72 research outputs found
Many-Body Expanded Full Configuration Interaction. II. Strongly Correlated Regime
In this second part of our series on the recently proposed many-body expanded
full configuration interaction (MBE-FCI) method, we introduce the concept of
multideterminantal expansion references. Through theoretical arguments and
numerical validations, the use of this class of starting points is shown to
result in a focussed compression of the MBE decomposition of the FCI energy,
thus allowing chemical problems dominated by strong correlation to be addressed
by the method. The general applicability and performance enhancements of
MBE-FCI are verified for standard stress tests such as the bond dissociations
in HO, N, C, and a linear H chain. Furthermore, the benefits
of employing a multideterminantal expansion reference in accelerating
calculations of high accuracy are discussed, with an emphasis on calculations
in extended basis sets. As an illustration of this latter quality of the
MBE-FCI method, results for HO and C in basis sets ranging from double-
to pentuple- quality are presented, demonstrating near-ideal parallel
scaling on up to almost processing units.Comment: 41 pages, 4 tables, 10 figures, 1 SI attached as an ancillary fil
Many-Body Expanded Full Configuration Interaction. I. Weakly Correlated Regime
Over the course of the past few decades, the field of computational chemistry
has managed to manifest itself as a key complement to more traditional
lab-oriented chemistry. This is particularly true in the wake of the recent
renaissance of full configuration interaction (FCI)-level methodologies, albeit
only if these can prove themselves sufficiently robust and versatile to be
routinely applied to a variety of chemical problems of interest. In the present
series of works, performance and feature enhancements of one such avenue
towards FCI-level results for medium to large one-electron basis sets, the
recently introduced many-body expanded full configuration interaction (MBE-FCI)
formalism [J. Phys. Chem. Lett., 8, 4633 (2017)], will be presented.
Specifically, in this opening part of the series, the capabilities of the
MBE-FCI method in producing near-exact ground state energies for weakly
correlated molecules of any spin multiplicity will be demonstrated.Comment: 38 pages, 7 tables, 3 figures, 1 SI attached as an ancillary fil
Decomposed Mean-Field Simulations of Local Properties in Condensed Phases
The present work demonstrates a robust protocol for probing localized
electronic structure in condensed-phase systems, operating in terms of a
recently proposed theory for decomposing the results of Kohn-Sham density
functional theory in a basis of spatially localized molecular orbitals
[Eriksen, J. Chem. Phys. 153, 214109 (2020)]. In an initial application to
liquid, ambient water and the assessment of the solvation energy and the
embedded dipole moment of HO in solution, we find that both properties are
amplified on average -- in accordance with expectation -- and that correlations
are indeed observed to exist between them. However, the simulated
solvent-induced shift to the dipole moment of water is found to be
significantly dampened with respect to typical literature values. The local
nature of our methodology has further allowed us to evaluate the convergence of
bulk properties with respect to the extent of the underlying one-electron basis
set, ranging from single- to full (augmented) quadruple- quality.
Albeit a pilot example, our work paves the way towards future studies of local
effects and defects in more complex phases, e.g., liquid mixtures and even
solid-state crystals.Comment: 17+7 pages, 5 figures, 1 SI attached as an ancillary fil
Virtual orbital many-body expansions: A possible route towards the full configuration interaction limit
In the present letter, it is demonstrated how full configuration interaction
(FCI) results in extended basis sets may be obtained to within sub-kJ/mol
accuracy by decomposing the energy in terms of many-body expansions in the
virtual orbitals of the molecular system at hand. This extension of the FCI
application range lends itself to two unique features of the current approach,
namely that the total energy calculation can be performed entirely within
considerably reduced orbital subspaces and may be so by means of embarrassingly
parallel programming. Facilitated by a rigorous and methodical screening
protocol and further aided by expansion points different from the Hartree-Fock
solution, all-electron numerical results are reported for HO in polarized
core-valence basis sets ranging from double- (10 , 28 ) to
quadruple- (10 , 144 ) quality.Comment: 20 pages, 3 figures, 1 table. * With respect to the original arXiv
version (v1), the present version of the letter contains updated results. The
original TZ and QZ values were unfortunately in error due to a subtle PySCF
bug, which has since then been fixe
Properties of Local Electronic Structures
The simulation of intrinsic contributions to molecular properties holds the
potential to allow for chemistry to be directly inferred from changes to
electronic structures at the atomic level. In the present study, we demonstrate
how such local properties can be readily derived from suitable molecular
orbitals to yield effective fingerprints of various types of atoms in organic
molecules. In contrast, corresponding inferences from schemes that instead make
use of individual atomic orbitals for this purpose are generally found to fail
in expressing much uniqueness in atomic environments. By further studying the
extent to which entire chemical reactions may be decomposed into meaningful and
continuously evolving atomic contributions, schemes based on molecular rather
than atomic orbitals are once again found to be the more consistent, even
allowing for intricate differences between seemingly uniform nucleophilic
substitutions to be probed.Comment: 20+6 pages, 7 figures. SI as an ancillary fil
Decomposing Chemical Space: Applications to the Machine Learning of Atomic Energies
We apply a number of atomic decomposition schemes across the standard QM7
dataset -- a small model set of organic molecules at equilibrium geometry -- to
inspect the possible emergence of trends among contributions to atomization
energies from distinct elements embedded within molecules. Specifically, a
recent decomposition scheme of ours based on spatially localized molecular
orbitals is compared to alternatives that instead partition molecular energies
on account of which nuclei individual atomic orbitals are centred on. We find
these partitioning schemes to expose the composition of chemical compound space
in very dissimilar ways in terms of the grouping, binning, and heterogeneity of
discrete atomic contributions, e.g., those associated with hydrogens bonded to
different heavy atoms. Furthermore, unphysical dependencies on the one-electron
basis set are found for some, but not all of these schemes. The relevance and
importance of these compositional factors for training tailored neural network
models based on atomic energies are next assessed. We identify both limitations
and possible advantages with respect to contemporary machine learning models
and discuss the design of potential counterparts based on atoms and the
intrinsic energies of these as the principal decomposition units.Comment: 21+7 pages, 6 figures. SI as an ancillary file. Version 2: All
PhysNet-based results are now based on NN models trained on a combination of
atomic and molecular energies (as opposed to only the former in Version 1).
SI also updated with a total of four figure
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