1,721,248 research outputs found
Physical adsorption at the nanoscale: Towards controllable scaling of the substrate-adsorbate van der Waals interaction
Full text linkpeer reviewedThe Lifshitz-Zaremba-Kohn (LZK) theory is commonly considered as the correct large-distance limit for the van der Waals (vdW) interaction of adsorbates (atoms, molecules, or nanoparticles) with solid substrates. In the standard approximate form, implicitly based on local dielectric functions, the LZK approach predicts universal power laws for vdW interactions depending only on the dimensionality of the interacting objects. However, recent experimental findings are challenging the universality of this theoretical approach at finite distances of relevance for nanoscale assembly. Here, we present a combined analytical and numerical many-body study demonstrating that physical adsorption can be significantly enhanced at the nanoscale. Regardless of the band gap or the nature of the adsorbate specie, we find deviations from conventional LZK power laws that extend to separation distances
of up to 10–20 nm. Comparison with recent experimental observations of ultra-long-ranged vdW interactions in the delamination of graphene from a silicon substrate reveals qualitative agreement with the present theory. The sensitivity of vdW interactions to the substrate response and to the adsorbate characteristic excitation frequency also suggests that adsorption strength can be effectively tuned in experiments, paving the way to an improved control of physical adsorption at the nanoscale
Nanoscale π-π Stacked molecules are bound by collective charge fluctuations
Full text linkNon-covalent π-π interactions are central to chemical and biological processes, yet the full understanding of their origin that would unite the simplicity of empirical approaches with the accuracy of quantum calculations is still missing. Here we employ a quantum-mechanical Hamiltonian model for van der Waals interactions, to demonstrate that intermolecular electron correlation in large supramolecular complexes at equilibrium distances is appropriately described by collective charge fluctuations. We visualize these fluctuations and provide connections both to orbital-based approaches to electron correlation, as well as to the simple London pairwise picture. The reported binding energies of ten supramolecular complexes obtained from the quantum-mechanical fluctuation model joined with density functional calculations are within 5% of the reference energies calculated with the diffusion quantum Monte-Carlo method. Our analysis suggests that π-π stacking in supramolecular complexes can be characterized by strong contributions to the binding energy from delocalized, collective charge fluctuations-in contrast to complexes with other types of bonding
Wavelike charge density fluctuations and van der Waals interactions at the nanoscale
Full text linkpeer reviewedRecent experiments on noncovalent interactions at the nanoscale have challenged the basic assumptions of commonly used particle- or fragment-based models for describing van der Waals (vdW) or dispersion forces. We demonstrate that a qualitatively correct description of the vdW interactions between polarizable nanostructures over a wide range of finite distances can only be attained by accounting for the wavelike nature of charge density fluctuations. By considering a diverse set of materials and biological systems with markedly different dimensionalities, topologies, and polarizabilities, we find a visible enhancement in the nonlocality of the charge density response in the range of 10 to 20 nanometers. These collective wavelike fluctuations are responsible for the emergence of nontrivial modifications of the power laws that govern noncovalent interactions at the nanoscale
Bridging Quantum Drude Oscillators and Electronic-Structure Theory for van der Waals Dispersion Interactions
Full text linkVan der Waals (vdW) dispersion forces are fundamental to the structure and behavior of biomolecular, solid-state, and polymeric systems. These interactions, arising from Coulomb-correlated quantum fluctuations in charge density, in principle, demand sophisticated quantum chemistry methods, such as coupled cluster and quantum Monte Carlo. However, the high computational cost of these approaches limits their practical application to large and complex systems. Approximate methods, like classical force fields or semi-local density functional theory (DFT), fall short of capturing the intricacies of vdW dispersion forces. This thesis addresses these limitations by advancing the theoretical description of vdW dispersion interactions through the quantum Drude oscillators (QDOs) framework -- a versatile, coarse-grained model for electronic response.
To this end, we first develop a universal, analytical vdW potential based on the QDO model, applicable across the periodic table. With minimal parametrization, this potential is designed for noble gases and generalized to atomic and molecular dimers, achieving high accuracy compared to experimental data and high-level ab initio calculations. Relying on just two atomic parameters -- dipole polarizability and dipolar dispersion coefficient, our vdW-QDO potential is twice as accurate as the widely used Lennard-Jones potential. This marks a significant advance for biomolecular force fields, where accurate vdW modeling is critical yet remains challenging.
While vdW interaction energies are known to scale with system size, their broader influence on other properties remains less explored. Using the dipole-coupled QDO framework within the many-body dispersion (MBD) method, we examine how vdW dispersion interactions impact electron density. Our findings reveal that these interactions induce significant charge polarization even in systems as small as 100 atoms -- a phenomenon often overlooked in semi-local DFT, where vdW forces are usually treated as a post hoc correction. To address this, we propose a fully coupled, optimally tuned variant of the MBD model based on vdW-QDO parameters, effectively capturing vdW-induced polarization in diverse systems from small molecules to proteins. Our results indicate potential improvements in density functional approximations by incorporating vdW polarization effects.
While the one-body density contains full information about the ground state of a system in DFT, the universal functional required to extract this information remains elusive. In contrast, the two-body density matrix represents electronic correlations more directly. Building on this idea, this thesis introduces a density-matrix reformulation of the MBD method. This approach facilitates real-space visualization of vdW dispersion interactions, while also linking the dipole-coupled QDO framework of MBD to nonlocal correlation functionals in DFT. The resulting nonlocal MBD correlation kernel is critically assessed against existing nonlocal functionals, offering a deeper insight into the theoretical underpinnings of vdW dispersion interactions.
In conclusion, this thesis highlights the QDO model as a robust framework for advancing vdW dispersion modeling across multiple levels of theory. From developing accurate interatomic potentials to uncovering vdW-induced polarization effects and linking to nonlocal correlation functionals, the QDO framework provides a unified platform to address key challenges in the field. This work enhances the understanding of vdW forces and offers a versatile toolbox for future studies in computational chemistry, biomolecular modeling, and beyond
Development of Practical Non-Local Many-Body Polarization Functionals
Full text linkElectronic structure calculations can now achieve the highly coveted chemical accuracy (less than 1 kcal/mol average error in energy differences) for molecules with a few dozens of atoms; however, extending these approaches to larger systems is an area of active research. A major challenge is devising methods that are widely applicable to both molecules and materials. To achieve this goal, computational advances must be coupled to a deep understanding of the related physical principles. Response functions and, in particular, polarizability, play a central role in our conceptual understanding of both electron correlation and polarization/dispersion interactions -- quantum mechanical effects that are notably hard to properly capture due to the underlying non-local nature of the quantities needed to compute these interactions.
To develop a practical formalism for non-local polarizability, one first needs to deeply elaborate the corresponding local and semi-local approaches. This can be achieved by studying model systems, atoms, and molecules, as the numerical results for molecular systems can be complemented by the physical understanding from the model results. By analyzing quantum systems ranging from model Hamiltonians to real molecules, in this work it is shown that polarizability can be factored into a spectrum-dependent and geometry-dependent part. Notably, the geometry-dependent part influences polarizability by a four-dimensional scaling law, enabling the proper description of response properties of individual atoms within molecules. A novel parametrization for representing the response of atoms by an effective harmonic oscillator model is also introduced, showing that spatially resolved polarization potentials can be predicted using just integrated dipolar properties of atoms.
Moving from model systems and atoms to molecules, it is found that the corresponding polarizability and HOMO-LUMO (highest occupied molecular orbital -- lowest unoccupied molecular orbital) gap are independent. In parallel, the theoretical foundations of non-local polarizability are examined, presenting expressions for a range of model systems via the polarization field correlation function. By following a rigorous derivation of this response function, it is shown that not only existing methods can be obtained from it as limiting cases, but the design of a general non-expanded many-body dispersion energy functional is also feasible.
Overall, this thesis aims to show that combining the fundamental physics of model systems, atoms, and molecules with a theory of non-local polarizability can lead to practical functionals for electronic structure calculations based on the advanced non-local description of response functions
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Fast and accurate quantum Monte Carlo for molecular crystals
Full text linkpeer reviewedComputer simulation plays a central role in modern-day materials
science. The utility of a given computational approach depends
largely on the balance it provides between accuracy and computational
cost. Molecular crystals are a class of materials of great
technological importance which are challenging for even the most
sophisticated ab initio electronic structure theories to accurately
describe. This is partly because they are held together by a balance
of weak intermolecular forces but also because the primitive
cells of molecular crystals are often substantially larger than those
of atomic solids. Here, we demonstrate that diffusion quantum
Monte Carlo (DMC) delivers subchemical accuracy for a diverse
set of molecular crystals at a surprisingly moderate computational
cost. As such, we anticipate that DMC can play an important role
in understanding and predicting the properties of a large number
of molecular crystals, including those built from relatively
large molecules which are far beyond reach of other high-accuracy
methods
van der Waals Dispersion Interactions in Biomolecular Systems: Quantum-Mechanical Insights and Methodological Advances
No full textIntermolecular interactions are paramount for the stability, dynamics and response of systems
across chemistry, biology and materials science. In biomolecules they govern secondary structure
formation, assembly, docking, regulation and functionality. van der Waals (vdW) dispersion
contributes a crucial part to those interactions. As part of the long-range electron correlation,
vdW interactions arise from Coulomb-coupled quantum-mechanical fluctuations in the instan-
taneous electronic charge distribution and are thus inherently many-body in nature. Common
approaches to describe biomolecular systems (i.e., classical molecular mechanics) fail to capture
the full complexity of vdW dispersion by adapting a phenomenological, atom-pairwise formalism.
This thesis explores beyond-pairwise vdW forces and the collectivity of intrinsic electronic behav-
iors in biomolecular systems and discusses their role in the context of biomolecular processes
and function. To this end, the many-body dispersion (MBD) formalism parameterized from
density-functional tight-binding (DFTB) calculations is used.
The investigation of simple molecular solvents with particular focus on water gives insights into
the vdW energetics and electronic response properties in liquids and solvation as well as emergent
behavior for coarse-grained models. A detailed study of intra-protein and protein–water vdW
interactions highlights the role of many-body forces during protein folding and provides a funda-
mental explanation for the previously observed “unbalanced” description and over-compaction
of disordered protein states. Further analysis of the intrinsic electronic behaviors in explicitly
solvated proteins indicates a long-range persistence of electron correlation through the aque-
ous environment, which is discussed in the context of protein–protein interactions, long-range
coordination and biomolecular regulation and allostery. Based on the example of a restriction
enzyme, the potential role of many-body vdW forces and collective electronic behavior for the
long-range coordination of enzymatic activity is discussed. Introducing electrodynamic quantum
fluctuations into the classical picture of allostery opens up the path to a more holistic view on
biomolecular regulation beyond the traditional focus on merely local structural modifications.
Building on top of the MBD framework, which describes vdW dispersion within the interatomic
dipole-limit, a practical extension to higher-order terms is presented. The resulting Dipole-
Correlated Coulomb Singles account for multipolar as well as dispersion-polarization-like contri-
butions beyond the random phase approximation by means of first-order perturbation theory
over the dipole-coupled MBD state. It is shown that Dipole-Correlated Coulomb Singles become
particularly relevant for relatively larger systems and can alter qualitative trends in the long-range
interaction under (nano-)confinement. Bearing in mind the frequent presence of confinement in
biomolecular systems due to cellular crowding, in ion channels or for interfacial water, this so-far
neglected contribution is expected to have broad implications for systems of biological relevance.
Ultimately, this thesis introduces a hybrid approach of DFTB and machine learning for the accu-
rate description of large-scale systems on a robust, albeit approximate, quantum-mechanical
level. The developed DFTB-NN rep approach combines the semi-empirical DFTB Hamiltonian
with a deep tensor neural network model for localized many-body repulsive potentials. DFTB-
NN rep provides an accurate description of energetic, structural and vibrational properties of a
wide range of small organic molecules much superior to standard DFTB or machine learning.
Overall, this thesis aims to extend the current view of complex (bio)molecular systems being
governed by local, (semi-)classical interactions and develops methodological steps towards an
advanced description and understanding including non-local interaction mechanisms enabled
by quantum-mechanical phenomena such as long-range correlation forces arising from collective
electronic fluctuations
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