Search results for "PERTURBATION"
showing 10 items of 811 documents
Nuclear magnetic shielding constants in the CC2 model
1997
Abstract Test calculations of nuclear magnetic shielding constants in the CC2 model are performed using the gauge-including atomic orbital approach. Absolute shielding constants are reported for reprsentative first-row hydrides, a few multiply bonded molecules and some challenging cases. The performance of CC2 is analyzed by comparison with experimental data and results from calculations employing more sophisticated treatments of electron correlation. In most cases, CC2 shieldings and chemical shifts are close to those obtained at second-order perturbation theory, despite the fact that the CC2 model includes an approximate treatment of orbital relaxation effects at the correlated level.
An ab initio study of the electron affinity of O2
1993
Abstract Coupled pair functional, multiconfigurational second-order perturbation theory, and multireference CI methods have been applied in a calculation of the electron affinity of the oxygen molecule. The convergence of the theoretical result has been checked with respect to a systematic expansion of the one-electron basis and the multireference CI wavefunction. The best calculated value, 0.39 eV, is 0.06 eV smaller than the recent experimental value 0.45±0.01 eV.
Ab Initio Methods for Excited States
2005
This chapter focuses mainly on the performance of ab initio methods for the description of spectroscopic molecular properties of compounds. Most of the quantum-chemical methods developed up to date are based on the concept of the one-electron wave function. The electronic states of a system with N electrons are described by a double expansion. Molecular orbitals (MOs) are one-electron wave functions expressed as linear combinations of a known one-electron basis set (K) and the N electron wave function is formulated in a many-electron basis set formed by determinants (or linear combination of them to form spin-adapted wave functions), built as normalized antisymmetric products of MOs. Accord…
Quantitative prediction of gas-phase F19 nuclear magnetic shielding constants
2008
Benchmark calculations of (19)F nuclear magnetic shielding constants are presented for a set of 28 molecules. Near-quantitative accuracy (ca. 2 ppm deviation from experiment) is achieved if (1) electron correlation is adequately treated by employing the coupled-cluster singles and doubles (CCSD) model augmented by a perturbative correction for triple excitations [CCSD(T)], (2) large (uncontracted) basis sets are used, (3) gauge-including atomic orbitals are used to ensure gauge-origin independence, (4) calculations are performed at accurate equilibrium geometries [obtained from CCSD(T)/cc-pVTZ calculations correlating all electrons], and (5) vibrational averaging and temperature corrections…
Perturbative treatment of triple excitations in internally contracted multireference coupled cluster theory.
2012
Internally contracted multireference coupled cluster (ic-MRCC) methods with perturbative treatment of triple excitations are formulated based on Dyall's definition of a zeroth-order Hamiltonian. The iterative models ic-MRCCSDT-1, ic-MRCC3, and their variants ic-MRCCSD(T), ic-MRCC(3) which determine the energy correction from triples by a non-iterative step are consistent in the single-reference limit with CCSDT-1a, CC3, CCSD(T), and CC(3), respectively. Numerical tests on the potential energy surfaces of BeH(2), H(2)O, and N(2) as well as on the structure and harmonic vibrational frequencies of the ozone molecule show that these methods account very well for higher order correlation effects…
A two-scale approach to electron correlation in multiconfigurational perturbation theory.
2014
We present a new approach for the calculation of dynamic electron correlation effects in large molecular systems using multiconfigurational second-order perturbation theory (CASPT2). The method is restricted to cases where partitioning of the molecular system into an active site and an environment is meaningful. Only dynamic correlation effects derived from orbitals extending over the active site are included at the CASPT2 level of theory, whereas the correlation effects of the environment are retrieved at lower computational costs. For sufficiently large systems, the small errors introduced by this approximation are contrasted by the substantial savings in both storage and computational de…
The extended CC2 model ECC2
2013
We present a size-extensive extension to the CC2 model that avoids the complications with quasi-degeneracies that are present in the CC2 model and related perturbation theory-based approaches. The formulation also provides a consistent model for treating different parts of a molecular system at different levels of electron correlation. Such a subsystem approach leads to large reductions in the computational requirements without compromising the accuracy. In this initial study, we focus on static molecular properties. © 2013 Copyright Taylor and Francis Group, LLC.
High order normal form construction near the elliptic orbit of the Sitnikov problem
2011
We consider the Sitnikov problem; from the equations of motion we derive the approximate Hamiltonian flow. Then, we introduce suitable action–angle variables in order to construct a high order normal form of the Hamiltonian. We introduce Birkhoff Cartesian coordinates near the elliptic orbit and we analyze the behavior of the remainder of the normal form. Finally, we derive a kind of local stability estimate in the vicinity of the periodic orbit for exponentially long times using the normal form up to 40th order in Cartesian coordinates.
Nonlinear analysis of cylindrical and conical hysteretic whirl motions in rotor-dynamics
2014
Abstract The internal friction of a rotor–shaft-support system is mainly due to the shaft structural hysteresis and to some possible shrink-fit release of the assembly. The experimentation points out the destabilizing effect of the internal friction in the over-critical rotor running. Nevertheless, this detrimental influence may be efficiently counterbalanced by other external dissipative sources located in the supports or by a proper anisotropic configuration of the support stiffness. The present analysis considers a rotor–shaft system which is symmetric with respect to the mid-span and is constrained by viscous-flexible supports with different stiffness on two orthogonal planes. The cylin…
An enormous sulfur isotope excursion indicates marine anoxia during the end-Triassic mass extinction
2020
The role of ocean anoxia as a cause of the end-Triassic marine mass extinction is widely debated. Here, we present carbonate-associated sulfate δ34S data from sections spanning the Late Triassic–Early Jurassic transition, which document synchronous large positive excursions on a global scale occurring in ~50 thousand years. Biogeochemical modeling demonstrates that this S isotope perturbation is best explained by a fivefold increase in global pyrite burial, consistent with large-scale development of marine anoxia on the Panthalassa margin and northwest European shelf. This pyrite burial event coincides with the loss of Triassic taxa seen in the studied sections. Modeling results also indica…