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V. M. Shabaev - One of the best experts on this subject based on the ideXlab platform.
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Non-perturbative calculation of the two-loop Lamb shift in Li-like ions
Physical Review Letters, 2006Co-Authors: V. A. Yerokhin, P. Indelicato, V. M. ShabaevAbstract:A calculation valid to all orders in the nuclear-strength parameter is presented for the two-loop Lamb shift, notably for the two-loop self-Energy Correction, to the 2p-2s transition energies in heavy Li-like ions. The calculation removes the largest theoretical uncertainty for these transitions and yields the first experimental identification of two-loop QED effects in the region of the strong binding field.
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One-loop self-Energy Correction in a strong binding field
Physical Review A, 2005Co-Authors: Vladimir A. Yerokhin, Krzysztof Pachucki, V. M. ShabaevAbstract:A scheme for the numerical evaluation of the one-loop self-Energy Correction to all orders in Z{alpha} is presented. The scheme proposed inherits the attractive features of the standard potential-expansion method but yields a partial-wave expansion that converges more rapidly than in the other methods reported in the literature.
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Two-loop self-Energy Correction to the ground-state Lamb shift in H-like ions
Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms, 2005Co-Authors: Vladimir A. Yerokhin, Paul Indelicato, V. M. ShabaevAbstract:Abstract The two-loop self-Energy Correction is evaluated to all orders in Zα for the ground-state Lamb shift of H-like ions with Z ⩾ 20, where Z is the nuclear charge number and α is the fine structure constant. The results obtained are compared with analytical calculations performed within the Zα-expansion.
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Two-loop self-Energy Correction in a strong Coulomb nuclear field
Journal of Experimental and Theoretical Physics, 2005Co-Authors: Vladimir A. Yerokhin, Paul Indelicato, V. M. ShabaevAbstract:The two-loop self-Energy Correction to the ground-state Energy levels of hydrogen-like ions with nuclear charges Z≥10 is calculated without the Zα expansion, where α is the fine-structure constant. The data obtained are compared with the results of analytical calculations within the Zα expansion; significant disagreement with the analytical results of order α2(Zα)6 has been found. Extrapolation is used to obtain the most accurate value for the two-loop self-Energy Correction for the 1s state in hydrogen.
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Two-loop self-Energy Correction to the ground-state Lamb shift in H-like ions
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2005Co-Authors: Vladimir A. Yerokhin, P. Indelicato, V. M. ShabaevAbstract:The two-loop self-Energy Correction is evaluated to all orders in Z alpha for the ground-state Lamb shift of H-like ions with Z >= 20, where Z is the nuclear charge number and alpha is the fine structure constant. The results obtained are compared with analytical calculations performed within the Z alpha expansion.Comment: a contribution to the conference "Highly Charged Ions 2004
Vladimir A. Yerokhin - One of the best experts on this subject based on the ideXlab platform.
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One-loop self-Energy Correction in a strong binding field
Physical Review A, 2005Co-Authors: Vladimir A. Yerokhin, Krzysztof Pachucki, V. M. ShabaevAbstract:A scheme for the numerical evaluation of the one-loop self-Energy Correction to all orders in Z{alpha} is presented. The scheme proposed inherits the attractive features of the standard potential-expansion method but yields a partial-wave expansion that converges more rapidly than in the other methods reported in the literature.
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Two-loop self-Energy Correction to the ground-state Lamb shift in H-like ions
Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms, 2005Co-Authors: Vladimir A. Yerokhin, Paul Indelicato, V. M. ShabaevAbstract:Abstract The two-loop self-Energy Correction is evaluated to all orders in Zα for the ground-state Lamb shift of H-like ions with Z ⩾ 20, where Z is the nuclear charge number and α is the fine structure constant. The results obtained are compared with analytical calculations performed within the Zα-expansion.
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Two-loop self-Energy Correction in a strong Coulomb nuclear field
Journal of Experimental and Theoretical Physics, 2005Co-Authors: Vladimir A. Yerokhin, Paul Indelicato, V. M. ShabaevAbstract:The two-loop self-Energy Correction to the ground-state Energy levels of hydrogen-like ions with nuclear charges Z≥10 is calculated without the Zα expansion, where α is the fine-structure constant. The data obtained are compared with the results of analytical calculations within the Zα expansion; significant disagreement with the analytical results of order α2(Zα)6 has been found. Extrapolation is used to obtain the most accurate value for the two-loop self-Energy Correction for the 1s state in hydrogen.
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Two-loop self-Energy Correction to the ground-state Lamb shift in H-like ions
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2005Co-Authors: Vladimir A. Yerokhin, P. Indelicato, V. M. ShabaevAbstract:The two-loop self-Energy Correction is evaluated to all orders in Z alpha for the ground-state Lamb shift of H-like ions with Z >= 20, where Z is the nuclear charge number and alpha is the fine structure constant. The results obtained are compared with analytical calculations performed within the Z alpha expansion.Comment: a contribution to the conference "Highly Charged Ions 2004
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Screened self-Energy Correction to the 2p3/2-2s transition Energy in Li-like ions
Optics and Spectroscopy, 2005Co-Authors: Vladimir A. Yerokhin, V. M. Shabaev, A. N. Artemyev, G. Plunien, Gerhard SoffAbstract:We present an ab initio calculation of the screened self-Energy Correction for 1s2 2p3/2 and 1s2 2s states of Li-like ions with nuclear charge numbers in the range Z = 12−100. The evaluation is carried out to all orders in the nuclear strength parameter Zα. This investigation concludes our calculations of all two-electron QED Corrections for the 2p3/2-2s transition Energy in Li-like ions and thus considerably improves theoretical predictions for this transition for high-Z ions.
Charles L Brooks - One of the best experts on this subject based on the ideXlab platform.
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extending the treatment of backbone energetics in protein force fields limitations of gas phase quantum mechanics in reproducing protein conformational distributions in molecular dynamics simulations
Journal of Computational Chemistry, 2004Co-Authors: Alexander D Mackerell, Michael Feig, Charles L BrooksAbstract:Computational studies of proteins based on empirical force fields represent a powerful tool to obtain structure-function relationships at an atomic level, and are central in current efforts to solve the protein folding problem. The results from studies applying these tools are, however, dependent on the quality of the force fields used. In particular, accurate treatment of the peptide backbone is crucial to achieve representative conformational distributions in simulation studies. To improve the treatment of the peptide backbone, quantum mechanical (QM) and molecular mechanical (MM) calculations were undertaken on the alanine, glycine, and proline dipeptides, and the results from these calculations were combined with molecular dynamics (MD) simulations of proteins in crystal and aqueous environments. QM potential Energy maps of the alanine and glycine dipeptides at the LMP2/cc-pVxZ//MP2/6-31G* levels, where x = D, T, and Q, were determined, and are compared to available QM studies on these molecules. The LMP2/cc-pVQZ//MP2/6-31G* Energy surfaces for all three dipeptides were then used to improve the MM treatment of the dipeptides. These improvements included additional parameter optimization via Monte Carlo simulated annealing and extension of the potential Energy function to contain peptide backbone phi, psi dihedral crossterms or a phi, psi grid-based Energy Correction term. Simultaneously, MD simulations of up to seven proteins in their crystalline environments were used to validate the force field enhancements. Comparison with QM and crystallographic data showed that an additional optimization of the phi, psi dihedral parameters along with the grid-based Energy Correction were required to yield significant improvements over the CHARMM22 force field. However, systematic deviations in the treatment of phi and psi in the helical and sheet regions were evident. Accordingly, empirical adjustments were made to the grid-based Energy Correction for alanine and glycine to account for these systematic differences. These adjustments lead to greater deviations from QM data for the two dipeptides but also yielded improved agreement with experimental crystallographic data. These improvements enhance the quality of the CHARMM force field in treating proteins. This extension of the potential Energy function is anticipated to facilitate improved treatment of biological macromolecules via MM approaches in general.
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extending the treatment of backbone energetics in protein force fields limitations of gas phase quantum mechanics in reproducing protein conformational distributions in molecular dynamics simulations
Journal of Computational Chemistry, 2004Co-Authors: Alexander D Mackerell, Michael Feig, Charles L BrooksAbstract:Computational studies of proteins based on empirical force fields represent a powerful tool to obtain structure–function relationships at an atomic level, and are central in current efforts to solve the protein folding problem. The results from studies applying these tools are, however, dependent on the quality of the force fields used. In particular, accurate treatment of the peptide backbone is crucial to achieve representative conformational distributions in simulation studies. To improve the treatment of the peptide backbone, quantum mechanical (QM) and molecular mechanical (MM) calculations were undertaken on the alanine, glycine, and proline dipeptides, and the results from these calculations were combined with molecular dynamics (MD) simulations of proteins in crystal and aqueous environments. QM potential Energy maps of the alanine and glycine dipeptides at the LMP2/cc-pVxZ//MP2/6-31G* levels, where x = D, T, and Q, were determined, and are compared to available QM studies on these molecules. The LMP2/cc-pVQZ//MP2/6-31G* Energy surfaces for all three dipeptides were then used to improve the MM treatment of the dipeptides. These improvements included additional parameter optimization via Monte Carlo simulated annealing and extension of the potential Energy function to contain peptide backbone ϕ, ψ dihedral crossterms or a ϕ, ψ grid-based Energy Correction term. Simultaneously, MD simulations of up to seven proteins in their crystalline environments were used to validate the force field enhancements. Comparison with QM and crystallographic data showed that an additional optimization of the ϕ, ψ dihedral parameters along with the grid-based Energy Correction were required to yield significant improvements over the CHARMM22 force field. However, systematic deviations in the treatment of ϕ and ψ in the helical and sheet regions were evident. Accordingly, empirical adjustments were made to the grid-based Energy Correction for alanine and glycine to account for these systematic differences. These adjustments lead to greater deviations from QM data for the two dipeptides but also yielded improved agreement with experimental crystallographic data. These improvements enhance the quality of the CHARMM force field in treating proteins. This extension of the potential Energy function is anticipated to facilitate improved treatment of biological macromolecules via MM approaches in general. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 1400–1415, 2004
V. A. Yerokhin - One of the best experts on this subject based on the ideXlab platform.
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Non-perturbative calculation of the two-loop Lamb shift in Li-like ions
Physical Review Letters, 2006Co-Authors: V. A. Yerokhin, P. Indelicato, V. M. ShabaevAbstract:A calculation valid to all orders in the nuclear-strength parameter is presented for the two-loop Lamb shift, notably for the two-loop self-Energy Correction, to the 2p-2s transition energies in heavy Li-like ions. The calculation removes the largest theoretical uncertainty for these transitions and yields the first experimental identification of two-loop QED effects in the region of the strong binding field.
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Two-loop self-Energy contribution to the Lamb shift in H-like ions
Physical Review A, 2005Co-Authors: V. A. Yerokhin, Paul Indelicato, V. M. ShabaevAbstract:The two-loop self-Energy Correction is evaluated to all orders in Z\\alpha for the ground-state Lamb shift of H-like ions with Z >= 10, where Z is the nuclear charge number and \\alpha is the fine structure constant. The results obtained are compared with the analytical values for the Z\\alpha-expansion coefficients. An extrapolation of the all-order numerical results to Z=1 is presented and implications of our calculation for the hydrogen Lamb shift are discussed.
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dual kinetic balance approach to basis set expansions for the dirac equation
Physical Review Letters, 2004Co-Authors: V. M. Shabaev, V. A. Yerokhin, G. Plunien, I I Tupitsyn, G SoffAbstract:A new approach to finite basis sets for the Dirac equation is developed. It does not involve spurious states and improves the convergence properties of basis-set calculations. Efficiency of the method is demonstrated for finite basis sets constructed from $B$ splines by calculating the one-loop self-Energy Correction for a hydrogenlike ion.
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Self-Energy Correction to the hyperfine structure splitting of the 1s and 2s states in hydrogenlike ions
Journal of Experimental and Theoretical Physics Letters, 1997Co-Authors: V. A. Yerokhin, V. M. Shabaev, A. N. ArtemyevAbstract:The one-loop self-Energy Correction to the hyperfine structure splitting of the 1s and 2s states of hydrogenlike ions is calculated both for the point and finite nucleus. The results of the calculation are combined with other Corrections to find the ground state hyperfine splitting in lithiumlike ^{209}Bi^{80+} and ^{165}Ho^{64+}.
Alexander D Mackerell - One of the best experts on this subject based on the ideXlab platform.
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extending the treatment of backbone energetics in protein force fields limitations of gas phase quantum mechanics in reproducing protein conformational distributions in molecular dynamics simulations
Journal of Computational Chemistry, 2004Co-Authors: Alexander D Mackerell, Michael Feig, Charles L BrooksAbstract:Computational studies of proteins based on empirical force fields represent a powerful tool to obtain structure-function relationships at an atomic level, and are central in current efforts to solve the protein folding problem. The results from studies applying these tools are, however, dependent on the quality of the force fields used. In particular, accurate treatment of the peptide backbone is crucial to achieve representative conformational distributions in simulation studies. To improve the treatment of the peptide backbone, quantum mechanical (QM) and molecular mechanical (MM) calculations were undertaken on the alanine, glycine, and proline dipeptides, and the results from these calculations were combined with molecular dynamics (MD) simulations of proteins in crystal and aqueous environments. QM potential Energy maps of the alanine and glycine dipeptides at the LMP2/cc-pVxZ//MP2/6-31G* levels, where x = D, T, and Q, were determined, and are compared to available QM studies on these molecules. The LMP2/cc-pVQZ//MP2/6-31G* Energy surfaces for all three dipeptides were then used to improve the MM treatment of the dipeptides. These improvements included additional parameter optimization via Monte Carlo simulated annealing and extension of the potential Energy function to contain peptide backbone phi, psi dihedral crossterms or a phi, psi grid-based Energy Correction term. Simultaneously, MD simulations of up to seven proteins in their crystalline environments were used to validate the force field enhancements. Comparison with QM and crystallographic data showed that an additional optimization of the phi, psi dihedral parameters along with the grid-based Energy Correction were required to yield significant improvements over the CHARMM22 force field. However, systematic deviations in the treatment of phi and psi in the helical and sheet regions were evident. Accordingly, empirical adjustments were made to the grid-based Energy Correction for alanine and glycine to account for these systematic differences. These adjustments lead to greater deviations from QM data for the two dipeptides but also yielded improved agreement with experimental crystallographic data. These improvements enhance the quality of the CHARMM force field in treating proteins. This extension of the potential Energy function is anticipated to facilitate improved treatment of biological macromolecules via MM approaches in general.
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extending the treatment of backbone energetics in protein force fields limitations of gas phase quantum mechanics in reproducing protein conformational distributions in molecular dynamics simulations
Journal of Computational Chemistry, 2004Co-Authors: Alexander D Mackerell, Michael Feig, Charles L BrooksAbstract:Computational studies of proteins based on empirical force fields represent a powerful tool to obtain structure–function relationships at an atomic level, and are central in current efforts to solve the protein folding problem. The results from studies applying these tools are, however, dependent on the quality of the force fields used. In particular, accurate treatment of the peptide backbone is crucial to achieve representative conformational distributions in simulation studies. To improve the treatment of the peptide backbone, quantum mechanical (QM) and molecular mechanical (MM) calculations were undertaken on the alanine, glycine, and proline dipeptides, and the results from these calculations were combined with molecular dynamics (MD) simulations of proteins in crystal and aqueous environments. QM potential Energy maps of the alanine and glycine dipeptides at the LMP2/cc-pVxZ//MP2/6-31G* levels, where x = D, T, and Q, were determined, and are compared to available QM studies on these molecules. The LMP2/cc-pVQZ//MP2/6-31G* Energy surfaces for all three dipeptides were then used to improve the MM treatment of the dipeptides. These improvements included additional parameter optimization via Monte Carlo simulated annealing and extension of the potential Energy function to contain peptide backbone ϕ, ψ dihedral crossterms or a ϕ, ψ grid-based Energy Correction term. Simultaneously, MD simulations of up to seven proteins in their crystalline environments were used to validate the force field enhancements. Comparison with QM and crystallographic data showed that an additional optimization of the ϕ, ψ dihedral parameters along with the grid-based Energy Correction were required to yield significant improvements over the CHARMM22 force field. However, systematic deviations in the treatment of ϕ and ψ in the helical and sheet regions were evident. Accordingly, empirical adjustments were made to the grid-based Energy Correction for alanine and glycine to account for these systematic differences. These adjustments lead to greater deviations from QM data for the two dipeptides but also yielded improved agreement with experimental crystallographic data. These improvements enhance the quality of the CHARMM force field in treating proteins. This extension of the potential Energy function is anticipated to facilitate improved treatment of biological macromolecules via MM approaches in general. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 1400–1415, 2004