7,234 research outputs found
Multireference Correlation in Long Molecules with the Quadratic Scaling Density Matrix Renormalization Group
We have devised and implemented a local ab initio Density Matrix
Renormalization Group (DMRG) algorithm to describe multireference nondynamic
correlations in large systems. For long molecules that are extended in one of
their spatial dimensions, this method allows us to obtain an exact
characterisation of correlation, in the given basis, with a cost that scales
only quadratically with the size of the system. The reduced scaling is achieved
solely through integral screening and without the construction of correlation
domains. We demonstrate the scaling, convergence, and robustness of the
algorithm in polyenes and hydrogen chains. We converge to exact correlation
energies (with 1-10 microhartree precision) in all cases and correlate up to
100 electrons in 100 active orbitals. We further use our algorithm to obtain
exact energies for the metal-insulator transition in hydrogen chains and
compare and contrast our results with those from conventional quantum chemical
methods.Comment: 14 pages, 12 figures, tciLaTeX, aip-BibTeX styl
Simple and inexpensive perturbative correction schemes for antisymmetric products of nonorthogonal geminals
A new multireference perturbation approach has been developed for the recently proposed AP1roG scheme, a computationally facile parametrization of an antisymmetric product of nonorthogonal geminals. This perturbation theory of second-order closely follows the biorthogonal treatment from multiconfiguration perturbation theory as introduced by Surjan et al., but makes use of the additional feature of AP1roG that the expansion coefficients within the space of closed-shell determinants are essentially correct already, which further increases the predictive power of the method. Building upon the ability of AP1roG to model static correlation, the perturbation correction accounts for dynamical electron correlation, leading to absolute energies close to full configuration interaction results. Potential surfaces for multiple bond dissociation in H2O and N-2 are predicted with high accuracy up to bond breaking. The computational cost of the method is the same as that of conventional single-reference MP2
New Approaches for ab initio Calculations of Molecules with Strong Electron Correlation
Reliable quantum chemical methods for the description of molecules with
dense-lying frontier orbitals are needed in the context of many chemical
compounds and reactions. Here, we review developments that led to our
newcomputational toolbo x which implements the quantum chemical density matrix
renormalization group in a second-generation algorithm. We present an overview
of the different components of this toolbox.Comment: 19 pages, 1 tabl
A quantum-mechanical perspective on linear response theory within polarizable embedding
The derivation of linear response theory within polarizable embedding is
carried out from a rigorous quantum-mechanical treatment of a composite system.
Two different subsystem decompositions (symmetric and nonsymmetric) of the
linear response function are presented, and the pole structures as well as
residues of the individual terms are analyzed and discussed. This theoretical
analysis clarifies which form of the response function to use in polarizable
embedding, and we highlight complications in separating out subsystem
contributions to molecular properties. For example, based on the nonsymmetric
decomposition of the complex linear response function, we derive conservation
laws for integrated absorption cross sections, providing a solid basis for
proper calculations of the intersubsystem intensity borrowing inherent to
coupled subsystems and how that can lead to negative subsystem intensities. We
finally identify steps and approximations required to achieve the transition
from a quantum-mechanical description of the composite system to polarizable
embedding with a classical treatment of the environment, thus providing a
thorough justification for the descriptions used in polarizable embedding
models
The Dalton quantum chemistry program system
Dalton is a powerful general\u2010purpose program system for the study of molecular electronic structure at the Hartree\u2013Fock, Kohn\u2013Sham, multiconfigurational self\u2010consistent\u2010field, M\uf8ller\u2013Plesset, configuration\u2010interaction, and coupled\u2010cluster levels of theory. Apart from the total energy, a wide variety of molecular properties may be calculated using these electronic\u2010structure models. Molecular gradients and Hessians are available for geometry optimizations, molecular dynamics, and vibrational studies, whereas magnetic resonance and optical activity can be studied in a gauge\u2010origin\u2010invariant manner. Frequency\u2010dependent molecular properties can be calculated using linear, quadratic, and cubic response theory. A large number of singlet and triplet perturbation operators are available for the study of one\u2010, two\u2010, and three\u2010photon processes. Environmental effects may be included using various dielectric\u2010medium and quantum\u2010mechanics/molecular\u2010mechanics models. Large molecules may be studied using linear\u2010scaling and massively parallel algorithms. Dalton is distributed at no cost from http://www.daltonprogram.org for a number of UNIX platform
Triggering redox activity in a thiophene compound: radical stabilization and coordination chemistry
The synthesis, metalation, and redox properties of an acyclic bis(iminothienyl)methene L− are presented. This π-conjugated anion displays pronounced redox activity, undergoing facile one-electron oxidation to the acyclic, metal-free, neutral radical L* on reaction with FeBr2. In contrast, reaction of L− with CuI forms the unique, neutral Cu2I2(L*) complex of a ligand-centered radical, whereas reaction with the stronger oxidant AgBF4 forms the metal-free radical dication L*2+
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