101 research outputs found
Comprehensive Benchmark of Association (Free) Energies of Realistic HostâGuest Complexes
The
S12L test set for supramolecular Gibbs free energies of association
Î<i>G</i><sub><i>a</i></sub> (Grimme, S. Chem. Eur. J. 2012, 18, 9955â9964) is extended
to 30 complexes (S30L), featuring more diverse interaction motifs,
anions, and higher charges (â1 up to +4) as well as larger
systems with up to 200 atoms. Various typical noncovalent interactions
like hydrogen and halogen bonding, ÏâÏ stacking,
nonpolar dispersion, and CHâÏ and cationâdipolar
interactions are represented by ârealâ complexes. The
experimental Gibbs free energies of association (Î<i>G</i><sub><i>a</i></sub><sup><i>exp</i></sup>) cover a wide range from â0.7 to
â24.7 kcal mol<sup>â1</sup>. In order to obtain a theoretical
best estimate for Î<i>G</i><sub><i>a</i></sub>, we test various dispersion corrected density functionals
in combination with quadruple-ζ basis sets for calculating the
association energies in the gas phase. Further, modern semiempirical
methods are employed to obtain the thermostatistical corrections from
energy to Gibbs free energy, and the COSMO-RS model with several parametrizations
as well as the SMD model are used to include solvation contributions.
We investigate the effect of including counterions for the charged
systems (S30L-CI), which is found to overall improve the results.
Our best method combination consists of PW6B95-D3 (for neutral and
charged systems) or ÏB97X-D3 (for systems with counterions)
energies, HF-3c thermostatistical corrections, and Gibbs free energies
of solvation obtained with the COSMO-RS 2012 parameters for nonpolar
solvents and 2013-fine for water. This combination gives a mean absolute
deviation for Î<i>G</i><sub><i>a</i></sub> of only 2.4 kcal mol<sup>â1</sup> (S30L) and 2.1 kcal mol<sup>â1</sup> (S30L-CI), with a mean deviation of almost zero compared
to experiment. Regarding the relative Gibbs free energies of association
for the 13 pairs of complexes which share the same host, the correct
trend in binding affinities could be reproduced except for two cases.
The MAD compared to experiment amounts to 1.2 kcal mol<sup>â1</sup>, and the MD is almost zero. The best-estimate theoretical corrections
are used to back-correct the experimental Î<i>G</i><sub><i>a</i></sub> values in order to get an empirical
estimate for the âexperimentalâ, zero-point vibrational
energy exclusive, gas phase binding energies. These are then utilized
to benchmark the performance of various âlow-costâ quantum
chemical methods for noncovalent interactions in large systems. The
performance of other common DFT methods as well as the use of semiempirical
methods for structure optimizations is discussed
DFT-D3 Study of Some Molecular Crystals
We investigate the performance of
the dispersion correction D3 with and without an explicit three-body
dispersion term for the energetic and structural properties of rare
gas and molecular crystals. Therefore, the two- and three-body gradient
of the dispersion energy is implemented in the periodic plane-wave
program VASP. It is combined with different density functionals at
the level of the general gradient approximation (GGA) and hybrid functionals.
Cohesive energies and lattice parameters for the rare gas crystals
Ar, Kr, and Xe and a set of 23 molecular crystals are calculated and
compared to experimental reference values. In general, all tested
methods yield very good results. For the molecular crystals the mean
absolute deviation of lattice energies from reference data (about
1â2 kcal/mol) is close to or below their uncertainties. The
influence of the three-body AxilrodâTellerâMuto dispersion
term on energy and structure is found to be rather small. While on
a GGA level cohesive energies become slightly worse, for hybrid functionals
the three-body term improves the results
Performance of Non-Local and Atom-Pairwise Dispersion Corrections to DFT for Structural Parameters of Molecules with Noncovalent Interactions
The nonlocal, electron density dependent dispersion correction
of Vydrov and Van Voorhis (Vydrov, O. A.; Van Voorhis, T. <i>J. Chem. Phys.</i> <b>2010</b>, <i>133</i>,
244103), termed VV10 or DFT-NL, has been implemented for structural
optimizations of molecules. It is tested in combination with the four
(hybrid)ÂGGA density functionals TPSS, TPSS0, B3LYP, and revPBE38 for <i>inter</i>- and <i>intra</i>molecular noncovalent interactions
(NCI) and compared to results from atom-pairwise dispersion corrected
DFT-D3. The methods are applied to a wide range of different problems,
namely the S22 and S66 test sets, large transition metal complexes,
water hexamer clusters, hexahelicene, and four other difficult cases
of intramolecular NCI. Critical interatomic distances are computed
remarkably accurately by both dispersion corrections compared to theoretical
or experimental reference data and inter- and intramolecular interactions
are treated on equal footing. The methods can be recommended as reliable
and robust tools for geometry optimizations of large systems in which
long-range dispersion forces are crucial
Electronic Circular Dichroism of Highly Conjugated ÏâSystems: Breakdown of the TammâDancoff/Configuration Interaction Singles Approximation
We show that the electronic circular
dichroism (ECD) of delocalized
Ï-systems represents a worst-case scenario for TammâDancoff
approximated (TDA) linear response methods. We mainly consider density
functional theory (TDA-DFT) variants together with range-separated
hybrids, but the conclusions also apply for other functionals as well
as the configuration interaction singles (CIS) approaches. We study
the effect of the TDA for the computation of ECD spectra in some prototypical
extended Ï-systems. The C<sub>76</sub> fullerene, a chiral carbon
nanotube fragment, and [11]Âhelicene serve as model systems for inherently
chiral, Ï-chromophores. Solving the full linear response problem
is inevitable in order to obtain accurate ECD spectra for these systems.
For the C<sub>76</sub> fullerene and the nanotube fragment, TDA and
CIS approximated methods yield spectra in the origin-independent velocity
gauge formalism of incorrect sign which would lead to the assignment
of the opposite (wrong) absolute configuration. As a counterexample,
we study the ECD of an α-helix polypeptide chain. Here, the
lowest-energy transitions are dominated by localized excitations within
the individual peptide units, and TDA methods perform satisfactorily.
The results may have far-reaching implications for simple semiempirical
methods which often employ TDA and CIS for huge molecules. Our recently
presented simplified time-dependent DFT approach proves to be an excellent
low-cost linear response method which together with range-separated
density functionals like ÏB97X-D3 produces ECD spectra in very
good agreement with experiment
Comprehensive Benchmark of Association (Free) Energies of Realistic HostâGuest Complexes
The
S12L test set for supramolecular Gibbs free energies of association
Î<i>G</i><sub><i>a</i></sub> (Grimme, S. Chem. Eur. J. 2012, 18, 9955â9964) is extended
to 30 complexes (S30L), featuring more diverse interaction motifs,
anions, and higher charges (â1 up to +4) as well as larger
systems with up to 200 atoms. Various typical noncovalent interactions
like hydrogen and halogen bonding, ÏâÏ stacking,
nonpolar dispersion, and CHâÏ and cationâdipolar
interactions are represented by ârealâ complexes. The
experimental Gibbs free energies of association (Î<i>G</i><sub><i>a</i></sub><sup><i>exp</i></sup>) cover a wide range from â0.7 to
â24.7 kcal mol<sup>â1</sup>. In order to obtain a theoretical
best estimate for Î<i>G</i><sub><i>a</i></sub>, we test various dispersion corrected density functionals
in combination with quadruple-ζ basis sets for calculating the
association energies in the gas phase. Further, modern semiempirical
methods are employed to obtain the thermostatistical corrections from
energy to Gibbs free energy, and the COSMO-RS model with several parametrizations
as well as the SMD model are used to include solvation contributions.
We investigate the effect of including counterions for the charged
systems (S30L-CI), which is found to overall improve the results.
Our best method combination consists of PW6B95-D3 (for neutral and
charged systems) or ÏB97X-D3 (for systems with counterions)
energies, HF-3c thermostatistical corrections, and Gibbs free energies
of solvation obtained with the COSMO-RS 2012 parameters for nonpolar
solvents and 2013-fine for water. This combination gives a mean absolute
deviation for Î<i>G</i><sub><i>a</i></sub> of only 2.4 kcal mol<sup>â1</sup> (S30L) and 2.1 kcal mol<sup>â1</sup> (S30L-CI), with a mean deviation of almost zero compared
to experiment. Regarding the relative Gibbs free energies of association
for the 13 pairs of complexes which share the same host, the correct
trend in binding affinities could be reproduced except for two cases.
The MAD compared to experiment amounts to 1.2 kcal mol<sup>â1</sup>, and the MD is almost zero. The best-estimate theoretical corrections
are used to back-correct the experimental Î<i>G</i><sub><i>a</i></sub> values in order to get an empirical
estimate for the âexperimentalâ, zero-point vibrational
energy exclusive, gas phase binding energies. These are then utilized
to benchmark the performance of various âlow-costâ quantum
chemical methods for noncovalent interactions in large systems. The
performance of other common DFT methods as well as the use of semiempirical
methods for structure optimizations is discussed
Elucidation of Electron Ionization Induced Fragmentations of Adenine by Semiempirical and Density Functional Molecular Dynamics
The
gas phase fragmentation pathways of the nucleobase adenine
upon 70 eV electron ionization are investigated by means of a combined
stochastic and first-principles based molecular dynamics approach.
We employ no preconceived fragmentation channels in our calculations,
which simulate standard electron ionization mass spectrometry (EI-MS)
conditions. The reactions observed compare well to a wealth of experimental
and theoretical data available for this important nucleic acid building
block. All significant peaks in the experimental mass spectrum of
adenine are reproduced. Additionally, the fragment ion connectivities
obtained from our simulations at least partially concur with results
from previous experimental studies on selectively isotope labeled
adenines. Moreover, we are able to assign noncyclic structures that
are entropically favored and have not been proposed in nondynamic
quantum chemical studies before to the decomposition products, which
result automatically from our molecular dynamics procedure. From simulations
under various conditions it is evident that most of the fragmentation
reactions even at low internal excess energy (<10 eV) occur very
fast within a few picoseconds
Accurate Modeling of Organic Molecular Crystals by Dispersion-Corrected Density Functional Tight Binding (DFTB)
The ambitious goal of organic crystal
structure prediction challenges
theoretical methods regarding their accuracy and efficiency. Dispersion-corrected
density functional theory (DFT-D) in principle is applicable, but
the computational demands, for example, to compute a huge number of
polymorphs, are too high. Here, we demonstrate that this task can
be carried out by a dispersion-corrected density functional tight
binding (DFTB) method. The semiempirical Hamiltonian with the D3 correction
can accurately and efficiently model both solid- and gas-phase inter-
and intramolecular interactions at a speed up of 2 orders of magnitude
compared to DFT-D. The mean absolute deviations for interaction (lattice)
energies for various databases are typically 2â3 kcal/mol (10â20%),
that is, only about two times larger than those for DFT-D. For zero-point
phonon energies, small deviations of <0.5 kcal/mol compared to
DFT-D are obtained
Why the Standard B3LYP/6-31G* Model Chemistry Should Not Be Used in DFT Calculations of Molecular Thermochemistry: Understanding and Correcting the Problem
We analyze the error compensations that are responsible
for the
relatively good performance of the popular B3LYP/6-31G* model chemistry
for molecular thermochemistry. We present the B3LYP-gCP-D3/6-31G*
scheme, which corrects for missing London dispersion and basis set
superposition error (BSSE) in a physically sound manner. Benchmark
results for the general main group thermochemistry, kinetics, and
noncovalent interactions set (GMTKN30) are presented. A detailed look
is cast on organic reactions of several arenes with C<sub>60</sub>, DielsâAlder reactions, and barriers to [4 + 3] cycloadditions.
We demonstrate the practical advantages of the new B3LYP-gCP-D3/6-31G*
scheme and show its higher robustness over standard B3LYP/6-31G*.
B3LYP-gCP-D3/6-31G* is meant to fully substitute standard B3LYP/6-31G*
calculations in the same black-box sense at essentially no increase
in computational cost. The energy corrections are made available by
a Web service (http://www.thch.uni-bonn.de/tc/gcpd3) and
by freely available software
Fast and Reasonable Geometry Optimization of Lanthanoid Complexes with an Extended Tight Binding Quantum Chemical Method
The
recently developed tight binding electronic structure approach
GFN-xTB is tested in a comprehensive and diverse lanthanoid geometry
optimization benchmark containing 80 lanthanoid complexes. The results
are evaluated with reference to high-quality X-ray molecular structures
obtained from the Cambridge Structural Database and theoretical DFT-D3Â(BJ)
optimized structures for a few Pm (<i>Z</i> = 61) containing
systems. The average structural heavy-atom root-mean-square deviation
of GFN-xTB (0.65 Ă
) is smaller compared to its competitors, the
Sparkle/PM6 (0.86 Ă
) and HF-3c (0.68 Ă
) quantum chemical
methods. It is shown that GFN-xTB yields chemically reasonable structures,
less outliers, and performs well in terms of overall computational
speed compared to other low-cost methods. The good reproduction of
large lanthanoid complex structures corroborates the wide applicability
of the GFN-xTB approach and its value as an efficient low-cost quantum
chemical method. Its main purpose is the search for energetically
low-lying complex conformations in the elucidation of reaction mechanisms
Either Accurate SingletâTriplet Gaps or Excited-State Structures: Testing and Understanding the Performance of TD-DFT for TADF Emitters
The energy gap between the lowest singlet and triplet
excited states
(ÎEST) is a key property of thermally
activated delayed fluorescence (TADF) emitters, where these states
are dominated by charge-transfer (CT) character. Despite its well-known
shortcomings concerning CT states, time-dependent density functional
theory (TD-DFT) is widely used to predict this gap and study TADF.
Moreover, polar CT states exhibit a strong interaction with their
molecular environment, which further complicates their computational
description. Addressing these two major challenges, this work studies
the performance of TammâDancoff-approximated TD-DFT (TDA-DFT)
on the recent STGABS27 benchmark set,1 exploring different
strategies to include orbital and structural relaxation, as well as
dielectric embedding. The results show that the best-performing strategy
is to calculate ÎEST at the ground-state
structure using functionals with a surprisingly small amount of Fock
exchange of â10% and without a (complete) solvent model. However,
as this approach heavily relies on error cancellation to mimic dielectric
relaxation, it is not robust and exhibits large systematic deviations
in excited state energies, state characters, and structures. More
rigorous approaches, including state-specific solvation, do not share
these systematic deviations, but their predicted ÎEST values exhibit larger statistical errors. We thus conclude
that for the description of CT states in dielectric environments,
none of the tested TDA-DFT methods is competitive with the recently
presented ROKS/PCM approach regarding robustness, accuracy, and computational
efficiency
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