2,320 research outputs found
How accurate is density functional theory at predicting dipole moments? An assessment using a new database of 200 benchmark values
Dipole moments are a simple, global measure of the accuracy of the electron
density of a polar molecule. Dipole moments also affect the interactions of a
molecule with other molecules as well as electric fields. To directly assess
the accuracy of modern density functionals for calculating dipole moments, we
have developed a database of 200 benchmark dipole moments, using coupled
cluster theory through triple excitations, extrapolated to the complete basis
set limit. This new database is used to assess the performance of 88 popular or
recently developed density functionals. The results suggest that double hybrid
functionals perform the best, yielding dipole moments within about 3.6-4.5%
regularized RMS error versus the reference values---which is not very different
from the 4% regularized RMS error produced by coupled cluster singles and
doubles. Many hybrid functionals also perform quite well, generating
regularized RMS errors in the 5-6% range. Some functionals however exhibit
large outliers and local functionals in general perform less well than hybrids
or double hybrids.Comment: Added several double hybrid functionals, most of which turned out to
be better than any functional from Rungs 1-4 of Jacob's ladder and are
actually competitive with CCS
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Unraveling substituent effects on frontier orbitals of conjugated molecules using an absolutely localized molecular orbital based analysis.
It is common to introduce electron-donating or electron-withdrawing substituent groups into functional conjugated molecules (such as dyes) to tune their electronic structure properties (such as frontier orbital energy levels) and photophysical properties (such as absorption and emission wavelengths). However, there lacks a generally applicable tool that can unravel the underlying interactions between orbitals from a substrate molecule and those from its substituents in modern electronic structure calculations, despite the long history of qualitative molecular orbital theory. In this work, the absolutely localized molecular orbitals (ALMO) based analysis is extended to analyze the effects of substituent groups on the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of a given system. This provides a bottom-up avenue towards quantification of effects from distinct physical origins (e.g. permanent electrostatics/Pauli repulsion, mutual polarization, inter-fragment orbital mixing). For the example case of prodan (a typical dye molecule), it is found that inter-fragment orbital mixing plays a key role in narrowing the HOMO-LUMO gap of the naphthalene core. Specifically, an out-of-phase mixing of high-lying occupied orbitals on the naphthalene core and the dimethylamino group leads to an elevated HOMO, whereas an in-phase combination of LUMOs on the naphthalene core and the propionyl group lowers the LUMO energy of the entire molecule. We expect this ALMO-based analysis to bridge the gap between concepts from qualitative orbital interaction analysis and quantitative electronic structure calculations
Orbital optimization in the perfect pairing hierarchy. Applications to full-valence calculations on linear polyacenes
We describe the implementation of orbital optimization for the models in the
perfect pairing hierarchy [Lehtola et al, J. Chem. Phys. 145, 134110 (2016)].
Orbital optimization, which is generally necessary to obtain reliable results,
is pursued at perfect pairing (PP) and perfect quadruples (PQ) levels of theory
for applications on linear polyacenes, which are believed to exhibit strong
correlation in the {\pi} space. While local minima and {\sigma}-{\pi} symmetry
breaking solutions were found for PP orbitals, no such problems were
encountered for PQ orbitals. The PQ orbitals are used for single-point
calculations at PP, PQ and perfect hextuples (PH) levels of theory, both only
in the {\pi} subspace, as well as in the full {\sigma}{\pi} valence space. It
is numerically demonstrated that the inclusion of single excitations is
necessary also when optimized orbitals are used. PH is found to yield good
agreement with previously published density matrix renormalization group (DMRG)
data in the {\pi} space, capturing over 95% of the correlation energy.
Full-valence calculations made possible by our novel, efficient code reveal
that strong correlations are weaker when larger bases or active spaces are
employed than in previous calculations. The largest full-valence PH
calculations presented correspond to a (192e,192o) problem.Comment: 19 pages, 4 figure
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Compressed intramolecular dispersion interactions.
The feasibility of the compression of localized virtual orbitals is explored in the context of intramolecular long-range dispersion interactions. Singular value decomposition (SVD) of coupled cluster doubles amplitudes associated with the dispersion interactions is analyzed for a number of long-chain systems, including saturated and unsaturated hydrocarbons and a silane chain. Further decomposition of the most important amplitudes obtained from these SVDs allows for the analysis of the dispersion-specific virtual orbitals that are naturally localized. Consistent with previous work on intermolecular dispersion interactions in dimers, it is found that three important geminals arise and account for the majority of dispersion interactions at the long range, even in the many body intramolecular case. Furthermore, it is shown that as few as three localized virtual orbitals per occupied orbital can be enough to capture all pairwise long-range dispersion interactions within a molecule
Delocalization errors in density functional theory are essentially quadratic in fractional occupation number
Approximate functionals used in practical density functional theory (DFT)
deviate from the piecewise linear behavior of the exact functional for
fractional charges. This deviation causes excess charge delocalization, which
leads to incorrect densities, molecular properties, barrier heights, band gaps
and excitation energies. We present a simple delocalization function for
characterizing this error and find it to be almost perfectly linear vs the
fractional electron number for systems spanning in size from the H atom to the
CH polyene. This causes the delocalization energy error to be a
quadratic polynomial in the fractional electron number, which permits us to
assess the comparative performance of 47 popular and recent functionals through
the curvature. The quadratic form further suggests that information about a
single fractional charge is sufficient to eliminate the principal source of
delocalization error. Generalizing traditional two-point information like
ionization potentials or electron affinities to account for a third, fractional
charge based data point could therefore permit fitting/tuning of functionals
with lower delocalization error.Comment: Discussion about fractional binding issues in anions have been added,
with other minor fixes/elaboration
The Doubles Connected Moments Expansion: A Tractable Approximate Horn-Weinstein Approach for Quantum Chemistry
Ab initio methods based on the second-order and higher connected moments, or
cumulants, of a reference function have seen limited use in the determination
of correlation energies of chemical systems throughout the years. Moment-based
methods have remained unattractive relative to more ubiquitous methods, such as
perturbation theory and coupled cluster theory, due in part to the intractable
cost of assembling moments of high-order and poor performance of low-order
expansions. Many of the traditional quantum chemical methodologies can be
recast as a selective summation of perturbative contributions to their energy;
using this familiar structure as a guide in selecting terms, we develop a
scheme to approximate connected moments limited to double excitations. The
tractable Double Connected Moments (DCM(N)) approximation is developed and
tested against a multitude of common single-reference methods to determine its
efficacy in the determination of the correlation energy of model systems and
small molecules. The DCM(N) sequence of energies exhibits smooth convergence,
with compute costs that scale as a non-iterative O(N^6) with molecule size, M.
Numerical tests on correlation energy recovery for 55 small molecules
comprising the G1 test set in the cc-pVDZ basis show that DCM(N) strongly
outperforms MP2 and even CCD with a Hartree-Fock reference. When using an
approximate Brueckner reference from orbital-optimized (oo) MP2, the resulting
oo:DCM(N) energies converge to values more accurate than CCSD for 49 of 55
molecules
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