The Experts below are selected from a list of 25197 Experts worldwide ranked by ideXlab platform
Stefan Grimme - One of the best experts on this subject based on the ideXlab platform.
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small atomic Orbital Basis set first principles quantum chemical methods for large molecular and periodic systems a critical analysis of error sources
ChemistryOpen, 2016Co-Authors: Rebecca Sure, Jan Gerit Brandenburg, Stefan GrimmeAbstract:In quantum chemical computations the combination of Hartree-Fock or a density functional theory (DFT) approximation with relatively small atomic Orbital Basis sets of double-zeta quality is still widely used, for example, in the popular B3LYP/6-31G* approach. In this Review, we critically analyze the two main sources of error in such computations, that is, the Basis set superposition error on the one hand and the missing London dispersion interactions on the other. We review various strategies to correct those errors and present exemplary calculations on mainly noncovalently bound systems of widely varying size. Energies and geometries of small dimers, large supramolecular complexes, and molecular crystals are covered. We conclude that it is not justified to rely on fortunate error compensation, as the main inconsistencies can be cured by modern correction schemes which clearly outperform the plain mean-field methods.
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Small Atomic Orbital Basis Set First‐Principles Quantum Chemical Methods for Large Molecular and Periodic Systems: A Critical Analysis of Error Sources
ChemistryOpen, 2015Co-Authors: Rebecca Sure, Jan Gerit Brandenburg, Stefan GrimmeAbstract:In quantum chemical computations the combination of Hartree-Fock or a density functional theory (DFT) approximation with relatively small atomic Orbital Basis sets of double-zeta quality is still widely used, for example, in the popular B3LYP/6-31G* approach. In this Review, we critically analyze the two main sources of error in such computations, that is, the Basis set superposition error on the one hand and the missing London dispersion interactions on the other. We review various strategies to correct those errors and present exemplary calculations on mainly noncovalently bound systems of widely varying size. Energies and geometries of small dimers, large supramolecular complexes, and molecular crystals are covered. We conclude that it is not justified to rely on fortunate error compensation, as the main inconsistencies can be cured by modern correction schemes which clearly outperform the plain mean-field methods.
Miquel Solà - One of the best experts on this subject based on the ideXlab platform.
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Spin-state-corrected Gaussian-type Orbital Basis sets.
The journal of physical chemistry. A, 2010Co-Authors: Marcel Swart, Mireia Güell, Josep M. Luis, Miquel SolàAbstract:Recently, we reported that the Basis set has a profound influence on the computed values for spin-state splittings [J. Phys. Chem. A 2008, 112, 6384]. In particular, small Gaussian-type Orbital (GTO) Basis sets were shown to be unreliable for the prediction of them. Here, we report simple modifications of the small Pople-type Gaussian-type Orbital Basis sets (3-21G, 3-21G*, 6-31G, 6-31G*), which correct their faulty behavior for the spin-state energies. We have investigated the Basis sets for a set of 13 first-row transition-metal complexes for which reliable reference data have been obtained at the OPBE/TZ2P(STO) level. For several systems, we have used single and double spin-contamination corrections to avoid ambiguity of the results because of spin contamination, that is, the energies and geometries were obtained for the pure spin states. The spin ground states as predicted by the spin-state-corrected GTO Basis sets (s6-31G, s6-31G*) are in complete agreement with the reference Slater-type Orbital (STO...
Tatsuo Shimizu - One of the best experts on this subject based on the ideXlab platform.
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Density functional theory calculations of isotropic hyperfine coupling constants of atoms and monohydrides
Chemical Physics Letters, 1995Co-Authors: Nobuhiko Ishii, Tatsuo ShimizuAbstract:Abstract Isotropic hyperfine coupling constants of atoms (H, B, C, N, O, F) and monohydrides (CH, NH, OH) are calculated using density functional theory with a gradient-corrected local-spin-density approximation and a Slater-type-Orbital Basis set. The agreement between the calculated and observed results is fairly good. It is important to use a Slater-type Orbital Basis set which satisfies the cusp condition for electron density in order to obtain reliable results.
Peter R. Taylor - One of the best experts on this subject based on the ideXlab platform.
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Accurate quantum-chemical calculations using Gaussian-type geminal and Gaussian-type Orbital Basis sets: applications to atoms and diatomics
Physical chemistry chemical physics : PCCP, 2007Co-Authors: Pål Dahle, Trygve Helgaker, D. Jonsson, Peter R. TaylorAbstract:We have implemented the use of mixed Basis sets of Gaussian one- and two-electron (geminal) functions for the calculation of second-order Moller-Plesset (MP2) correlation energies. In this paper, we describe some aspects of this implementation, including different forms chosen for the pair functions. Computational results are presented for some closed-shell atoms and diatomics. Our calculations indicate that the method presented is capable of yielding highly accurate second-order correlation energies with rather modest Gaussian Orbital Basis sets, providing an alternative route to highly accurate wave functions. For the neon atom, the hydrogen molecule, and the hydrogen fluoride molecule, our calculations yield the most accurate MP2 energies published so far. A critical comparison is made with established MP2-R12 methods, revealing an erratic behaviour of some of these methods, even in large Basis sets.
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The harmonic frequencies of benzene. A case for atomic natural Orbital Basis sets
Chemical Physics Letters, 1997Co-Authors: Jan M. L. Martin, Peter R. Taylor, Timothy J. LeeAbstract:Abstract The geometry and harmonic force field of benzene have been computed at the CCSD(T) level with Basis sets of spdf quality. Two out-of-plane modes, ω4 and ω5 (and to a lesser extent ω17), exhibit a pathological Basis set dep to Basis set superposition error. Using an atomic natural Orbital (ANO) Basis set of [4s3p2d1η/4s2p] quality, the best available experimentally derived harmonic frequencies can be reproduced with an RMS error of 6 cm−1 without any empirical corrections. We strongly recommend the use of ANO Basis sets for accurate frequency calculations on unsaturated and aromatic systems. Our best estimate for the equilibrium geometry is r e ( CC ) = 1.392(2), r e ( CH ) = 1.081 A .
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Atomic natural Orbital Basis sets for transition metals
Theoretica chimica acta, 1993Co-Authors: Charles W. Bauschlicher, Peter R. TaylorAbstract:We show that atomic natural Orbitals are an excellent way to contract transition-metal Basis sets, even though the different low-lying electronic states may have very different Basis set requirements.
Quazi D. M. Khosru - One of the best experts on this subject based on the ideXlab platform.
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Parametrization of a silicon nanowire effective mass model from sp3d5s* Orbital Basis calculations
Semiconductor Science and Technology, 2009Co-Authors: R.n. Sajjad, Khairul Alam, Quazi D. M. KhosruAbstract:We parameterize a silicon nanowire effective mass model to facilitate device simulation, where the mass depends on the wire dimension. Parametrization is performed for n-channel silicon nanowire transistors from sp3d5s* atomic Orbital Basis tight-binding calculations. The nanowires used in this study are grown in 1 0 0 and 1 1 0 directions. With the parameterized nanowire effective masses, we then calculate the current and compare against the full band I–V. The full band I–V is calculated for 1 1 0 wires of cross sections 0.82 nm × 0.82 nm and 1.2 nm× 1.2 nm due to computational resource limitation. The full-band and effective-mass I–V characteristics of 1.2 nm × 1.2 nm wire show very good agreement. However, a relatively larger mismatch is observed for the 0.82 nm × 0.82 nm wire, especially at the lower gate biases. This is because the current has both the thermal and tunneling components, and the nanowire effective-mass model overestimates the tunneling current. This overestimation is relatively larger for thinner wires. The thermal component of current is the same in both the nanowire effective-mass and full-band models. The performance metrics, namely the intrinsic switching delay and the unity current gain frequency are evaluated from the full-band calculations. The device has a near ideal subthreshold slope, a fraction of picosecond switching delay and a tera Hertz unity current gain frequency.
