729 research outputs found
Unified Treatment of Asymptotic van der Waals Forces
In a framework for long-range density-functional theory we present a unified
full-field treatment of the asymptotic van der Waals interaction for atoms,
molecules, surfaces, and other objects. The only input needed consists of the
electron densities of the interacting fragments and the static polarizability
or the static image plane, which can be easily evaluated in a ground-state
density-functional calculation for each fragment. Results for separated atoms,
molecules, and for atoms/molecules outside surfaces are in agreement with those
of other, more elaborate, calculations.Comment: 6 pages, 5 figure
van der Waals density functional calculations of binding in molecular crystals
A recent paper [J. Chem. Phys. 132, 134705 (2010)] illustrated the potential
of the van der Waals density functional (vdW-DF) method [Phys. Rev. Lett. 92,
246401 (2004)] for efficient first-principle accounts of structure and cohesion
in molecular crystals. Since then, modifications of the original vdW-DF version
(identified as vdW-DF1) has been proposed, and there is also a new version
called vdW-DF2 [ArXiv 1003.5255], within the vdW-DF framework. Here we
investigate the performance and nature of the modifications and the new version
for the binding of a set of simple molecular crystals: hexamine, dodecahedrane,
C60, and graphite. These extended systems provide benchmarks for computational
methods dealing with sparse matter. We show that a previously documented
enhancement of non-local correlations of vdW-DF1 over an asymptotic atom-based
account close to and a few A, beyond binding separation persists in vdW-DF2.
The calculation and analysis of the binding in molecular crystals requires
appropriate computational tools. In this paper, we also present details on our
real-space parallel implementation of the vdW-DF correlation and on the method
used to generate asymptotic atom-based pair potentials based on vdW-DF.Comment: 5 pages, 4 figure
Long-range interactions in the ozone molecule: spectroscopic and dynamical points of view
Using the multipolar expansion of the electrostatic energy, we have
characterized the asymptotic interactions between an oxygen atom O and
an oxygen molecule O, both in their electronic ground state.
We have calculated the interaction energy induced by the permanent electric
quadrupoles of O and O and the van der Waals energy. On one hand we
determined the 27 electronic potential energy surfaces including spin-orbit
connected to the O + O dissociation limit of the
O--O complex. On the other hand we computed the potential energy curves
characterizing the interaction between O and a O
molecule in its lowest vibrational level and in a low rotational level. Such
curves are found adiabatic to a good approximation, namely they are only weakly
coupled to each other. These results represent a first step for modeling the
spectroscopy of ozone bound levels close to the dissociation limit, as well as
the low energy collisions between O and O thus complementing the knowledge
relevant for the ozone formation mechanism.Comment: Submitted to J. Chem. Phys. after revisio
Structure and binding in crystals of cage-like molecules: hexamine and platonic hydrocarbons
In this paper, we show that first-principle calculations using a van der
Waals density functional (vdW-DF), [Phys. Rev. Lett. , 246401
(2004)] permits determination of molecular crystal structure. We study the
crystal structures of hexamine and the platonic hydrocarbons (cubane and
dodecahedrane). The calculated lattice parameters and cohesion energy agree
well with experiments. Further, we examine the asymptotic accounts of the van
der Waals forces by comparing full vdW-DF with asymptotic atom-based pair
potentials extracted from vdW-DF. The character of the binding differ in the
two cases, with vdW-DF giving a significant enhancement at intermediate and
relevant binding separations. We analyze consequences of this result for
methods such as DFT-D, and question DFT-D's transferability over the full range
of separations
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
Tractable non-local correlation density functionals for flat surfaces and slabs
A systematic approach for the construction of a density functional for van
der Waals interactions that also accounts for saturation effects is described,
i.e. one that is applicable at short distances. A very efficient method to
calculate the resulting expressions in the case of flat surfaces, a method
leading to an order reduction in computational complexity, is presented.
Results for the interaction of two parallel jellium slabs are shown to agree
with those of a recent RPA calculation (J.F. Dobson and J. Wang, Phys. Rev.
Lett. 82, 2123 1999). The method is easy to use; its input consists of the
electron density of the system, and we show that it can be successfully
approximated by the electron densities of the interacting fragments. Results
for the surface correlation energy of jellium compare very well with those of
other studies. The correlation-interaction energy between two parallel jellia
is calculated for all separations d, and substantial saturation effects are
predicted.Comment: 10 pages, 6 figure
FDE-vdW: A van der Waals Inclusive Subsystem Density-Functional Theory
We present a formally exact van der Waals inclusive electronic structure
theory, called FDE-vdW, based on the Frozen Density Embedding formulation of
subsystem Density-Functional Theory. In subsystem DFT, the energy functional is
composed of subsystem additive and non-additive terms. We show that an
appropriate definition of the long-range correlation energy is given by the
value of the non-additive correlation functional. This functional is evaluated
using the Fluctuation-Dissipation Theorem aided by a formally exact
decomposition of the response functions into subsystem contributions. FDE-vdW
is derived in detail and several approximate schemes are proposed, which lead
to practical implementations of the method. We show that FDE-vdW is
Casimir-Polder consistent, i.e. it reduces to the generalized Casimir-Polder
formula for asymptotic inter-subsystems separations. Pilot calculations of
binding energies of 13 weakly bound complexes singled out from the S22 set show
a dramatic improvement upon semilocal subsystem DFT, provided that an
appropriate exchange functional is employed. The convergence of FDE-vdW with
basis set size is discussed, as well as its dependence on the choice of
associated density functional approximant
Universal Pairwise Interatomic van der Waals Potentials Based on Quantum Drude Oscillators
Repulsive short-range and attractive long-range van der Waals (vdW) forces
have an appreciable role in the behavior of extended molecular systems. When
using empirical force fields - the most popular computational methods applied
to such systems - vdW forces are typically described by Lennard-Jones-like
potentials, which unfortunately have a limited predictive power. Here, we
present a universal parameterization of a quantum-mechanical vdW potential,
which requires only two free-atom properties - the static dipole polarizability
and the dipole-dipole dispersion coefficient. This is achieved
by deriving the functional form of the potential from the quantum Drude
oscillator (QDO) model, employing scaling laws for the equilibrium distance and
the binding energy as well as applying the microscopic law of corresponding
states. The vdW-QDO potential is shown to be accurate for vdW binding energy
curves, as demonstrated by comparing to ab initio binding curves of 21
noble-gas dimers. The functional form of the vdW-QDO potential has the correct
asymptotic behavior both at zero and infinite distances. In addition, it is
shown that the damped vdW-QDO potential can accurately describe vdW
interactions in dimers consisting of group II elements. Finally, we demonstrate
the applicability of the atom-in-molecule vdW-QDO model for predicting accurate
dispersion energies for molecular systems. The present work makes an important
step towards constructing universal vdW potentials, which could benefit
(bio)molecular computational studies
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