151 research outputs found
Towards quantum-chemical method development for arbitrary basis functions
We present the design of a flexible quantum-chemical method development
framework, which supports employing any type of basis function. This design has
been implemented in the light-weight program package molsturm, yielding a
basis-function-independent self-consistent field scheme. Versatile interfaces,
making use of open standards like python, mediate the integration of molsturm
with existing third-party packages. In this way both rapid extension of the
present set of methods for electronic structure calculations as well as adding
new basis function types can be readily achieved. This makes molsturm
well-suitable for testing novel approaches for discretising the electronic wave
function and allows comparing them to existing methods using the same software
stack. This is illustrated by two examples, an implementation of
coupled-cluster doubles as well as a gradient-free geometry optimisation, where
in both cases, an arbitrary basis functions could be used. molsturm is
open-source and can be obtained from https://molsturm.org.Comment: 15 pages and 7 figure
Quantum chemistry with Coulomb Sturmians:Construction and convergence of Coulomb Sturmian basis sets at the Hartree-Fock level
The first discussion of basis sets consisting of exponentially decaying
Coulomb Sturmian functions for modelling electronic structures is presented.
The proposed basis set construction selects Coulomb Sturmian functions using
separate upper limits to their principle, angular momentum and magnetic quantum
numbers. Their common Coulomb Sturmian exponent is taken as a fourth parameter.
The convergence properties of such basis sets are investigated for second and
third row atoms at the Hartree-Fock level. Thereby important relations between
the values of the basis set parameters and the physical properties of the
electronic structure are recognised. For example, an unusually large limit for
the angular momentum quantum number in unrestricted Hartree-Fock calculations
can be linked to the breaking of spherical symmetry in such cases. Furthermore,
a connection between the optimal, i.e. minimum-energy, Coulomb Sturmian
exponent and the average Slater exponents values obtained by Clementi and
Raimondi (E. Clementi and D. L. Raimondi, J. Chem. Phys. 38, 2686 (1963)) is
made. These features of Coulomb Sturmian basis sets emphasise their ability to
correctly reproduce the physical features of Hartree-Fock wave functions.Comment: 16 pages, 14 figures, supporting inf
The inverted singletâtriplet gap: a vanishing myth?
Molecules with an inverted singletâtriplet gap (STG) between the first excited singlet and triplet states, for example, heptazine, have recently been reported and gained substantial attention since they violate the famous Hundâs rule. Utilizing state-of-the-art high-level ab initio methods, the singletâtriplet gap vanishes and approaches zero from below whatever is improved in the theoretical description of the molecules: the basis set or the level of electron correlation. Seemingly, the phenomenon of inverted singletâtriplet gaps tends to vanish the closer we observe
KineticNet: Deep learning a transferable kinetic energy functional for orbital-free density functional theory
Orbital-free density functional theory (OF-DFT) holds the promise to compute
ground state molecular properties at minimal cost. However, it has been held
back by our inability to compute the kinetic energy as a functional of the
electron density only. We here set out to learn the kinetic energy functional
from ground truth provided by the more expensive Kohn-Sham density functional
theory. Such learning is confronted with two key challenges: Giving the model
sufficient expressivity and spatial context while limiting the memory footprint
to afford computations on a GPU; and creating a sufficiently broad distribution
of training data to enable iterative density optimization even when starting
from a poor initial guess. In response, we introduce KineticNet, an equivariant
deep neural network architecture based on point convolutions adapted to the
prediction of quantities on molecular quadrature grids. Important contributions
include convolution filters with sufficient spatial resolution in the vicinity
of the nuclear cusp, an atom-centric sparse but expressive architecture that
relays information across multiple bond lengths; and a new strategy to generate
varied training data by finding ground state densities in the face of
perturbations by a random external potential. KineticNet achieves, for the
first time, chemical accuracy of the learned functionals across input densities
and geometries of tiny molecules. For two electron systems, we additionally
demonstrate OF-DFT density optimization with chemical accuracy.Comment: 10 pages, 8 figure
A simple monomer-based model-Hamiltonian approach to combine excitonic coupling and Jahn-Teller theory
The interplay of excitonic and vibronic coupling in coupled chromophores determines the efficiency of exciton localization vs delocalization, or in other words, coherent excitation energy transfer vs exciton hopping. For the investigation of exciton localization in large coupled dimers, a model Hamiltonian approach is derived, the ingredients of which can all be obtained from monomer ab initio calculations alone avoiding costly ab initio computation of the full dimer. The accuracy and applicability of this model are exemplified for the benzene dimer by rigorous comparison to ab initio results
Geometry dependence of excitonic couplings and the consequences for configuration-space sampling
Excitonic coupling plays a key role for the understanding of excitonic energy transport (EET) in, for example, organic photovoltaics. However, the calculation of realistic systems is often beyond the applicability range of accurate wavefunction methods so that lower-scaling semi-empirical methods are used to model EET events. In the present work, the distance and angle dependence of excitonic couplings of dimers of selected organic molecules are evaluated for the semi-empirical long-range corrected density functional based tight binding (LC-DFTB) method and spin opposite scaled second order approximate coupled cluster singles and doubles (SOS-CC2). While semi-empirically scaled methods can lead to slightly increased deviations for excitation energies, the excitonic couplings and their dependence on the dimer geometry are reproduced. LC-DFTB yields a similar accuracy range as density-functional theory (DFT) employing the ÏB97X functional while the computation time is reduced by several orders of magnitude. The dependence of the exchange contributions to the excitonic couplings on the dimer geometry is analyzed assessing the calculation of Coulombic excitonic couplings from monomer local excited states only, which reduces the computational effort significantly. The present work is a necessary first step toward the simulation of excitonic energy transport using semi-empirical methods
Interatomic and Intermolecular Coulombic Decay
Interatomic or intermolecular Coulombic decay (ICD) is a nonlocal electronic decay mechanism occurring in weakly bound matter. In an ICD process, energy released by electronic relaxation of an excited atom or molecule leads to ionization of a neighboring one via Coulombic electron interactions. ICD has been predicted theoretically in the mid nineties of the last century, and its existence has been confirmed experimentally approximately ten years later. Since then, a number of fundamental and applied aspects have been studied in this quickly growing field of research. This review provides an introduction to ICD and draws the connection to related energy transfer and ionization processes. The theoretical approaches for the description of ICD as well as the experimental techniques developed and employed for its investigation are described. The existing body of literature on experimental and theoretical studies of ICD processes in different atomic and molecular systems is reviewed. © 2020 American Chemical Societ
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