91 research outputs found
A Novel Intermolecular Potential to Describe the Interaction Between the Azide Anion and Carbon Nanotubes
47 P.International audienceIn this contribution we propose a novel and accurate intermolecular potential that can be used for the simulation of the azide anion confined inside carbon nanotubes of arbitrary size. The peculiarity of our approach is to include an explicit term, modeling the induction attractive contributions from the negatively charged azide ion, that can be generalized to other ions confined in carbon nanotubes of different size and length. Through a series of accurate DLPNO-CCSD(T) calculations, we show that this potential reproduces the ab initio interaction energy to within a few kcal/mol. The potential is implemented in a molecular dynamics program, with which we carried out illustrative simulations to demonstrate the effectiveness of our approach. At last, the guidelines provided by this investigation can be applied to build up force fields for many neutral/ionic molecular species confined within carbon nanotubes; a crucial requirement to carry out molecular dynamics simulations under a variety of conditions
Spin partition
The Total Position Spread (TPS) tensor, defined as the second moment cumulant
of the position operator, is a key quantity to describe the mobility of
electrons in a molecule or an extended system. In the present investigation,
the partition of the TPS tensor according to spin variables is derived and
discussed. It is shown that, while the spin-summed TPS gives information on
charge mobility, the spin-partitioned TPS tensor becomes a powerful tool that
provides information about spin fluctuations. The case of the hydrogen
molecule is treated, both analytically, by using a 1s Slater-type orbital, and
numerically, at Full Configuration Interaction (FCI) level with a V6Z basis
set. It is found that, for very large inter-nuclear distances, the partitioned
tensor growths quadratically with the distance in some of the low-lying
electronic states. This fact is related to the presence of entanglement in the
wave function. Non-dimerized open chains described by a model Hubbard
Hamiltonian and linear hydrogen chains H n (n ≥ 2), composed of equally spaced
atoms, are also studied at FCI level. The hydrogen systems show the presence
of marked maxima for the spin-summed TPS (corresponding to a high charge
mobility) when the inter-nuclear distance is about 2 bohrs. This fact can be
associated to the presence of a Mott transition occurring in this region. The
spin-partitioned TPS tensor, on the other hand, has a quadratical growth at
long distances, a fact that corresponds to the high spin mobility in a
magnetic system
Publisher’s Note: “The total position-spread tensor: Spin partition” [J. Chem. Phys. 142, 094113 (2015)]
An application to Heisenberg spin chains
The spin partition of the Total Position-Spread (TPS) tensor has been
performed for one-dimensional Heisenberg chains with open boundary conditions.
Both the cases of a ferromagnetic (high-spin) and an anti-ferromagnetic (low-
spin) ground-state have been considered. In the case of a low-spin ground-
state, the use of alternating magnetic couplings allowed to investigate the
effect of spin-pairing. The behavior of the spin-partitioned TPS (SP-TPS)
tensor as a function of the number of sites turned to be closely related to
the presence of an energy gap between the ground-state and the first excited-
state at the thermodynamic limit. Indeed, a gapped energy spectrum is
associated to a linear growth of the SP-TPS tensor with the number of sites.
On the other hand, in gapless situations, the spread presents a faster-than-
linear growth, resulting in the divergence of its per-site value. Finally, for
the case of a high-spin wave function, an analytical expression of the
dependence of the SP-TPS on the number of sites n and the total spin-
projection Sz has been derived
Distributed Gaussian orbitals for molecular calculations: application to simple systems
International audienc
The Electronic Structure of Short Carbon Nanotubes: The Effects of Correlation
This paper presents a tight binding and ab initio study of finite zig-zag nanotubes of various diameters and lengths. The vertical energy spectra of such nanotubes are presented, as well as their spin multiplicities. The calculations performed using the tight binding approach show the existence of quasi-degenerate orbitals located around the Fermi level, thus suggesting the importance of high-quality ab initio methods, capable of a correct description of the nondynamical correlation. Such approaches (Complete Active Space SCF and Multireference Perturbation Theory calculations) were used in order to get accurate ground and nearest excited-state energies, along with the corresponding spin multiplicities
A simple position operator for periodic systems
International audienceWe present a position operator that is compatible with periodic boundary conditions (PBC). It is a one-body operator that can be applied in calculations of correlated materials by simply replacing the traditional position vector by the new definition. We show that it satisfies important fundamental as well as practical constraints. To illustrate the usefulness of the PBC position operator we apply it to the localization tensor, a key quantity that is able to differentiate metallic from insulating states. In particular, we show that the localization tensor given in terms of the PBC position operator yields the correct expression in the thermodynamic limit. Moreover, we show that it correctly distinguishes between finite precursors of metals and insulators
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