121 research outputs found
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
General formulation of polarizable embedding models and of their coupling
We propose a general formalism for polarizable embedding models that can be applied to either continuum or atomistic polarizable models. After deriving such a formalism for both variational and non-variational models, we address the problem of coupling two polarizable models among themselves and to a quantum mechanical (QM) description in the spirit of multiscale quantum chemistry. We discuss general, model-independent coupling hypotheses and derive coupled polarization equations for all combinations of variational and non-variational models and discuss the embedding contributions to the analytical derivatives of the energy, with a particular focus on the elements of the Fock or Kohn-Sham matrix. We apply the general formalism to the derivation of the working equations for a three-layered, fully polarizable QM/MM/continuum strategy using the non-variational atomic multipole optimized energetics for biomolecular applications polarizable force field and the domain decomposition conductor-like screening model
Is There a Quadruple Fe-C Bond in FeC(CO)3?
A recent computational paper (Kalita et al., Phys. Chem. Chem. Phys. 2020, 22, 24178–24180) reports the existence of a quadruple bond between a carbon and an iron atom in the FeC(CO)3 molecule. In this communication, we perform several computations on the same system, using both density functional theory and post-Hartree–Fock methods and find that the results, and in particular the Fe-C bond length and stretching frequency depend strongly on the method used. We ascribe this behavior to a strong multireference character of the FeC(CO)3 ground state, which explains the non-conclusive results obtained with single-reference methods. We therefore conclude that, while the existence of a Fe-C quadruple bond is not disproved, further investigation is required before a conclusion can be drawn
Hybrid QM/classical models: Methodological advances and new applications
Hybrid methods that combine quantum mechanical descriptions with classical models are very popular in molecular modeling. Such a large diffusion reflects their effectiveness, which over the years has allowed the quantum mechanical description to extend its boundaries to systems
of increasing size and to processes of increasing complexity. Despite this success, research in this field is still very active and a number of advances have been made recently, further extending the range of their applications. In this review, we describe such advances and discuss how
hybrid methods may continue to improve in the future. The various formulations proposed so far are presented here in a coherent way to underline their common methodological aspects. At the same time, the specificities of the different classical models and of their coupling with the quantum mechanical domain are highlighted and discussed, with special attention to the computational and numerical aspects
A Second-Order CASSCF Algorithm with the Cholesky Decomposition of the Two-Electron Integrals
In this contribution, we present the implementation of a second-order CASSCF
algorithm in conjunction with the Cholesky decomposition of the two-electron
repulsion integrals. The algorithm, called Norm-Extended Optimization,
guarantees convergence of the optimization, but it involves the full Hessian of
the wavefunction and is therefore computationally expensive. Coupling the
second-order procedure with the Cholesky decomposition leads to a significant
reduction in the computational cost, reduced memory requirements, and an
improved parallel performance. As a result, CASSCF calculations of larger
molecular systems become possible as a routine task. The performance of the new
implementation is illustrated by means of benchmark calculations on molecules
of increasing size, with up to about 3000 basis functions and 14 active
orbitals
Perspective: Polarizable continuum models for quantum-mechanical descriptions
Polarizable continuum solvation models are nowadays the most popular approach to describe solvent effects in the context of quantum mechanical calculations. Unexpectedly, despite their widespread use in all branches of quantum chemistry and beyond, important aspects of both their theoretical formulation and numerical implementation are still not completely understood. In particular, in this perspective we focus on the numerical issues of their implementation when applied to large systems and on the theoretical framework needed to treat time dependent problems and excited states or to deal with electronic correlation. Possible extensions beyond a purely electrostatic model and generalizations to environments beyond common solvents are also critically presented and discussed. Finally, some possible new theoretical approaches and numerical strategies are suggested to overcome the obstacles which still prevent a full exploitation of these models
Achieving Linear Scaling in Computational Cost for a Fully Polarizable MM/Continuum Embedding
In this paper, we present a new, efficient implementation of a fully polarizable QM/MM/continuum model based on an induced-dipoles polarizable force field and on the Conductor-like Screening Model as a polarizable continuum in combination with a self-consistent field QM method. The paper focuses on the implementation of the MM/continuum embedding, where the two polarizable methods are fully coupled to take into account their mutual polarization. With respect to previous implementations, we achieve for the first time a linear scaling with respect to both the computational cost and the memory requirements without limitations on the molecular cavity shape. This is achieved thanks to the use of the recently developed ddCOSMO model for the continuum and the Fast Multipole Method for the force field, together with an efficient iterative procedure. Therefore, it becomes possible to include in the classical layer as much as several tens of thousands of atoms with a limited computational effort
CASSCF response equations revisited: a simple and efficient iterative algorithm
We present an algorithm to solve the CASSCF linear response equations that is
both simple and efficient. The algorithm makes use of the well established
symmetric and antisymmetric combinations of trial vectors, but further
orthogonalizes them with respect to the scalar product induced by the response
matrix. This leads to a standard, symmetric, block eigenvalue problem in the
expansion subspace that can be solved by diagonalizing a symmetric, positive
definite matrix half the size of the expansion space. Preliminary numerical
tests show that the algorithm is robust and stable
A Cholesky decomposition-based implementation of relativistic two-component coupled-cluster methods for medium-sized molecules
A Cholesky decomposition (CD)-based implementation of relativistic
two-component coupled-cluster (CC) and equation-of-motion CC (EOM-CC) methods
using an exact two-component Hamiltonian augmented with atomic-mean-field
spin-orbit integrals (the X2CAMF scheme) is reported. The present CD-based
implementation of X2CAMF-CC and EOM-CC methods employs atomic-orbital-based
algorithms to avoid the construction of two-electron integrals and
intermediates involving three and four virtual indices. Our CD-based
implementation extends the applicability of X2CAMF-CC and EOM-CC methods to
medium-sized molecules with the possibility to correlate around 1000 spinors.
Benchmark calculations for uranium-containing small molecules have been
performed to assess the dependence of the CC results on the Cholesky threshold.
A Cholesky threshold of is shown to be sufficient to maintain
chemical accuracy. Example calculations to illustrate the capability of the
CD-based relativistic CC methods are reported for the bond-dissociation energy
of the uranium hexafluoride molecule, UF, with up to quadruple-zeta basis
sets, and the lowest excitation energy in solvated uranyl ion
[UO(HO)]
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