennis L. LichtenbergerAbstract:The Rydberg states in the vacuum ultraviolet photoabsorption spectrum of 1,2,3-triazole have been measured and analyzed with the aid of comparison to the UV valence photoelectron ionizations and the results of ab initio Configuration Interaction (CI) calculations. Calculated electronic ionization and excitation energies for singlet, triplet valence, and Rydberg states were obtained using multireference multiroot CI procedures with an aug-cc-pVTZ [5s3p3d1f] basis set and a set of Rydberg [4s3p3d3f] functions. Adiabatic excitation energies obtained for several electronic states using coupled-cluster (singles, doubles, and triples) and complete active space self-consistent field procedures agree well with experimental values. Variations in bond lengths with the electronic state are discussed. The lowest energy UV band (∼5.5–6.5 eV) is assigned to three electronically excited states and demonstrates the occurrence of a nonplanar upper state on the low energy side. A UV photoelectron spectrum with an improved ...