共查询到20条相似文献,搜索用时 15 毫秒
1.
M. Häser R. Ahlrichs H. P. Baron P. Weis H. Horn 《Theoretical chemistry accounts》1992,83(5-6):455-470
Summary The ab initio SCF computation of second-order properties of large molecules (with 50 atoms or more) on workstation computers is demonstrated for static dipole polarizabilities and nuclear magneting shieldings. The magnetic shieldings are calculated on the basis of gauge including atomic orbitals (GIAO). Algorithmic advances (semi-direct algorithms with efficient integral pre-screening, and use of a quadratically convergent functional for the polarizabilities) are presented together with an illustrative application to the fullerenes C60 and C70. 相似文献
2.
Matjaž Žaucer Danilo Pumpernik Milan Hladnik Andrej Ažman 《Chemical physics letters》1976,44(1):58-60
The magnetic susceptibility of hydrogen fluoride is calculated with four different basis sets of gauge invariant atomic orbitals (GIAO's). The Roothaan equations are solved for various values of the magnetic field strength and the susceptibility is deduced by a numerical differentiation of the energy. 相似文献
3.
A novel procedure for calculating magnetic susceptibilities and nuclear magnetic shieldings in molecules is outlined, based on formal annihilation of transverse paramagnetic contribution to quantum mechanical current density induced within the electron cloud by an external homogeneous, static magnetic field. Within this method all the components of nuclear magnetic shielding are independent of a gauge translation, in any calculation relying on the algebraic approximation, irrespective of size and quality of the gaugeless basis set adopted; magnetic susceptibilities are invariant for center-symmetric molecules (virtual invariance is actually observed for molecules of arbitrary symmetry). Large basis set calculations of near-Hartree-Fock magnetic properties and maps describing a current density field of acetylene molecule carried out via the new procedure are compared with corresponding ones adopting the common origin-coupled Hartree-Fock approach and methods formally annihilating the diamagnetic contribution to the current density. © 1996 John Wiley & Sons, Inc. 相似文献
4.
Nuclear magnetic shielding and spin–spin coupling constants are intrinsically all-electron relativistic properties and demand
in principle fully relativistic treatments. Here, the magnetic balance (MB) condition plays an essential role, both conceptually
and computationally. The various formulations can be unified in terms of the idea of "orbital decomposition." Further combined
with the ansatz of "gauge-including atomic orbitals" (GIAO) for distributed gauge origins leads to very efficient four-component
relativistic methods at both the mean-field and correlated levels. To illustrate the latter, the no-pair MB-GIAO-MP2 expressions
for nuclear shieldings are derived in two different ways, one with the derivative technique and the other through the induced
current. Due to the non-variational nature of MP2, the two expressions are not identical. The current-dependent expression
is much simpler and appears more natural in view of the experimental measurement. 相似文献
5.
Matjaž Žaucer Danilo Pumpernik Milan Hladnik Andrej Ažman 《Chemical physics letters》1977,48(1):139-140
Susceptibility and magnetic shieldings of both nuclei of BH are calculated by use of the finite perturbation method and gauge invariant gaussian basis sets. The results provide further evidence for the temperature independent paramagnetism of BH. 相似文献
6.
D.B. Chesnut B.E. Rusiloski K.D. Moore D.A. Egolf 《Journal of computational chemistry》1993,14(11):1364-1375
The locally dense basis set approach to the calculation of nuclear magnetic resonance shieldings is one in which a sufficiently large or dense set of basis functions is used for an atom or molecular fragment containing the resonant nucleus or nuclei of interest and fewer or attenuated sets of basis functions employed elsewhere. Provided the dense set is of sufficient size, this approach is capable of determining chemical shieldings nearly as well as a calculation with a balanced basis set of quality equal to the locally dense set, but with considerable savings of CPU time. Detailed comparisons are provided of locally dense and balanced calculations in the gauge including atomic orbital (GIAO) method for the individual principal values, the isotropic shieldings, and the tensor orientations for hydrogen, carbon, nitrogen, oxygen, fluorine, and phosphorus nuclei. It is seen that chemical functional groups can often define the appropriate molecular fragment to be taken locally dense. While the present test cases are for the most part small molecules, the value of the method is that it will allow calculations on systems that would otherwise presently be computationally expensive or inaccessible. © John Wiley & Sons, Inc. 相似文献
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8.
Ann Strömberg 《Chemical physics letters》1983,99(2):122-128
The valence interactions in two effective core potential (ECP) methods, the frozen orbital ECP and the method of Sakai and Huzinaga, are shown to yield atomic valence orbital Coulomb and exchange interaction integrals closely approximating all-electron calculations. The ECP approximation is studied in some detail with special application to the ScO molecule. The too short bond distance in ScO when the 3s. 3p orbitals are included in the core is shown to stem from a long-range attraction of the ECP. 相似文献
9.
10.
The spatial magnetic properties (through-space NMR shieldings, or TSNMRSs) of the antiaromatic 9-oxaanthracene anion 12(-) and of the corresponding 9-dimeric dianion 11(2-) have been calculated by the gauge-invariant atomic orbitals (GIAO) perturbation method employing the nucleus independent chemical shift (NICS) concept and visualized as iso-chemical-shielding surfaces (ICSSs) of various size and direction. The TSNMRS values, thus obtained, can be employed to indicate antiaromaticity by paratropic ring currents of the anionic compounds of 11(2-) and 12(-) studied and other neutral and ionic antiaromatic molecules from previous studies because anisotropic effects of functional groups in (1)H NMR spectra have quantitatively proven to be the molecular response property of theoretical spatial nucleus independent chemical shieldings (NICS). 相似文献
11.
A theoretical NMR study of selected benzazoles: Comparison of GIPAW and GIAO‐PCM (DMSO) calculations
《Magnetic resonance in chemistry : MRC》2018,56(3):164-171
This paper compares the absolute shieldings obtained by gauge‐including‐projected‐augmented‐wave (GIPAW) to those obtained by gauge‐invariant atomic orbital/Becke, 3‐parameter, Lee‐Yang‐Parr (GIAO/B3LYP)/6–311++G(d,p)‐polarizable continuum model (PCM, dimethyl sulfoxide) for nine benzazoles (benzimidazoles, indazoles, and benzotriazoles) recorded in the solid‐state. Three nuclei were explored, 13C, 15N, and 19F, and the gauge‐including‐projected‐augmented‐wave approach only proved better for 15N MAS NMR. 相似文献
12.
13.
A. V. Titov 《International journal of quantum chemistry》1996,57(3):453-463
An approach for a space-separated calculation of the wave function in the valence and core regions of a molecule is proposed. As the first step, the calculation of the orbitals (or two-component spinors in the relativistic case) in the valence region by the effective core potential (ECP ) method is performed. Then, it is followed by a restoration of orbitals (four-component spinors) expanded on spherical harmonics in the core regions of heavy atoms. Theoretical questions of the variational calculation of the molecular orbitals are considered in some core region limited by a sphere. Inclusion from the electronic cloud outside this region is reduced by the necessity of taking into account the orthonormality and boundary conditions together with an effective external field in respect to the selected core region. This method may be used for calculation of matrix elements of operators that are singular near nuclei (P, T-odd interactions, hyperfine structure, etc.). A substantial computational saving can be reached because the method enables, by the most optimal way, to combine the advantages of two well-developed approaches: molecular ECP calculations in the Gaussian basis set and one-center numerical atomic calculations with an external field. It is especially important when the relativistic effects are taken into account. © 1996 John Wiley & Sons, Inc. 相似文献
14.
Gauge‐independent atomic orbital (GIAO) method at Hartree‐Fock (HF) and density functional theory (DFT) levels, respectively, was employed to calculate 19F NMR chemical shieldings of solid state alkaline‐earth‐metal fluorides MF2 (M = Mg, Ca, Sr, Ba). The results show that, although the calculated 19F chemical shieldings tend to be larger than the experimental values, they have a fairly good linear relationship with the observed ones. The calculated results based on different combinations of basis sets show that the B3LYP (a hybrid of DFT with HF) predictions are greatly superior to the HF predictions. When a basis set of metal atom with effective core potential (ECP) has well representation of valence wavefunction, especially wavefunction of d component, and proper definition of core electron number, it can be applied to obtain 19F chemical shielding which is close to that of all‐electron calculation. The variation of 19F chemical shielding of alkaline‐earth‐metal fluorides correlates well with the lattice factor A/R2. 相似文献
15.
Two expressions for nuclear-magnetic-shielding tensor components based on analytically differentiating the electronic energy of a system are presented. The first is based on a second-order Douglas-Kroll-Hess approach, in which the off-diagonal block terms of the transformed Dirac Hamiltonian are diminished to second order with respect to both the electrostatic nuclear attraction potential V and the magnetic vector potential A. The second expression is based on the method of Barysz-Sadlej-Snijders, in which the off-diagonal block terms in the transformed Dirac Hamiltonian are completely eliminated with respect to purely V terms, while they are diminished to second order with respect to terms including A. The two approaches are applied to the calculation of nuclear magnetic shieldings of HX (X=F, Cl, Br, I), H2X (X=O, S, Se, Te), and noble gas X (X =He,Ne,Ar,Kr,Xe) systems with common gauge origins. The results show that relativistic corrections of higher than second order are negligibly small, except for the paramagnetic parts of I, Te, and Xe shieldings. The present calculations yield very large positive values for the anisotropy of proton shielding, deltasigma(H) = sigmaparallel(H)-sigmaperpendicular(H), of HI compared to previous reports. Unfortunately, no experimental values for the anisotropy of proton shielding in HI are available for verification. 相似文献
16.
Keld L. Bak Aage E. Hansen Kenneth Ruud Trygve Helgaker Jeppe Olsen Poul Jørgensen 《Theoretical chemistry accounts》1995,90(5-6):441-458
Summary The second-quantization magnetic dipole operator that arises when London atomic orbitals are used as basis functions is derived. In atomic units, the magnetic dipole operator is defined as the negative of the first derivative of the electronic Hamiltonian containing the interaction with the external magnetic field. It is shown that for finite basis sets, the gauge origin dependence of the resulting magnetic dipole operator is analogous to that of the exact operator, and that the derived operator converges to the exact operator in the limit of a complete basis set. It is also demonstrated that the length expression for the rotatory strength in linear response calculations gives gauge-origin-independent results. Sample calculations ontrans-cyclooctene and its fragments are presented. Compared to conventional orbitals, the basis set convergence of the rotatory strengths calculated in the length form using London atomic orbitals is favourable. The rotatory strength calculated fortrans-cyclooctene agrees nicely with the corresponding experimental circular dichroism spectrum, but the spectra for the fragment molecules show little resemblance with that oftrans-cyclooctene.Dedicated to Prof. Jan Linderberg 相似文献
17.
Robert Ditchfield 《Chemical physics letters》1976,40(1):53-56
Proton and fluorine magnetic shielding tensors are calculated for FHF- and HF using coupled Hartree-Fock (CHF) theory with gauge-invariant atomic orbitals (GIAO). The theoretical value of the hydrogen bond shift in FHF- determined from slightly extended basis set studies is in good agreement with experimental results. Thus a recent suggestion that the GIAO CHF approach will suffer from the same deficiencies as the ordinary CHF method for the FHF- ion does not appear to be correct. 相似文献
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19.
The interaction Hamiltonian within the Bloch gauge for the potentials of the electromagnetic field has been used to define magnetic multipole moment operators and operators for the magnetic field of electrons acting on the nuclei of a molecule in the presence of nonhomogeneous external magnetic field. Perturbation theory has been applied to evaluate the induced electronic moments and magnetic field at the nuclei. Multipole magnetic susceptibility and nuclear magnetic shielding tensors have been introduced to describe the contributions arising in nonuniform fields, and their origin dependence has been analyzed. Extended numerical tests on the ammonia molecule in a static, nonuniform magnetic field have been carried out, using the random-phase approximation within the framework of accurate Hartree-Fock zero-order wavefunctions, and allowing for both angular momentum and torque formalisms in the calculation of paramagnetic contributions. 相似文献
20.
Massimo Ragazzi Paola Carbone Dino R. Ferro 《International journal of quantum chemistry》2002,88(5):663-669
Cycloolefin copolymers (COC) have recently raised much interest because of their excellent thermal and optical properties, largely determined by the chain composition and stereochemistry. Previous force‐field computations allowed us to define the main conformational characteristics of ethylene–norbornene (E–N) copolymers and to contribute to the elucidation of their microstructure on the basis of empirical relationships between conformation and 13C nuclear magnetic resonance (NMR) chemical shifts. A thorough test of ab initio 13C chemical shifts computations [gauge‐invariant atomic orbitals (GIAO)] on known cases shows that the agreement with experimental data is quite good, especially with the MPW1PW91 density functional theory (DFT), using the 6‐311+G(2d,p) basis set on properly energy‐minimized structures. We applied this method on proper model compounds to confirm the signal assignment of the spectra of E–N copolymers in the presence of norbornene microblocks, where strong effects arising from ring distortions are expected to occur. The results nicely confirm the latest assignment of norbornene signals belonging to ENNE sequences. This shows the great potentialities of GIAO/DFT computations with regard to complex spectra interpretation and polymer microstructural investigations. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002 相似文献