5,096 research outputs found
Ab initio study of mixed clusters of water and N,N′-dimethylethyleneurea
Intermolecular interactions between a single water and two N,N′-dimethylethyleneurea (DMEU) molecules have been investigated using local and density-fitting approximations of the standard Moller-Plesset perturbation theory (DF-LMP2) with the aug-ccpVTZ basis set. Six stable configurations have been found. In the first three, the water molecule intercalates between two DMEU molecules. In the next three configurations, the water molecule is attached to a stacked DMEU dimer, and these structures are more stable than the first three. These results support the view that DMEU molecules can form contact pairs in dilute aqueous solutions. © 2011
Group theoretical analysis of symmetry breaking in two-dimensional quantum dots
We present a group theoretical study of the symmetry-broken unrestricted
Hartree-Fock orbitals and electron densities in the case of a two-dimensional
N-electron single quantum dot (with and without an external magnetic field).
The breaking of rotational symmetry results in canonical orbitals that (1) are
associated with the eigenvectors of a Hueckel hamiltonian having sites at the
positions determined by the equilibrium molecular configuration of the
classical N-electron problem, and (2) transform according to the irreducible
representations of the point group specified by the discrete symmetries of this
classical molecular configuration. Through restoration of the total-spin and
rotational symmetries via projection techniques, we show that the point-group
discrete symmetry of the unrestricted Hartree-Fock wave function underlies the
appearance of magic angular momenta (familiar from exact-diagonalization
studies) in the excitation spectra of the quantum dot. Furthermore, this
two-step symmetry-breaking/symmetry-restoration method accurately describes the
energy spectra associated with the magic angular momenta.Comment: A section VI.B entitled "Quantitative description of the lowest
rotational band" has been added. 16 pages. Revtex with 10 EPS figures. A
version of the manuscript with high quality figures is available at
http://calcite.physics.gatech.edu/~costas/uhf_group.html For related papers,
see http://www.prism.gatech.edu/~ph274c
Triplet-Tuning: A Novel Family of Non-Empirical Exchange-Correlation Functionals
In the framework of DFT, the lowest triplet excited state, T, can be
evaluated using multiple formulations, the most straightforward of which are
UDFT and TDDFT. Assuming the exact XC functional is applied, UDFT and TDDFT
provide identical energies for T (), which is also a constraint
that we require our XC functionals to obey. However, this condition is not
satisfied by most of the popular XC functionals, leading to inaccurate
predictions of low-lying, spectroscopically and photochemically important
excited states, such as T and S. Inspired by the optimal tuning
strategy for frontier orbital energies [Stein, Kronik, and Baer, {\it J. Am.
Chem. Soc.} {\bf 2009}, 131, 2818], we proposed a novel and non-empirical
prescription of constructing an XC functional in which the agreement between
UDFT and TDDFT in is strictly enforced. Referred to as "triplet
tuning", our procedure allows us to formulate the XC functional on a
case-by-case basis using the molecular structure as the exclusive input,
without fitting to any experimental data. The first triplet tuned XC
functional, TT-PBEh, is formulated as a long-range-corrected hybrid of
PBE and HF functionals [Rohrdanz, Martins, and Herbert, {\it J. Chem. Phys.}
{\bf 2009}, 130, 054112] and tested on four sets of large organic molecules.
Compared to existing functionals, TT-PBEh manages to provide more
accurate predictions for key spectroscopic and photochemical observables,
including but not limited to , , , and
, as it adjusts the effective electron-hole interactions to arrive at the
correct excitation energies. This promising triplet tuning scheme can be
applied to a broad range of systems that were notorious in DFT for being
extremely challenging
On the accuracy of DFT exchange-correlation functionals for H bonds in small water clusters II: The water hexamer and van der Waals interactions
Second order Moeller-Plesset perturbation theory (MP2) at the complete basis
set (CBS) limit and diffusion quantum Monte Carlo (DMC) are used to examine
several low energy isomers of the water hexamer. Both approaches predict the
so-called "prism" to be the lowest energy isomer, followed by "cage", "book",
and "cyclic" isomers. The energies of the four isomers are very similar, all
being within 10-15 meV/H2O. This reference data is then used to evaluate the
performance of several density-functional theory (DFT) exchange-correlation
(xc) functionals. A subset of the xc functionals tested for smaller water
clusters [I: Santra et al., J. Chem. Phys. 127, 184104 (2007)] has been
considered. Whilst certain functionals do a reasonable job at predicting the
absolute dissociation energies of the various isomers (coming within 10-20
meV/H2O), none predict the correct energetic ordering of the four isomers, nor
does any predict the correct low total energy isomer. All xc functionals tested
either predict the book or cyclic isomers to have the largest dissociation
energies. A many-body decomposition of the total interaction energies within
the hexamers leads to the conclusion that the failure lies in the poor
description of van der Waals (dispersion) forces in the xc functionals
considered. It is shown that the addition of an empirical pairwise (attractive)
C6/R6 correction to certain functionals allows for an improved energetic
ordering of the hexamers. The relevance of these results to density-functional
simulations of liquid water is also briefly discussed.Comment: 15 pages including 4 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
Calculation of the static and dynamical correlation energy of pseudo-one-dimensional beryllium systems via a many-body expansion
Low-dimensional beryllium systems constitute interesting case studies for the
test of correlation methods because of the importance of both static and
dynamical correlation in the formation of the bond. Aiming to describe the
whole dissociation curve of extended Be systems we chose to apply the method of
increments (MoI) in its multireference (MR) formalism. However, in order to do
so an insight into the wave function was necessary. Therefore we started by
focusing on the description of small Be chains via standard quantum chemical
methods and gave a brief analysis of the main characteristics of their wave
functions. We then applied the MoI to larger beryllium systems, starting from
the Be6 ring. First, the complete active space formalism (CAS-MoI) was employed
and the results were used as reference for local MR calculations of the whole
dissociation curve. Despite this approach is well established for the
calculation of systems with limited multireference character, its application
to the description of whole dissociation curves still requires further testing.
After discussing the role of the basis set, the method was finally applied to
larger rings and extrapolated to an infinite chain
An ab initio path integral Monte Carlo simulation method for molecules and clusters: application to Li_4 and Li_5^+
A novel method for simulating the statistical mechanics of molecular systems
in which both nuclear and electronic degrees of freedom are treated quantum
mechanically is presented. The scheme combines a path integral description of
the nuclear variables with a first-principles adiabatic description of the
electronic structure. The electronic problem is solved for the ground state
within a density functional approach, with the electronic orbitals expanded in
a localized (Gaussian) basis set. The discretized path integral is computed by
a Metropolis Monte Carlo sampling technique on the normal modes of the
isomorphic ring-polymer. An effective short-time action correct to order
is used. The validity and performance of the method are tested in two
small Lithium clusters, namely Li and Li. Structural and electronic
properties computed within this fully quantum-mechanical scheme are presented
and compared to those obtained within the classical nuclei approximation.
Quantum delocalization effects are significant but tunneling turns out to be
irrelevant at low temperatures.Comment: 11 text pages, 7 figures, to be published in J. Chem. Phy
Electronic structure of conducting organic polymers: insights from time-dependent density functional theory
Cataloged from PDF version of article.Conducting organic polymers (COPs) became an active field of research after it was discovered how thin films rather than insoluble infusible powders can be produced. The combination of the properties of plastics with those of semiconductors opened the research field of organic electronics. COPs share many electronic properties with inorganic semiconductors, but there are also major differences, e. g., the nature of the charge carriers and the amount of the exciton binding energy. Theoretical analysis has been used to interpret experimental observations early on. The polaron model that was developed from one-electron theories is still the most widely used concept. In the 1990s, time-dependent density functional theory (TDDFT) became available for routine calculations. Using TDDFT, electronic states of long oligomers can be calculated. Now UV spectra of neutral and oxidized or reduced species can be compared with in situ UV spectra recorded during doping. Likewise states of cations can be used to model photoelectron spectra. Analysis of states has resolved several puzzles which cannot be understood with the polaron model, e. g., the origin of the dual absorption band of green polymers and the origin of a 'vestigial neutral band' upon doping of long oligomers. DFT calculations also established that defect localization is not crucial for spectral changes observed during doping and that there are no bound bipolarons in COPs. (C) 2014 John Wiley & Sons, Ltd
Dispersion, rehybridization, and pentacoordination: keys to understand clustering of boron and aluminum hydrides and halides
The structure, stability, and bonding characteristics of dimers and trimers involving BX3 and AlX3 (X = H, F, Cl) in the gas phase, many of them explored for the first time, were investigated using different DFT (B3LYP, B3LYP/D3BJ, and M06-2X) and ab initio (MP2 and G4) methods together with different energy decomposition formalisms, namely, many-body interaction-energy and localized molecular orbital energy decomposition analysis. The electron density of the clusters investigated was analyzed with QTAIM, electron localization function, NCIPLOT, and adaptive natural density partitioning approaches. Our results for triel hydride dimers and Al2X6 (X = F, Cl) clusters are in good agreement with previous studies in the literature, but in contrast with the general accepted idea that B2F6 and B2Cl6 do not exist, we have found that they are predicted to be weakly bound systems if dispersion interactions are conveniently accounted for in the theoretical schemes used. Dispersion interactions are also dominant in both homo- and heterotrimers involving boron halide monomers. Surprisingly, B3F9 and B3Cl9 C3v cyclic trimers, in spite of exhibiting rather strong B-X (X = F, Cl) interactions, were found to be unstable with respect to the isolated monomers due to the high energetic cost of the rehybridization of the B atom, which is larger than the two- and three-body stabilization contributions when the cyclic is formed. Another important feature is the enhanced stability of both homo- and heterotrimers in which Al is the central atom because Al is systematically pentacoordinated, whereas this is not the case when the central atom is B, which is only tri- or tetra-coordinatedPID2021-125207NB-C31, PID2021-125207NB-C32, PID2019-110091GB-I0
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