1,369 research outputs found
Photoionization of few electron systems with a hybrid Coupled Channels approach
We present the hybrid anti-symmetrized coupled channels method for the
calculation of fully differential photo-electron spectra of multi-electron
atoms and small molecules interacting with strong laser fields. The method
unites quantum chemical few-body electronic structure with strong-field
dynamics by solving the time dependent Schr\"odinger equation in a fully
anti-symmetrized basis composed of multi-electron states from quantum chemistry
and a one-electron numerical basis. Photoelectron spectra are obtained via the
time dependent surface flux (tSURFF) method. Performance and accuracy of the
approach are demonstrated for spectra from the helium and berryllium atoms and
the hydrogen molecule in linearly polarized laser fields at wavelength from 21
nm to 400 nm. At long wavelengths, helium and the hydrogen molecule at
equilibrium inter-nuclear distance can be approximated as single channel
systems whereas beryllium needs a multi-channel description
Configuration Interaction calculations of positron binding to Be(3Po)
The Configuration Interaction method is applied to investigate the
possibility of positron binding to the metastable beryllium (1s^22s2p 3Po)
state. The largest calculation obtained an estimated energy that was unstable
by 0.00014 Hartree with respect to the Ps + Be^+(2s) lowest dissociation
channel. It is likely that positron binding to parent states with non-zero
angular momentum is inhibited by centrifugal barriers.Comment: 12 pages, 2 figures, Elsevier tex format, In press
Nucl.Instrum.Meth.Phys.Res.B positron issu
1D states of the beryllium atom: Quantum mechanical nonrelativistic calculations employing explicitly correlated Gaussian functions
Very accurate finite-nuclear-mass variational nonrelativistic calculations are performed for the lowest five
1D states (1s2 2p2, 1s2 2s1 3d1, 1s2 2s1 4d1, 1s2 2s1 5d1, and 1s2 2s1 6d1) of the beryllium atom (9Be). The
wave functions of the states are expanded in terms of all-electron explicitly correlated Gaussian functions. The
exponential parameters of the Gaussians are optimized using the variational method with the aid of the analytical
energy gradient determined with respect to those parameters. The calculations exemplify the level of accuracy
that is now possible with Gaussians in describing bound states of a four-electron system where some of the
electrons are excited into higher angular state
Prediction of 1P Rydberg energy levels of beryllium based on calculations with explicitly correlated Gaussians
Benchmark variational calculations are performed for the seven lowest 1s22s np (1P), n = 2. . . 8, states of the beryllium atom. The calculations explicitly include the effect of finite mass of 9Be nucleus and account perturbatively for the mass-velocity, Darwin, and spin-spin relativistic corrections. The wave functions of the states are expanded in terms of all-electron explicitly correlated Gaussian functions. Basis sets of up to 12 500 optimized Gaussians are used. The maximum discrepancy between the calculated nonrelativistic and experimental energies of 1s22s np(1P)→1s22s2 (1S) transition
is about 12 cm−1. The inclusion of the relativistic corrections reduces the discrepancy to bellow 0.8 cm−
Assessment of the accuracy the experimental energies of the 1Po 1s22s6p and 1s22s7p states of 9Be based on variational calculations with explicitly correlated Gaussians
Benchmark variational calculations are performed for the six lowest states of the 1Po 1s22snp state series of the 9Be atom. The wave functions of the states are expanded in terms of all-particle, explicitly correlated Gaussian basis functions and the effect of the finite nuclear mass is directly included in the calculations. The exponential parameters of the Gaussians are variationally optimized using the analytical energy gradient determined with respect to those parameters. Besides providing reference non-relativistic energies for the considered states, the calculations also allow to assess the accuracy of the experimental energies of the 1Po 1s22s6p and 1s22s7p states and suggest their refinemen
The New Resonating Valence Bond Method for Ab-Initio Electronic Simulations
The Resonating Valence Bond theory of the chemical bond was introduced soon
after the discovery of quantum mechanics and has contributed to explain the
role of electron correlation within a particularly simple and intuitive
approach where the chemical bond between two nearby atoms is described by one
or more singlet electron pairs. In this chapter Pauling's resonating valence
bond theory of the chemical bond is revisited within a new formulation,
introduced by P.W. Anderson after the discovery of High-Tc superconductivity.
It is shown that this intuitive picture of electron correlation becomes now
practical and efficient, since it allows us to faithfully exploit the locality
of the electron correlation, and to describe several new phases of matter, such
as Mott insulators, High-Tc superconductors, and spin liquid phases
Assessment of the accuracy the experimental energies of the 1Po 1s22s6p and 1s22s7p states of 9Be based on variational calculations with explicitly correlated Gaussians
Benchmark variational calculations are performed for the six lowest states of the 1Po 1s22snp state series of the 9Be atom. The wave functions of the states are expanded in terms of all-particle, explicitly correlated Gaussian basis functions and the effect of the finite nuclear mass is directly included in the calculations. The exponential parameters of the Gaussians are variationally optimized using the analytical energy gradient determined with respect to those parameters. Besides providing reference non-relativistic energies for the considered states, the calculations also allow to assess the accuracy of the experimental energies of the 1Po 1s22s6p and 1s22s7p states and suggest their refinemen
Spectroscopic accuracy directly from quantum chemistry: application to ground and excited states of beryllium dimer
We combine explicit correlation via the canonical transcorrelation approach
with the density matrix renormalization group and initiator full configuration
interaction quantum Monte Carlo methods to compute a near-exact beryllium dimer
curve, {\it without} the use of composite methods. In particular, our direct
density matrix renormalization group calculations produce a well-depth of
=931.2 cm which agrees very well with recent experimentally derived
estimates =929.7~cm [Science, 324, 1548 (2009)] and
=934.6~cm [Science, 326, 1382 (2009)]], as well the best composite
theoretical estimates, =938~cm [J. Phys. Chem. A, 111,
12822 (2007)] and =935.1~cm [Phys. Chem. Chem. Phys., 13,
20311 (2011)]. Our results suggest possible inaccuracies in the functional form
of the potential used at shorter bond lengths to fit the experimental data
[Science, 324, 1548 (2009)]. With the density matrix renormalization group we
also compute near-exact vertical excitation energies at the equilibrium
geometry. These provide non-trivial benchmarks for quantum chemical methods for
excited states, and illustrate the surprisingly large error that remains for
1 state with approximate multi-reference configuration
interaction and equation-of-motion coupled cluster methods. Overall, we
demonstrate that explicitly correlated density matrix renormalization group and
initiator full configuration interaction quantum Monte Carlo methods allow us
to fully converge to the basis set and correlation limit of the
non-relativistic Schr\"odinger equation in small molecules
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