1,165 research outputs found
Properties of the water to boron nitride interaction: from zero to two dimensions with benchmark accuracy
Molecular adsorption on surfaces plays an important part in catalysis,
corrosion, desalination, and various other processes that are relevant to
industry and in nature. As a complement to experiments, accurate adsorption
energies can be obtained using various sophisticated electronic structure
methods that can now be applied to periodic systems. The adsorption energy of
water on boron nitride substrates, going from zero to 2-dimensional
periodicity, is particularly interesting as it calls for an accurate treatment
of polarizable electrostatics and dispersion interactions, as well as posing a
practical challenge to experiments and electronic structure methods. Here, we
present reference adsorption energies, static polarizabilities, and dynamic
polarizabilities, for water on BN substrates of varying size and dimension.
Adsorption energies are computed with coupled cluster theory, fixed-node
quantum Monte Carlo (FNQMC), the random phase approximation (RPA), and second
order M{\o}ller-Plesset (MP2) theory. These explicitly correlated methods are
found to agree in molecular as well as periodic systems. The best estimate of
the water/h-BN adsorption energy is meV from FNQMC. In addition, the
water adsorption energy on the BN substrates could be expected to grow
monotonically with the size of the substrate due to increased dispersion
interactions but interestingly, this is not the case here. This peculiar
finding is explained using the static polarizabilities and molecular dispersion
coefficients of the systems, as computed from time-dependent density functional
theory (DFT). Dynamic as well as static polarizabilities are found to be highly
anisotropic in these systems. In addition, the many-body dispersion method in
DFT emerges as a particularly useful estimation of finite size effects for
other expensive, many-body wavefunction based methods
Theory and applications of atomic and ionic polarizabilities
Atomic polarization phenomena impinge upon a number of areas and processes in
physics. The dielectric constant and refractive index of any gas are examples
of macroscopic properties that are largely determined by the dipole
polarizability. When it comes to microscopic phenomena, the existence of
alkaline-earth anions and the recently discovered ability of positrons to bind
to many atoms are predominantly due to the polarization interaction. An
imperfect knowledge of atomic polarizabilities is presently looming as the
largest source of uncertainty in the new generation of optical frequency
standards. Accurate polarizabilities for the group I and II atoms and ions of
the periodic table have recently become available by a variety of techniques.
These include refined many-body perturbation theory and coupled-cluster
calculations sometimes combined with precise experimental data for selected
transitions, microwave spectroscopy of Rydberg atoms and ions, refractive index
measurements in microwave cavities, ab initio calculations of atomic structures
using explicitly correlated wave functions, interferometry with atom beams, and
velocity changes of laser cooled atoms induced by an electric field. This
review examines existing theoretical methods of determining atomic and ionic
polarizabilities, and discusses their relevance to various applications with
particular emphasis on cold-atom physics and the metrology of atomic frequency
standards.Comment: Review paper, 44 page
Time-dependent density functional theory calculation of van der Waals coefficient of sodium clusters
In this paper we employ all-electron \textit{ab-initio} time-dependent
density functional theory based method to calculate the long range
dipole-dipole dispersion coefficient (van der Waals coefficient) of
sodium atom clusters containing even number of atoms ranging from 2 to 20
atoms. The dispersion coefficients are obtained via Casimir-Polder relation.
The calculations are carried out with two different exchange-correlation
potentials: (i) the asymptotically correct statistical average of orbital
potential (SAOP) and (ii) Vosko-Wilk-Nusair representation of
exchange-correlation potential within local density approximation. A comparison
with the other theoretical results has been performed. We also present the
results for the static polarizabilities of sodium clusters and also compare
them with other theoretical and experimental results. These comparisons reveal
that the SAOP results for C_{6} and static polarizability are quite accurate
and very close to the experimental results. We examine the relationship between
volume of the cluster and van der Waals coefficient and find that to a very
high degree of correlation C_{6} scales as square of the volume. We also
present the results for van der Waals coefficient corresponding to cluster-Ar
atom and cluster-N_{2} molecule interactions.Comment: 22 pages including 6 figures. To be published in Journal of Chemical
Physic
Toward transferable interatomic van der Waals interactions without electrons: The role of multipole electrostatics and many-body dispersion
We estimate polarizabilities of atoms in molecules without electron density,
using a Voronoi tesselation approach instead of conventional density
partitioning schemes. The resulting atomic dispersion coefficients are
calculated, as well as many-body dispersion effects on intermolecular potential
energies. We also estimate contributions from multipole electrostatics and
compare them to dispersion. We assess the performance of the resulting
intermolecular interaction model from dispersion and electrostatics for more
than 1,300 neutral and charged, small organic molecular dimers. Applications to
water clusters, the benzene crystal, the anti-cancer drug
ellipticine---intercalated between two Watson-Crick DNA base pairs, as well as
six macro-molecular host-guest complexes highlight the potential of this method
and help to identify points of future improvement. The mean absolute error made
by the combination of static electrostatics with many-body dispersion reduces
at larger distances, while it plateaus for two-body dispersion, in conflict
with the common assumption that the simple correction will yield proper
dissociative tails. Overall, the method achieves an accuracy well within
conventional molecular force fields while exhibiting a simple parametrization
protocol.Comment: 13 pages, 8 figure
Long-range correlation energy calculated from coupled atomic response functions
An accurate determination of the electron correlation energy is essential for
describing the structure, stability, and function in a wide variety of systems,
ranging from gas-phase molecular assemblies to condensed matter and
organic/inorganic interfaces. Even small errors in the correlation energy can
have a large impact on the description of chemical and physical properties in
the systems of interest. In this context, the development of efficient
approaches for the accurate calculation of the long-range correlation energy
(and hence dispersion) is the main challenge. In the last years a number of
methods have been developed to augment density functional approximations via
dispersion energy corrections, but most of these approaches ignore the
intrinsic many-body nature of correlation effects, leading to inconsistent and
sometimes even qualitatively incorrect predictions. Here we build upon the
recent many-body dispersion (MBD) framework, which is intimately linked to the
random-phase approximation for the correlation energy. We separate the
correlation energy into short-range contributions that are modeled by
semi-local functionals and long-range contributions that are calculated by
mapping the complex all-electron problem onto a set of atomic response
functions coupled in the dipole approximation. We propose an effective
range-separation of the coupling between the atomic response functions that
extends the already broad applicability of the MBD method to non-metallic
materials with highly anisotropic responses, such as layered nanostructures.
Application to a variety of high-quality benchmark datasets illustrates the
accuracy and applicability of the improved MBD approach, which offers the
prospect of first-principles modeling of large structurally complex systems
with an accurate description of the long-range correlation energy.Comment: 15 pages, 3 figure
Non-covalent interactions across organic and biological subsets of chemical space: Physics-based potentials parametrized from machine learning
Classical intermolecular potentials typically require an extensive
parametrization procedure for any new compound considered. To do away with
prior parametrization, we propose a combination of physics-based potentials
with machine learning (ML), coined IPML, which is transferable across small
neutral organic and biologically-relevant molecules. ML models provide
on-the-fly predictions for environment-dependent local atomic properties:
electrostatic multipole coefficients (significant error reduction compared to
previously reported), the population and decay rate of valence atomic
densities, and polarizabilities across conformations and chemical compositions
of H, C, N, and O atoms. These parameters enable accurate calculations of
intermolecular contributions---electrostatics, charge penetration, repulsion,
induction/polarization, and many-body dispersion. Unlike other potentials, this
model is transferable in its ability to handle new molecules and conformations
without explicit prior parametrization: All local atomic properties are
predicted from ML, leaving only eight global parameters---optimized once and
for all across compounds. We validate IPML on various gas-phase dimers at and
away from equilibrium separation, where we obtain mean absolute errors between
0.4 and 0.7 kcal/mol for several chemically and conformationally diverse
datasets representative of non-covalent interactions in biologically-relevant
molecules. We further focus on hydrogen-bonded complexes---essential but
challenging due to their directional nature---where datasets of DNA base pairs
and amino acids yield an extremely encouraging 1.4 kcal/mol error. Finally, and
as a first look, we consider IPML in denser systems: water clusters,
supramolecular host-guest complexes, and the benzene crystal.Comment: 15 pages, 9 figure
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