排序方式: 共有12条查询结果,搜索用时 15 毫秒
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Peach MJ Helgaker T Sałek P Keal TW Lutnaes OB Tozer DJ Handy NC 《Physical chemistry chemical physics : PCCP》2006,8(5):558-562
The recently proposed CAM-B3LYP exchange-correlation energy functional, based on a partitioning of the r operator in the exchange interaction into long- and short-range components, is assessed for the determination of molecular thermochemistry, structures, and second order response properties. Rydberg and charge transfer excitation energies and static electronic polarisabilities are notably improved over the standard B3LYP functional; classical reaction barriers also improve. Ionisation potentials, bond lengths, NMR shielding constants and indirect spin-spin coupling constants are comparable with the two functionals. CAM-B3LYP atomisation energies and diatomic harmonic vibrational wavenumbers are less accurate than those of B3LYP. Future research directions are outlined. 相似文献
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It is demonstrated that there is still scope for improvement in the quality of conventional, semiempirical hybrid exchange-correlation functionals in density-functional theory. A new functional, denoted B97-3, is determined from a fit to eight chemical properties (316 data points). For a series of 25 chemical assessments (850 data points) including 17 assessments and 10 chemical properties absent from the fitting data, B97-3 provides the lowest or joint-lowest mean absolute error on 15 occasions, compared to 6, 5, and 4 occasions for B3LYP, PBE0, and B97-2, respectively [A. D. Becke, J. Chem. Phys. 98, 5648 (1993); M. Ernzerhof and G. E. Scuseria, J. Chem. Phys. 110, 5029 (1999); C. Adamo and V. Barone, J. Chem. Phys. 110, 6158 (1999); P. J. Wilson, T. J. Bradley, and D. J. Tozer, J. Chem. Phys. 115, 9233 (2001)]. Mean absolute errors from B97-3 are, on average, 21%, 18%, and 12% smaller than from these three functionals. The most notable improvements are obtained for classical reaction barriers, where the error reductions are 60%, 54%, and 27%. 相似文献
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We present a comparison of three previously published algorithms for optimising the minimum energy crossing point between
two Born–Oppenheimer electronic states. The algorithms are implemented in a development version of the MNDO electronic structure
package for use with semiempirical configuration interaction methods. The penalty function method requires only the energies
and gradients of the states involved, whereas the gradient projection and Lagrange–Newton methods also require the calculation
of non-adiabatic coupling terms. The performance of the algorithms is measured against a set of well-known small molecule
conical intersections. The Lagrange–Newton method is found to be the most efficient, with the projected gradient method also
competitive. The penalty function method can only be recommended for situations where non-adiabatic coupling terms cannot
be calculated.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
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The potential energy surfaces and non-adiabatic dynamics of the C5H6NH 2 + protonated Schiff base (PSB3) have been investigated using the OM2 semiempirical Hamiltonian with GUGA configuration interaction. Three approaches to selecting the GUGA-CI active space are evaluated using closed-shell and open-shell molecular orbitals. Energy minima and minimum energy crossing points (MECPs) have been compared with ab initio CASSCF and CASPT2 results. Only the open-shell calculations give a qualitatively correct MECP. Minimum energy path (MEP) calculations demonstrate that a minimal active space gives a barrierless path from the planar S1 minimum to the ground state, whereas larger active spaces result in a small barrier to torsional motion. Surface hopping dynamics calculations indicate that this barrier induces bi-exponential dynamics. The comparable CASSCF S1 energy surface is barrierless, but the CASPT2 surface features an energy plateau, which may also lead to more complex dynamics. 相似文献
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Robbins AJ Ng WT Jochym D Keal TW Clark SJ Tozer DJ Hodgkinson P 《Physical chemistry chemical physics : PCCP》2007,9(19):2389-2396
Advances in solid-state NMR methodology and computational chemistry are applied to the (19)F NMR of solid octafluoronaphthalene. It is demonstrated experimentally, and confirmed by density functional theory (DFT) calculations, that the spectral resolution in the magic-angle spinning spectrum is limited by the anisotropy of the bulk magnetic susceptibility (ABMS). This leads to the unusual observation that the resolution improves as the sample is diluted. DFT calculations provide assignments of each of the peaks in the (19)F spectrum, but the predictions are close to the limits of accuracy and correlation information from 2-D NMR is invaluable in confirming the assignments. The effects of non-Gaussian lineshapes on the use of 2-D NMR for mapping correlations of spectral frequencies (e.g. due to the ABMS) are also discussed. 相似文献