263,041 research outputs found
High accuracy calculations of the rotation-vibration spectrum of H
Calculation of the rotation-vibration spectrum of H3+, as well as of its
deuterated isotopologues, with near-spectroscopic accuracy requires the
development of sophisticated theoretical models, methods, and codes. The
present paper reviews the state-of-the-art in these fields. Computation of
rovibrational states on a given potential energy surface (PES) has now become
standard for triatomic molecules, at least up to intermediate energies, due to
developments achieved by the present authors and others. However, highly
accurate Born--Oppenheimer energies leading to highly accurate PESs are not
accessible even for this two-electron system using conventional electronic
structure procedures e.g., configuration-interaction or coupled-cluster
techniques with extrapolation to the complete basis set limit). For this
purpose highly specialized techniques must be used, e.g., those employing
explicitly correlated Gaussians and nonlinear parameter optimizations. It has
also become evident that a very dense grid of \ai\ points is required to obtain
reliable representations of the computed points extending from the minimum to
the asymptotic limits. Furthermore, adiabatic, relativistic, and QED correction
terms need to be considered to achieve near-spectroscopic accuracy during
calculation of the rotation-vibration spectrum of H3+. The remaining and most
intractable problem is then the treatment of the effects of non-adiabatic
coupling on the rovibrational energies, which, in the worst cases, may lead to
corrections on the order of several \cm. A promising way of handling this
difficulty is the further development of effective, motion- or even
coordinate-dependent, masses and mass surfaces. Finally, the unresolved
challenge of how to describe and elucidate the experimental pre-dissociation
spectra of H and its isotopologues is discussed.Comment: Topical review to be published in J Phys B: At Mol Opt Phy
Torsional anharmonicity in the conformational thermodynamics of flexible molecules
We present an algorithm for calculating the conformational thermodynamics of large, flexible molecules that combines ab initio electronic structure theory calculations with a torsional path integral Monte Carlo (TPIMC) simulation. The new algorithm overcomes the previous limitations of the TPIMC method by including the thermodynamic contributions of non-torsional vibrational modes and by affordably incorporating the ab initio calculation of conformer electronic energies, and it improves the conventional ab initio treatment of conformational thermodynamics by accounting for the anharmonicity of the torsional modes. Using previously published ab initio results and new TPIMC calculations, we apply the algorithm to the conformers of the adrenaline molecule
Multiple-Scattering Series For Color Transparency
Color transparency CT depends on the formation of a wavepacket of small
spatial extent. It is useful to interpret experimental searches for CT with a
multiple scattering scattering series based on wavepacket-nucleon scattering
instead of the standard one using nucleon-nucleon scattering. We develop
several new techniques which are valid for differing ranges of energy. These
techniques are applied to verify some early approximations; study new forms of
the wave-packet-nucleon interaction; examine effects of treating wave packets
of non-zero size; and predict the production of 's in electron scattering
experiments.Comment: 26 pages, U.Wa. preprint 40427-23-N9
Three applications of path integrals: equilibrium and kinetic isotope effects, and the temperature dependence of the rate constant of the [1,5] sigmatropic hydrogen shift in (Z)-1,3-pentadiene
Recent experiments have confirmed the importance of nuclear quantum effects
even in large biomolecules at physiological temperature. Here we describe how
the path integral formalism can be used to describe rigorously the nuclear
quantum effects on equilibrium and kinetic properties of molecules.
Specifically, we explain how path integrals can be employed to evaluate the
equilibrium (EIE) and kinetic (KIE) isotope effects, and the temperature
dependence of the rate constant. The methodology is applied to the [1,5]
sigmatropic hydrogen shift in pentadiene. Both the KIE and the temperature
dependence of the rate constant confirm the importance of tunneling and other
nuclear quantum effects as well as of the anharmonicity of the potential energy
surface. Moreover, previous results on the KIE were improved by using a
combination of a high level electronic structure calculation within the
harmonic approximation with a path integral anharmonicity correction using a
lower level method.Comment: 9 pages, 4 figure
An Effective Semiclassical Approach to IR Spectroscopy
We present a novel approach to calculate molecular IR spectra based on
semiclassical molecular dynamics. The main advance from a previous
semiclassical method [M. Micciarelli, R. Conte, J. Suarez, M. Ceotto J. Chem.
Phys. 149, 064115 (2018)] consists in the possibility to avoid state-to-state
calculations making applications to systems characterized by sizable densities
of vibrational states feasible. Furthermore, this new method accounts not only
for positions and intensities of the several absorption bands which make up the
IR spectrum, but also for their shapes. We show that accurate semiclassical IR
spectra including quantum effects and anharmonicities for both frequencies and
intensities can be obtained starting from semiclassical power spectra. The
approach is first tested against the water molecule, and then applied to the
10-atom glycine aminoacid
"Divide and Conquer" Semiclassical Molecular Dynamics: A practical method for Spectroscopic calculations of High Dimensional Molecular Systems
We extensively describe our recently established "divide-and-conquer"
semiclassical method [M. Ceotto, G. Di Liberto and R. Conte, Phys. Rev. Lett.
119, 010401 (2017)] and propose a new implementation of it to increase the
accuracy of results. The technique permits to perform spectroscopic
calculations of high dimensional systems by dividing the full-dimensional
problem into a set of smaller dimensional ones. The partition procedure,
originally based on a dynamical analysis of the Hessian matrix, is here more
rigorously achieved through a hierarchical subspace-separation criterion based
on Liouville's theorem. Comparisons of calculated vibrational frequencies to
exact quantum ones for a set of molecules including benzene show that the new
implementation performs better than the original one and that, on average, the
loss in accuracy with respect to full-dimensional semiclassical calculations is
reduced to only 10 wavenumbers. Furthermore, by investigating the challenging
Zundel cation, we also demonstrate that the "divide-and-conquer" approach
allows to deal with complex strongly anharmonic molecular systems. Overall the
method very much helps the assignment and physical interpretation of
experimental IR spectra by providing accurate vibrational fundamentals and
overtones decomposed into reduced dimensionality spectra
Charge Symmetry Violation in Nuclear Physics
The study of charge symmetry violation in nuclear physics is a potentially
enormous subject. Through a few topical examples we aim to show that it is not
a subject of peripheral interest but rather goes to the heart of our
understanding of hadronic systems.Comment: Invited talk at the Int. Conference on Weak and Electromagnetic
Interactions in Nuclei, Osaka, June 12-16 199
Embedded finite-element solver for computation of brushless permanent-magnet motors
This paper describes the theory underlying the formulation of a âminimum setâ of finite-element solutions to be used in the design and analysis of saturated brushless permanent-magnet motors. The choice of finite-element solutions is described in terms of key points on the fluxâMMF diagram. When the diagram has a regular shape, a huge reduction in finite-element analysis is possible with no loss of accuracy. If the loop is irregular, many more solutions are needed. This paper describes an efficient technique in which a finite-element solver is associated with a classical â -axis circuit model in such a way that the number of finite-element solutions in one electrical half-cycle can be varied between 1 and 360. The finite-element process is used to determine not only the average torque but also the saturated inductances as the rotor rotates
- âŠ