4,618 research outputs found
Two Single-Reference Approaches to Singlet Biradicaloid Problems: Complex, Restricted Orbitals and Approximate Spin-Projection Combined With Regularized Orbital-Optimized M{\o}ller-Plesset Perturbation Theory
We present a comprehensive study of two single-reference approaches to
singlet biradicaloids. These two approaches are based on the recently developed
regularized orbital-optimized M{\o}ller-Plesset method (-OOMP2). The
first approach is to combine the Yamaguchi's approximate projection (AP) scheme
and -OOMP2 with unrestricted (U) orbitals (-UOOMP2). By
capturing only essential symmetry breaking, -UOOMP2 can serve as a
suitable basis for AP. The second approach is -OOMP2 with complex,
restricted (cR) orbitals (-cROOMP2). Though its applicability is more
limited due to the comparative rarity of cR solutions, -cROOMP2 offers
a simple framework for describing singlet biradicaloids with complex
polarization while removing artificial spatial symmetry breaking. We compare
the scope of these two methods with numerical studies. We show that
AP+-UOOMP2 and -cROOMP2 can perform similarly well in the TS12
set, a data set that includes 12 data points for triplet-singlet gaps of
several atoms and diatomic molecules with a triplet ground state. This was also
found to be true for the barrier height of a reaction involving attack on a
cysteine ion by a singlet oxygen molecule. However, we also demonstrate that in
highly symmetric systems like ()
-cROOMP2 is more suitable as it conserves spatial symmetry. Lastly, we
present an organic biradicaloid that does not have a -cROOMP2 solution
in which case only AP+-UOOMP2 is applicable. We recommend
-cROOMP2 whenever complex polarization is essential and
AP+-UOOMP2 for biradicaloids without essential complex polarization but
with essential spin-polarization
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
Water is not a Dynamic Polydisperse Branched Polymer
The contributed paper by Naserifar and Goddard reports that their RexPoN
water model under ambient conditions simulates liquid water as a dynamic
polydisperse branched polymer, which they speculate explains the existence of
the liquid-liquid critical point (LLCP) in the supercooled region. Our work
addresses several serious factual errors and needless speculation in their
paper about their interpretation of their model and its implication for the
LLCP in supercooled water.Comment: Lette
Development of an Advanced Force Field for Water using Variational Energy Decomposition Analysis
Given the piecewise approach to modeling intermolecular interactions for
force fields, they can be difficult to parameterize since they are fit to data
like total energies that only indirectly connect to their separable functional
forms. Furthermore, by neglecting certain types of molecular interactions such
as charge penetration and charge transfer, most classical force fields must
rely on, but do not always demonstrate, how cancellation of errors occurs among
the remaining molecular interactions accounted for such as exchange repulsion,
electrostatics, and polarization. In this work we present the first generation
of the (many-body) MB-UCB force field that explicitly accounts for the
decomposed molecular interactions commensurate with a variational energy
decomposition analysis, including charge transfer, with force field design
choices that reduce the computational expense of the MB-UCB potential while
remaining accurate. We optimize parameters using only single water molecule and
water cluster data up through pentamers, with no fitting to condensed phase
data, and we demonstrate that high accuracy is maintained when the force field
is subsequently validated against conformational energies of larger water
cluster data sets, radial distribution functions of the liquid phase, and the
temperature dependence of thermodynamic and transport water properties. We
conclude that MB-UCB is comparable in performance to MB-Pol, but is less
expensive and more transferable by eliminating the need to represent
short-ranged interactions through large parameter fits to high order
polynomials
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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
Hydration Water Dynamics and Instigation of Protein Structural Relaxation
The molecular mechanism of the solvent motion that is required to instigate
the protein structural relaxation above a critical hydration level or
transition temperature has yet to be determined. In this work we use
quasi-elastic neutron scattering (QENS) and molecular dynamics simulation to
investigate hydration water dynamics near a greatly simplified protein surface.
We consider the hydration water dynamics near the completely deuterated
N-acetyl-leucine-methylamide (NALMA) solute, a hydrophobic amino acid side
chain attached to a polar blocked polypeptide backbone, as a function of
concentration between 0.5M-2.0M, under ambient conditions. In this
Communication, we focus our results of hydration dynamics near a model protein
surface on the issue of how enzymatic activity is restored once a critical
hydration level is reached, and provide a hypothesis for the molecular
mechanism of the solvent motion that is required to trigger protein structural
relaxation when above the hydration transition.Comment: 2 pages, 2 figures, Communicatio
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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