37,760 research outputs found
Majorana and the investigation of infrared spectra of ammonia
An account is given on the first studies on the physics of ammonia, focusing
on the infrared spectra of that molecule. Relevant contributions from several
authors, in the years until 1932, are pointed out, discussing also an unknown
study by E.Majorana on this topic.Comment: 13 page
Anharmonic Vibrational Eigenfunctions and Infrared Spectra from Semiclassical Molecular Dynamics
We describe a new approach based on semiclassical molecular dynamics that
allows to simulate infrared absorption or emission spectra of molecular systems
with inclusion of anharmonic intensities. This is achieved from semiclassical
power spectra by computing first the vibrational eigenfunctions as a linear
combination of harmonic states, and then the oscillator strengths associated to
the vibrational transitions. We test the approach against a 1D Morse potential
and apply it to the water molecule with results in excellent agreement with
discrete variable representation quantum benchmarks. The method does not
require any grid calculations and it is directly extendable to high dimensional
systems. The usual exponential scaling of the basis set size with the
dimensionality of the system can be avoided by means of an appropriate
truncation scheme. Furthermore, the approach has the advantage to provide IR
spectra beyond the harmonic approximation without losing the possibility of an
intuitive assignment of absorption peaks in terms of normal modes of vibration
Vibrational signature of a single water molecule adsorbed on Pt(111): toward a reliable anharmonic description
In this study, we present a thorough benchmarking of our direct anharmonic vibrational variation-perturbation approach for adsorbed molecules on surfaces. We then use our method to describe the vibrational structure of a water molecule adsorbed on a Pt(111) surface and compare our results with the available experimental data. By using an explicitly correlated hybrid method to describe the molecule-surface interaction, we improve on the initial periodic PBE/DZP potential energy landscape and obtain vibrational frequencies that are of near-experimental accuracy. We introduce an implementation of anharmonic z-polarized IR intensity calculation and explain the absence of antisymmetric O-H stretch in the experimental data for the adsorbed water molecule, while the symmetric O-H stretch is predicted to be visible
Electromagnetic selection rules in the triangular alpha-cluster model of 12C
After recapitulating the procedure to find the bands and the states occurring
in the alpha-cluster model of C in which the clusters
are placed at the vertexes of an equilateral triangle, we obtain the selection
rules for electromagnetic transitions. While the alpha cluster structure leads
to the cancellation of E1 transitions, the approximations carried out in
deriving the roto-vibrational hamiltonian lead to the disappearance of M1
transitions. Furthermore, although in general the lowest active modes are E2,
E3, and M2, M3, , the cancellation of M2, M3 and M5
transitions between certain bands also occurs, as a result of the application
of group theoretical techniques drawn from molecular physics. These
implications can be very relevant for the spectroscopic analysis of
-ray spectra of C
Simulation of quantum zero-point effects in water using a frequency-dependent thermostat
Molecules like water have vibrational modes with a zero-point energy well
above room temperature. As a consequence, classical molecular dynamics
simulations of their liquids largely underestimate the energy of modes with a
higher zero-point temperature, which translates into an underestimation of
covalent interatomic distances due to anharmonic effects. Zero-point effects
can be recovered using path integral molecular dynamics simulations, but these
are computationally expensive, making their combination with ab initio
molecular dynamics simulations a challenge. As an alternative to path integral
methods, from a computationally simple perspective, one would envision the
design of a thermostat capable of equilibrating and maintaining the different
vibrational modes at their corresponding zero-point temperatures. Recently,
Ceriotti et al. (Phys. Rev. Lett. 102 020601 (2009)) introduced a framework to
use a custom-tailored Langevin equation with correlated noise that can be used
to include quantum fluctuations in classical molecular dynamics simulations.
Here we show that it is possible to use the generalized Langevin equation with
suppressed noise in combination with Nose-Hoover thermostats to efficiently
impose a zero-point temperature on independent modes in liquid water. Using our
simple and inexpensive method, we achieve excellent agreement for all atomic
pair correlation functions compared to the path integral molecular dynamics
simulation.Comment: 27 pages, 12 figs, Published versio
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