6 research outputs found
Surface Adsorption from the Exchange-Hole Dipole Moment Dispersion Model
The accurate calculation
of intermolecular interaction energies
with density functional theory requires methods that include a treatment
of long-range, nonlocal dispersion correlation. In this work, we explore
the ability of the exchange-hole dipole moment (XDM) dispersion correction
to model molecular surface adsorption. Adsorption energies are calculated
for six small aromatic molecules (benzene, furan, pyridine, thiophene,
thiophenol, and benzenediamine) and the four DNA nucleobases (adenine,
thymine, guanine, and cytosine) on the (111) surfaces of the three
coinage metals (copper, silver, and gold). For benzene, where the
experimental reference data is most precise, the mean absolute error
in the computed absorption energies is 0.04 eV. For the other aromatic
molecules, the computed binding energies are found to be within 0.09
eV of the available reference data, on average, which is well below
the expected experimental uncertainties for temperature-programmed
desorption measurements. Unlike other dispersion-corrected functionals,
adequate performance does not require changes to the canonical XDM
implementation, and the good performance of XDM is explained in terms
of the behavior of the exchange hole. Additionally, the base functional
employed (B86bPBE) is also optimal for molecular studies, making B86bPBE-XDM
an excellent candidate for studying chemistry on material surfaces.
Finally, the noncovalent interaction (NCI) plot technique is shown
to detect adsorption effects in real space on the order of tenths
of an eV
Atom-Centered Potentials with Dispersion-Corrected Minimal-Basis-Set Hartree–Fock: An Efficient and Accurate Computational Approach for Large Molecular Systems
We present a computational methodology
based on atom-centered potentials
(ACPs) for the efficient and accurate structural modeling of large
molecular systems. ACPs are atom-centered one-electron potentials
that have the same functional form as effective-core potentials. In
recent works, we showed that ACPs can be used to produce a correction
to the ground-state wave function and electronic energy to alleviate
shortcomings in the underlying model chemistry. In this work, we present
ACPs for H, C, N, and O atoms that are specifically designed to predict
accurate non-covalent binding energies and inter- and intramolecular
geometries when combined with dispersion-corrected Hartree–Fock
(HF-D3) and a minimal basis-set (scaled MINI or MINIs). For example,
the combined HF-D3/MINIs-ACP method demonstrates excellent performance,
with mean absolute errors of 0.36 and 0.28 kcal/mol for the S22x5
and S66x8 benchmark sets, respectively, relative to highly correlated
complete-basis-set data. The application of ACPs results in a significant
decrease in error compared to uncorrected HF-D3/MINIs for all benchmark
sets examined. In addition, HF-D3/MINIs-ACP, has a cost only slightly
higher than a minimal-basis-set HF calculation and can be used with
any electronic structure program for molecular quantum chemistry that
uses Gaussian basis sets and effective-core potentials
Atom-Centered Potentials with Dispersion-Corrected Minimal-Basis-Set Hartree–Fock: An Efficient and Accurate Computational Approach for Large Molecular Systems
We present a computational methodology
based on atom-centered potentials
(ACPs) for the efficient and accurate structural modeling of large
molecular systems. ACPs are atom-centered one-electron potentials
that have the same functional form as effective-core potentials. In
recent works, we showed that ACPs can be used to produce a correction
to the ground-state wave function and electronic energy to alleviate
shortcomings in the underlying model chemistry. In this work, we present
ACPs for H, C, N, and O atoms that are specifically designed to predict
accurate non-covalent binding energies and inter- and intramolecular
geometries when combined with dispersion-corrected Hartree–Fock
(HF-D3) and a minimal basis-set (scaled MINI or MINIs). For example,
the combined HF-D3/MINIs-ACP method demonstrates excellent performance,
with mean absolute errors of 0.36 and 0.28 kcal/mol for the S22x5
and S66x8 benchmark sets, respectively, relative to highly correlated
complete-basis-set data. The application of ACPs results in a significant
decrease in error compared to uncorrected HF-D3/MINIs for all benchmark
sets examined. In addition, HF-D3/MINIs-ACP, has a cost only slightly
higher than a minimal-basis-set HF calculation and can be used with
any electronic structure program for molecular quantum chemistry that
uses Gaussian basis sets and effective-core potentials
Accurate Modeling of Water Clusters with Density-Functional Theory Using Atom-Centered Potentials
The ability of atom-centered potentials
(ACPs) to improve the modeling
of water clusters using density-functional methods is explored. Water-specific
ACPs were developed using accurate <i>ab initio</i> reference
data to correct the deficiencies of the BHandHLYP density functional
in the calculation of absolute and relative binding energies of water
clusters. In conjunction with aug-cc-pVTZ basis sets and with or without
dispersion corrections, it is possible to obtain absolute binding
energies for water clusters containing up to 10 H<sub>2</sub>O molecules
to within 0.44 kcal/mol or 0.04 kcal/mol per water molecule. In contrast,
dispersion-corrected BHandHLYP/aug-cc-pVTZ predicts binding energies
with errors as large as 6 kcal/mol for (H<sub>2</sub>O)<sub>10</sub> in the absence of ACPs. Therefore, the ACPs improve predicted binding
energies in these clusters by more than an order of magnitude. The
conformers of (H<sub>2</sub>O)<sub>16</sub> and (H<sub>2</sub>O)<sub>17</sub> were used to validate the application of ACPs to larger
clusters. ACP-based approaches are able to predict the binding energies
in (H<sub>2</sub>O)<sub>16,17</sub> within a range of 0.3–2.2
kcal/mol (less than 1.3%) of recently revised <i>ab initio</i> wave function results. ACPs for basis sets smaller than aug-cc-pVTZ
are also presented. However, the ability of the BHandHLYP/ACP approach
to predict accurate binding energies deteriorates as the size of the
basis sets decreases. Nevertheless, ACPs improve predicted binding
energies by as much as a factor of 50 across the range of the basis
sets studied. The BHandHLYP/aug-cc-pVTZ-ACP method was applied to
(H<sub>2</sub>O)<sub>25</sub> in order to identify the minimum-energy
structure of a collection of proposed global minimum-energy structures.
The BHandHLYP/aug-cc-pVTZ-ACP approach is an accurate and computationally
affordable alternative to wave function theory methods for the prediction
of the binding energies and energy ranking of water clusters
Composite and Low-Cost Approaches for Molecular Crystal Structure Prediction
Molecular
crystal structure prediction (CSP) requires evaluating
differences in lattice energy between candidate crystal structures
accurately and efficiently. In this work, we explore and compare several
low-cost alternatives to dispersion-corrected density-functional theory
(DFT) in the plane-waves/pseudopotential approximation, the most accurate
and general approach used for CSP at present. Three types of low-cost
methods are considered: DFT with a small basis set of finite-support
numerical orbitals (the SIESTA method), dispersion-corrected Gaussian
small or minimal-basis-set Hartree–Fock and DFT with additional
empirical corrections (HF-3c and PBEh-3c), and self-consistent-charge
dispersion-corrected density-functional tight binding (SCC-DFTB3-D3).
In addition, we study the performance of composite methods that comprise
a geometry optimization using a low-cost approach followed by a single-point
calculation using the accurate but comparatively expensive B86bPBE-XDM
functional. All methods were tested for their abilities to produce
absolute lattice energies, relative lattice energies, and crystal
geometries. We show that assessing various methods by their ability
to produce absolute lattice energies can be misleading and that relative
lattice energies are a much better indicator of performance in CSP.
The EE14 set of relative solubilities of homochiral and heterochiral
chiral crystals is proposed for relative lattice-energy benchmarking.
Our results show that PBE-D2 plus a DZP basis set of numerical orbitals
coupled with a final B86bPBE-XDM single-point calculation gives excellent
performance at a fraction of the cost of a full B86bPBE-XDM calculation,
although the results are sensitive to the particular details of the
computational protocol. The B86bPBE-XDM//PBE-D2/DZP method was subsequently
tested in a practical CSP application from our recent work on the
crystal structure of the enantiopure and racemate forms of 1-aza[6]Âhelicene,
a chiral organic semiconductor. Our results show that this multilevel
method is able to correctly reproduce the energy ranking of both crystal
forms
Carbon Nanotube Chirality Determines Efficiency of Electron Transfer to Fullerene in All-Carbon Photovoltaics
Nanocarbon-based
photovoltaics offer a promising new architecture
for the next generation of solar cells. We demonstrate that a key
factor determining the efficiency of single-walled carbon nanotube
(SWCNT)/fullerene devices is the chirality of the SWCNT. This is shown
via current density vs voltage measurements of nanocarbon devices
prepared with (9,7), (7,6) and (6,5) SWCNTs, as well as density-functional
theory (DFT) density of states calculations of C<sub>60</sub> adsorbed
onto the corresponding SWCNTs. The trends in efficiency are rationalized
in terms of the relative energies of the fullerene and SWCNT conduction
band energy levels