The Experts below are selected from a list of 135 Experts worldwide ranked by ideXlab platform
Trygve Helgaker - One of the best experts on this subject based on the ideXlab platform.
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An efficient density-functional-theory force evaluation for large molecular systems.
The Journal of chemical physics, 2010Co-Authors: Simen Reine, Trygve Helgaker, Andreas Krapp, Maria Francesca Iozzi, Vebjørn Bakken, Filip Pawłowski, Paweł SałekAbstract:An efficient, linear-scaling implementation of Kohn–Sham density-functional theory for the calculation of molecular forces for systems containing hundreds of atoms is presented. The density-fitted Coulomb force contribution is calculated in linear time by combining Atomic Integral screening with the continuous fast multipole method. For higher efficiency and greater simplicity, the near-field Coulomb force contribution is calculated by expanding the solid-harmonic Gaussian basis functions in Hermite rather than Cartesian Gaussians. The efficiency and linear complexity of the molecular-force evaluation is demonstrated by sample calculations and applied to the geometry optimization of a few selected large systems.
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Integral direct coupled cluster calculations of frequency dependent polarizabilities transition probabilities and excited state properties
Journal of Chemical Physics, 1998Co-Authors: Ove Christiansen, Henrik Koch, Asger Halkier, Trygve HelgakerAbstract:An Atomic Integral-direct implementation of molecular linear-response properties and excited-state one-electron properties is presented for the coupled cluster models CCS, CC2, and CCSD. Sample calculations are presented for the polarizability of N2 and for excited-state one-electron properties and transition-properties of furan.
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Integral direct calculation of CC2 excitation energies: singlet excited states of benzene
Chemical Physics Letters, 1996Co-Authors: Ove Christiansen, Henrik Koch, Poul Jørgensen, Trygve HelgakerAbstract:The calculation of excitation energies has been implemented in the CC2 model using an Atomic Integral direct algorithm where Integrals are generated in distributions with one Atomic index fixed and three indices free. The calculations are dominated by the recalculation of Atomic Integrals. Singlet excitation energies have been calculated for benzene and a basis set analysis has been carried out with basis sets containing up to 432 basis functions. The CC2 excitation energies have been compared with other theoretical and experimental results. Basis set effects of the order of 0.2 eV are found in benzene going from augmented double-zeta basis sets to augmented triple-zeta quality. The differences between the CC2 results and results obtained by the CASPT2 method are of the same order of magnitude as the basis set errors except for the 2 1E28 state. For this state, large discrepancies are found between the results obtained by coupled cluster methods and CASPT2.
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Large-scale calculations of excitation energies in coupled cluster theory: The singlet excited states of benzene
The Journal of Chemical Physics, 1996Co-Authors: Ove Christiansen, Henrik Koch, Asger Halkier, Trygve Helgaker, Poul Jørgensen, Alfredo Sánchez De MerásAbstract:Algorithms for calculating singlet excitation energies in the coupled cluster singles and doubles (CCSD) model are discussed and an implementation of an Atomic‐Integral direct algorithm is presented. Each excitation energy is calculated at a cost comparable to that of the CCSD ground‐state energy. Singlet excitation energies are calculated for benzene using up to 432 basis functions. Basis‐set effects of the order of 0.2 eV are observed when the basis is increased from augmented polarized valence double‐zeta (aug‐cc‐pVDZ) to augmented polarized valence triple‐zeta (aug‐cc‐pVTZ) quality. The correlation problem is examined by performing calculations in the hierarchy of coupled cluster models CCS, CC2, CCSD, and CC3, as well as by using the CCSDR(3) perturbative triples corrections. The effect of triple excitations are less than 0.2 eV for all excitations except for the 2 1E2g state. The calculated excitation energies are compared with experiment and other theoretical results.
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The Integral‐direct coupled cluster singles and doubles model
The Journal of Chemical Physics, 1996Co-Authors: Henrik Koch, Trygve Helgaker, Alfredo Sánchez De Merás, Ove ChristiansenAbstract:An efficient and highly vectorized implementation of the coupled cluster singles and doubles (CCSD) model using a direct Atomic Integral technique is presented. The minimal number of n6 processes has been implemented for the most time consuming terms and point group symmetry is used to further reduce operation counts and memory requirements. The significantly increased application range of the CCSD method is illustrated with sample calculations on several systems with more than 500 basis functions. Furthermore, we present the basic trends of an open ended algorithm and discuss the use of Integral prescreening.
Ove Christiansen - One of the best experts on this subject based on the ideXlab platform.
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Atomic Integral driven second order polarization propagator calculations of the excitation spectra of naphthalene and anthracene
Journal of Chemical Physics, 2000Co-Authors: Keld L Bak, Henrik Koch, Jens Oddershede, Ove Christiansen, Stephan P A SauerAbstract:An Atomic Integral direct implementation of the second order polarization propagator approximation (SOPPA) for the calculation of electronic excitation energies and oscillator strengths is presented. The SOPPA equations are solved iteratively using an Integral direct approach and, contrary to previous implementations, the new algorithm does not require two-electron Integrals in the molecular orbital basis. The linear transformation of trial vectors are calculated directly from Integrals in the Atomic orbital basis. In addition, the eigenvalue solver is designed to work efficiently with only three trial vectors per eigenvalue. Both of these modifications dramatically reduce the amount of disk space required, thus, increasing the range of applicability of the SOPPA method. Calculations of the lowest singlet excitation energies and corresponding dipole oscillator strengths for naphthalene and anthracene employing basis sets of 238 and 329 Atomic orbitals, respectively, are presented. The overall agreement of...
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Integral direct coupled cluster calculations of frequency dependent polarizabilities transition probabilities and excited state properties
Journal of Chemical Physics, 1998Co-Authors: Ove Christiansen, Henrik Koch, Asger Halkier, Trygve HelgakerAbstract:An Atomic Integral-direct implementation of molecular linear-response properties and excited-state one-electron properties is presented for the coupled cluster models CCS, CC2, and CCSD. Sample calculations are presented for the polarizability of N2 and for excited-state one-electron properties and transition-properties of furan.
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Integral direct calculation of CC2 excitation energies: singlet excited states of benzene
Chemical Physics Letters, 1996Co-Authors: Ove Christiansen, Henrik Koch, Poul Jørgensen, Trygve HelgakerAbstract:The calculation of excitation energies has been implemented in the CC2 model using an Atomic Integral direct algorithm where Integrals are generated in distributions with one Atomic index fixed and three indices free. The calculations are dominated by the recalculation of Atomic Integrals. Singlet excitation energies have been calculated for benzene and a basis set analysis has been carried out with basis sets containing up to 432 basis functions. The CC2 excitation energies have been compared with other theoretical and experimental results. Basis set effects of the order of 0.2 eV are found in benzene going from augmented double-zeta basis sets to augmented triple-zeta quality. The differences between the CC2 results and results obtained by the CASPT2 method are of the same order of magnitude as the basis set errors except for the 2 1E28 state. For this state, large discrepancies are found between the results obtained by coupled cluster methods and CASPT2.
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Large-scale calculations of excitation energies in coupled cluster theory: The singlet excited states of benzene
The Journal of Chemical Physics, 1996Co-Authors: Ove Christiansen, Henrik Koch, Asger Halkier, Trygve Helgaker, Poul Jørgensen, Alfredo Sánchez De MerásAbstract:Algorithms for calculating singlet excitation energies in the coupled cluster singles and doubles (CCSD) model are discussed and an implementation of an Atomic‐Integral direct algorithm is presented. Each excitation energy is calculated at a cost comparable to that of the CCSD ground‐state energy. Singlet excitation energies are calculated for benzene using up to 432 basis functions. Basis‐set effects of the order of 0.2 eV are observed when the basis is increased from augmented polarized valence double‐zeta (aug‐cc‐pVDZ) to augmented polarized valence triple‐zeta (aug‐cc‐pVTZ) quality. The correlation problem is examined by performing calculations in the hierarchy of coupled cluster models CCS, CC2, CCSD, and CC3, as well as by using the CCSDR(3) perturbative triples corrections. The effect of triple excitations are less than 0.2 eV for all excitations except for the 2 1E2g state. The calculated excitation energies are compared with experiment and other theoretical results.
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The Integral‐direct coupled cluster singles and doubles model
The Journal of Chemical Physics, 1996Co-Authors: Henrik Koch, Trygve Helgaker, Alfredo Sánchez De Merás, Ove ChristiansenAbstract:An efficient and highly vectorized implementation of the coupled cluster singles and doubles (CCSD) model using a direct Atomic Integral technique is presented. The minimal number of n6 processes has been implemented for the most time consuming terms and point group symmetry is used to further reduce operation counts and memory requirements. The significantly increased application range of the CCSD method is illustrated with sample calculations on several systems with more than 500 basis functions. Furthermore, we present the basic trends of an open ended algorithm and discuss the use of Integral prescreening.
Henrik Koch - One of the best experts on this subject based on the ideXlab platform.
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Atomic Integral driven second order polarization propagator calculations of the excitation spectra of naphthalene and anthracene
Journal of Chemical Physics, 2000Co-Authors: Keld L Bak, Henrik Koch, Jens Oddershede, Ove Christiansen, Stephan P A SauerAbstract:An Atomic Integral direct implementation of the second order polarization propagator approximation (SOPPA) for the calculation of electronic excitation energies and oscillator strengths is presented. The SOPPA equations are solved iteratively using an Integral direct approach and, contrary to previous implementations, the new algorithm does not require two-electron Integrals in the molecular orbital basis. The linear transformation of trial vectors are calculated directly from Integrals in the Atomic orbital basis. In addition, the eigenvalue solver is designed to work efficiently with only three trial vectors per eigenvalue. Both of these modifications dramatically reduce the amount of disk space required, thus, increasing the range of applicability of the SOPPA method. Calculations of the lowest singlet excitation energies and corresponding dipole oscillator strengths for naphthalene and anthracene employing basis sets of 238 and 329 Atomic orbitals, respectively, are presented. The overall agreement of...
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Integral direct coupled cluster calculations of frequency dependent polarizabilities transition probabilities and excited state properties
Journal of Chemical Physics, 1998Co-Authors: Ove Christiansen, Henrik Koch, Asger Halkier, Trygve HelgakerAbstract:An Atomic Integral-direct implementation of molecular linear-response properties and excited-state one-electron properties is presented for the coupled cluster models CCS, CC2, and CCSD. Sample calculations are presented for the polarizability of N2 and for excited-state one-electron properties and transition-properties of furan.
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Integral direct calculation of CC2 excitation energies: singlet excited states of benzene
Chemical Physics Letters, 1996Co-Authors: Ove Christiansen, Henrik Koch, Poul Jørgensen, Trygve HelgakerAbstract:The calculation of excitation energies has been implemented in the CC2 model using an Atomic Integral direct algorithm where Integrals are generated in distributions with one Atomic index fixed and three indices free. The calculations are dominated by the recalculation of Atomic Integrals. Singlet excitation energies have been calculated for benzene and a basis set analysis has been carried out with basis sets containing up to 432 basis functions. The CC2 excitation energies have been compared with other theoretical and experimental results. Basis set effects of the order of 0.2 eV are found in benzene going from augmented double-zeta basis sets to augmented triple-zeta quality. The differences between the CC2 results and results obtained by the CASPT2 method are of the same order of magnitude as the basis set errors except for the 2 1E28 state. For this state, large discrepancies are found between the results obtained by coupled cluster methods and CASPT2.
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Large-scale calculations of excitation energies in coupled cluster theory: The singlet excited states of benzene
The Journal of Chemical Physics, 1996Co-Authors: Ove Christiansen, Henrik Koch, Asger Halkier, Trygve Helgaker, Poul Jørgensen, Alfredo Sánchez De MerásAbstract:Algorithms for calculating singlet excitation energies in the coupled cluster singles and doubles (CCSD) model are discussed and an implementation of an Atomic‐Integral direct algorithm is presented. Each excitation energy is calculated at a cost comparable to that of the CCSD ground‐state energy. Singlet excitation energies are calculated for benzene using up to 432 basis functions. Basis‐set effects of the order of 0.2 eV are observed when the basis is increased from augmented polarized valence double‐zeta (aug‐cc‐pVDZ) to augmented polarized valence triple‐zeta (aug‐cc‐pVTZ) quality. The correlation problem is examined by performing calculations in the hierarchy of coupled cluster models CCS, CC2, CCSD, and CC3, as well as by using the CCSDR(3) perturbative triples corrections. The effect of triple excitations are less than 0.2 eV for all excitations except for the 2 1E2g state. The calculated excitation energies are compared with experiment and other theoretical results.
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The Integral‐direct coupled cluster singles and doubles model
The Journal of Chemical Physics, 1996Co-Authors: Henrik Koch, Trygve Helgaker, Alfredo Sánchez De Merás, Ove ChristiansenAbstract:An efficient and highly vectorized implementation of the coupled cluster singles and doubles (CCSD) model using a direct Atomic Integral technique is presented. The minimal number of n6 processes has been implemented for the most time consuming terms and point group symmetry is used to further reduce operation counts and memory requirements. The significantly increased application range of the CCSD method is illustrated with sample calculations on several systems with more than 500 basis functions. Furthermore, we present the basic trends of an open ended algorithm and discuss the use of Integral prescreening.
Jorge A Morales - One of the best experts on this subject based on the ideXlab platform.
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Massively parallel implementations of coupled-cluster methods for electron spin resonance spectra. I. Isotropic hyperfine coupling tensors in large radicals
The Journal of chemical physics, 2013Co-Authors: Prakash Verma, Ajith Perera, Jorge A MoralesAbstract:Coupled cluster (CC) methods provide highly accurate predictions of molecular properties, but their high computational cost has precluded their routine application to large systems. Fortunately, recent computational developments in the ACES III program by the Bartlett group [the OED/ERD Atomic Integral package, the super instruction processor, and the super instruction architecture language] permit overcoming that limitation by providing a framework for massively parallel CC implementations. In that scheme, we are further extending those parallel CC efforts to systematically predict the three main electron spin resonance (ESR) tensors (A-, g-, and D-tensors) to be reported in a series of papers. In this paper inaugurating that series, we report our new ACES III parallel capabilities that calculate isotropic hyperfine coupling constants in 38 neutral, cationic, and anionic radicals that include the 11B, 17O, 9Be, 19F, 1H, 13C, 35Cl, 33S,14N, 31P, and 67Zn nuclei. Present parallel calculations are conducted at the Hartree-Fock (HF), second-order many-body perturbation theory [MBPT(2)], CC singles and doubles (CCSD), and CCSD with perturbative triples [CCSD(T)] levels using Roos augmented double- and triple-zeta Atomic natural orbitals basis sets. HF results consistently overestimate isotropic hyperfine coupling constants. However, inclusion of electron correlation effects in the simplest way via MBPT(2) provides significant improvements in the predictions, but not without occasional failures. In contrast, CCSD results are consistently in very good agreement with experimental results. Inclusion of perturbative triples to CCSD via CCSD(T) leads to small improvements in the predictions, which might not compensate for the extra computational effort at a non-iterative N7-scaling in CCSD(T). The importance of these accurate computations of isotropic hyperfine coupling constants to elucidate experimental ESR spectra, to interpret spin-density distributions, and to characterize and identify radical species is illustrated with our results from large organic radicals. Those include species relevant for organic chemistry, petroleum industry, and biochemistry, such as the cyclo-hexyl, 1-adamatyl, and Zn-porphycene anion radicals, inter alia.
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massively parallel implementations of coupled cluster methods for electron spin resonance spectra i isotropic hyperfine coupling tensors in large radicals
Journal of Chemical Physics, 2013Co-Authors: Prakash Verma, Ajith Perera, Jorge A MoralesAbstract:Coupled cluster (CC) methods provide highly accurate predictions of molecular properties, but their high computational cost has precluded their routine application to large systems. Fortunately, recent computational developments in the ACES III program by the Bartlett group [the OED/ERD Atomic Integral package, the super instruction processor, and the super instruction architecture language] permit overcoming that limitation by providing a framework for massively parallel CC implementations. In that scheme, we are further extending those parallel CC efforts to systematically predict the three main electron spin resonance (ESR) tensors (A-, g-, and D-tensors) to be reported in a series of papers. In this paper inaugurating that series, we report our new ACES III parallel capabilities that calculate isotropic hyperfine coupling constants in 38 neutral, cationic, and anionic radicals that include the 11B, 17O, 9Be, 19F, 1H, 13C, 35Cl, 33S,14N, 31P, and 67Zn nuclei. Present parallel calculations are conducted...
Prakash Verma - One of the best experts on this subject based on the ideXlab platform.
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Massively parallel implementations of coupled-cluster methods for electron spin resonance spectra. I. Isotropic hyperfine coupling tensors in large radicals
The Journal of chemical physics, 2013Co-Authors: Prakash Verma, Ajith Perera, Jorge A MoralesAbstract:Coupled cluster (CC) methods provide highly accurate predictions of molecular properties, but their high computational cost has precluded their routine application to large systems. Fortunately, recent computational developments in the ACES III program by the Bartlett group [the OED/ERD Atomic Integral package, the super instruction processor, and the super instruction architecture language] permit overcoming that limitation by providing a framework for massively parallel CC implementations. In that scheme, we are further extending those parallel CC efforts to systematically predict the three main electron spin resonance (ESR) tensors (A-, g-, and D-tensors) to be reported in a series of papers. In this paper inaugurating that series, we report our new ACES III parallel capabilities that calculate isotropic hyperfine coupling constants in 38 neutral, cationic, and anionic radicals that include the 11B, 17O, 9Be, 19F, 1H, 13C, 35Cl, 33S,14N, 31P, and 67Zn nuclei. Present parallel calculations are conducted at the Hartree-Fock (HF), second-order many-body perturbation theory [MBPT(2)], CC singles and doubles (CCSD), and CCSD with perturbative triples [CCSD(T)] levels using Roos augmented double- and triple-zeta Atomic natural orbitals basis sets. HF results consistently overestimate isotropic hyperfine coupling constants. However, inclusion of electron correlation effects in the simplest way via MBPT(2) provides significant improvements in the predictions, but not without occasional failures. In contrast, CCSD results are consistently in very good agreement with experimental results. Inclusion of perturbative triples to CCSD via CCSD(T) leads to small improvements in the predictions, which might not compensate for the extra computational effort at a non-iterative N7-scaling in CCSD(T). The importance of these accurate computations of isotropic hyperfine coupling constants to elucidate experimental ESR spectra, to interpret spin-density distributions, and to characterize and identify radical species is illustrated with our results from large organic radicals. Those include species relevant for organic chemistry, petroleum industry, and biochemistry, such as the cyclo-hexyl, 1-adamatyl, and Zn-porphycene anion radicals, inter alia.
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massively parallel implementations of coupled cluster methods for electron spin resonance spectra i isotropic hyperfine coupling tensors in large radicals
Journal of Chemical Physics, 2013Co-Authors: Prakash Verma, Ajith Perera, Jorge A MoralesAbstract:Coupled cluster (CC) methods provide highly accurate predictions of molecular properties, but their high computational cost has precluded their routine application to large systems. Fortunately, recent computational developments in the ACES III program by the Bartlett group [the OED/ERD Atomic Integral package, the super instruction processor, and the super instruction architecture language] permit overcoming that limitation by providing a framework for massively parallel CC implementations. In that scheme, we are further extending those parallel CC efforts to systematically predict the three main electron spin resonance (ESR) tensors (A-, g-, and D-tensors) to be reported in a series of papers. In this paper inaugurating that series, we report our new ACES III parallel capabilities that calculate isotropic hyperfine coupling constants in 38 neutral, cationic, and anionic radicals that include the 11B, 17O, 9Be, 19F, 1H, 13C, 35Cl, 33S,14N, 31P, and 67Zn nuclei. Present parallel calculations are conducted...