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In this review, rational design strategies for pure organic high-spin molecules with strong intramolecular magnetic interactions are presented, mostly focusing on the design of ferromagnetically coupled organic diradicals. After brief introduction of the calculation procedure for intramolecular magnetic coupling constant J using density functional theory (DFT), classification and standardization of magnetic character of well-known stable radicals and coupler units are discussed. Following the development of general strategy for the design of organic high-spin diradical by means of the classification and standardization scheme, applicability of the strategy for making pendent-type organic polyradical using zigzag graphene nanoribbons backbone is demonstrated. In a computational point of view, a scaling procedure and an optimization of Hartree–Fock exact exchange ratio of a hybrid DFT functional for better prediction of the J of diradicals are discussed. © 2015 Wiley Periodicals, Inc.

Using a linearized augmented cylindrical wave (LACW) approach taking into account the screw and rotational symmetries of carbon nanotubes (CNTs), the first principles technique for the spin-dependent band structure calculations of single-walled CNTs is developed. The method is applicable to any tubule independent on diameter and chirality. The calculations are based on the two-component relativistic Hamiltonian and muffin-tin and exchange approximations for potentials. As example, the band structures of the three chiral, one armchair, and one zigzag CNTs are calculated and presented as the functions of the screw wave vector and rotational quantum number. The spin-orbit coupling effects appear as splitting of some nonrelativistic electron bands equal to between the 0.01 and 1 meV depending on the CNTs structure, rotational quantum number, and Brillouin zone position. In agreement with previous empirical tight-binding theories, almost perfect polarization of spin is observed in the case of chiral tubules. © 2015 Wiley Periodicals, Inc.

The vibrational contribution to static and dynamic (hyper)polarizabilities for the zigzag (n ,0 ) family of BN nanotubes, with n ranging from (6,0) to (36,0), has been obtained. Calculations were done by the finite field nuclear relaxation (FF-NR) method for periodic systems, newly implemented in the CRYSTAL code, using the Coupled Perturbed Kohn-Sham (CPKS) scheme at the B3LYP/6-31G* level for the required electronic properties. Both transverse and transverse-longitudinal tensor components are determined by applying finite, i.e. static, fields in the transverse direction. The magnitude of the vibrational term increases with the radius of the nanotube as determined by the increase in the field-induced geometric deformation. The resulting vibrational (hyper)polarizability varies from being dominant to negligible, when compared with the corresponding static electronic contribution. This depends upon the radius, as well as the property and the component, in a systematic manner. The extension to longitudinal components, not yet available, will be implemented next. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem 00: 000-000, 2011

To address the choice of an appropriate value of electron smearing to facilitate self-consistent field (SCF) convergence, we studied the interaction of doxorubicin with short armchair and zigzag single-walled carbon nanotube models with closed caps, at the PWC/DNP level of density functional theory. By gradually reducing the electron smearing value from a large and most commonly used one of 0.005 Ha to zero (Fermi occupation), we monitored the changes in close contacts between the interacting species, total energy of the molecular system, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy and isosurfaces, HOMO-LUMO gap energy, and plots of electrostatic potential. It became evident that the commonly used smearing values of ≥0.001 Ha can alter the results significantly (for example, by one order of magnitude for HOMO–LUMO gap energy). We suggest the setting of electron smearing value at 0.0001 Ha, which does not imply too high computation cost and can guarantee the results close to the ones obtained with Fermi occupation. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

We investigate the transport properties (IxV curves and zero bias transmittance) of pristine graphene nanoribbons (GNRs) as well as doped with boron and nitrogen using an approach that combines nonequilibrium Green's functions and density functional theory (DFT) [NEGF-DFT]. Even for a pristine nanoribbon we verify a spin-filter effect under finite bias voltage when the leads have an antiparallel magnetization. The presence of the impurities at the edges of monohydrogenated zigzag GNRs changes dramatically the charge transport properties inducing a spin-polarized conductance. The IxV curves for these systems show that depending on the bias voltage the spin polarization can be inverted. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

We studied the dependence of mutual position of substitutional nitrogen atoms on the diameter and helicity of carbon nanotubes by using a hybrid B3LYP method. It was found that nitrogen atoms prefer occupying the para-site of a hexagon in armchair nanotubes and to be far apart in zigzag and chiral nanotubes. Even more preferable position for both zigzag and armchair nanotubes is nitrogen location in pentagonal rings at the tube caps. These results are supported by experimental data indicating low concentration of nitrogen (∼1 at%) in single-wall carbon nanotubes. On the basis of the results of calculations, we plotted current–voltage dependences for initial and nitrogen-doped carbon (CNx) nanotubes for elucidating the effect of nitrogen atoms configuration on field-emission characteristics of nanotubes. Nitrogen doping of carbon armchair nanotubes was shown to reduce the voltage threshold and to increase the current density significantly. The improvement of field-emission property for carbon zigzag nanotubes is expected when nitrogen atoms are near the tube tip. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

International audience; The static polarizabilities of a series of zigzag single walled carbon nanotubes [(m, 0) with m = 3n for m ¼ 7-25] were computed by the coupled perturbed Kohn-Sham (CPKS) coupled method recently implemented in the periodic CRYSTAL code. This method makes it possible to compute both the longitudinal and transverse polarizabilities with the same approach including the local field depolarization effects for the whole series of the investigated nanotubes. To quantify these effects, the unscreened longitudinal and transverse polarizabilities were also calculated within the linear response theory using the Sum Over States (SOS) method. Our results confirmed the inverse-square dependence on the bandgap of the longitudinal polarizabilities which are found weakly affected by the crystalline orbital relaxation. On the other hand, the comparison between the screened (CPKS) and unscreened (Sum Over States) transverse polarizabilities permit to calibrate more precisely the classical correction of Benedict et al.

Low-lying electronic terms of the hydrocarbons [C7]q (q = 1,3…9) were calculated within CASSCF approach. The [C7]q are H-terminated finite fragment of zigzag carbon nanotube (7,0) containing q cyclacene units. Each [C7]q with odd q has D7h point symmetry. All systems [Cn]q with arbitrary n and q have a specific set of the one-electron states characterized by the exponential decay along the cylinder axis that has been estimated within the Huckel model. These MOs are localized near bases of the cylinder and they are frontier in all systems [Cn]q. By hypothesis, the low-lying electronic terms of the hydrocarbons [C7]q (q = 1,3…9) are defined by four quasi-degenerate MOs e3′, e3″, which were included in the active space CASSCF. Low-lying electronic terms possess tier structure and systems with various q have the same order of states. The distance between tiers tends to asymptotic limit with increasing q. In this case, states in each tier become quasi-degenerate. The lowest tier consists of close set of states 1A, 3A, 5A, and each is characterized by single occupation of the localized MOs. Ground state of [C7]q is singlet; values of splitting are well described by Heisenberg hamiltonaian. The calculation results can be interpreted as reducing interaction of states localized on opposite cylinder bases with increasing fragment length. According to CASSCF, this interaction reduces exponentially in exact accordance with the similar characteristics of the localized MOs. There is a reason to believe that the value of splitting decreases less rapidly with increasing q if level of calculation is improved. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Constituting a part of odd electrons that are excluded from the covalent bonding, effectively unpaired electrons are posed by the singlet instability of the single-determinant broken spin-symmetry unrestricted Hartree–Fock (UBS HF) SCF solution. The correct determination of the total number of effectively unpaired electrons ND and its fraction on each NDA atom is well provided by the UBS HF solution. The NDA value is offered to be a quantifier of atomic chemical susceptibility (or equivalently, reactivity) thus highlighting targets that are the most favorable for addition reactions of any type. The approach is illustrated for two families involving fragments of arm-chair (n,n) and zigzag (m,0) single-walled nanotubes different by the length and end structure. Short and long tubes as well as tubes with capped end and open end, in the latter case, both hydrogen terminated and empty, are considered. Algorithms of the quantitative description of any length tubes are suggested. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Nonlinear mechanism of charge transport in low-dimensional nanosystems is considered, based on the account of the electron–phonon interaction that arises from the deformation dependencies of the on-site and the hopping interaction energies at intermediatevalues of the corresponding coupling constants. In particular, carbon-type hexagonal zigzag nanotubes are studied. It is shown that in the adiabatic approximation the electron–phonon coupling results in the self-trapping of carriers and formation of polaron (soliton) states. We show that the ground state of an electron in a nanotube is a low-dimensional polaron whose symmetry depends on the strength of the coupling. Namely, at relatively weak coupling, the polaron possesses quasi-one-dimensional properties and has an azimuthal symmetry. When the coupling constant exceeds some critical value, the azimuthal symmetry breaks down and two-dimensional polaron on the nanotube surface is formed. There are also polarons formed by the electrons in the conducting band (or by holes in the valence band) in semiconducting zigzag carbon nanotubes. Such polarons are described by the system of coupled nonlinear Schroedinger equations which admits single-band polarons, and entangled (hybridized) polarons. These two types of polarons possess different energies and symmetry properties. Single-band solutions are one-dimensional polarons: they are azimuthally symmetric and localized along the nanotube axis. The entangled polarons are also self-trapped along the nanotube axis, but possess an inner structure and are modulated around the nanotube. The entangled polarons break the azimuthal symmetry and their energy is lower than the energy of single-band polarons. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Within the framework of the surface Green's function matching method, we carried out calculations of the differential conductance of symmetrically and asymmetrically connected armchair (n, n)—zigzag (2n, 0)—armchair (n, n) carbon nanotubes with different lengths of the middle section and infinitely long ends. It is shown that the (n, n)/(2n, 0)/(n, n) segment, when n is not a multiple of 3, behaves as a quantum dot and has a conduction gap even for short middle segments. The position of the conductance peeks closest to the Fermi energy is determined by the interface states of the (n, n)/(2n, 0) junction. In addition to conductance peaks originating from the interface states of the (n, n)/(2n, 0) heterojunction, for sufficiently long middle zigzag nanotube, there are more conductance peaks in the vicinity of the Fermi energy, and those stem from the electronic structure of an individual finite (2n, 0) zigzag nanotube. However, positions of the peaks farther away from the Fermi energy cannot be found in such a simple way. Thus, such (n, n)/(2n, 0)/(n, n) quantum dot has singularities in its electronic properties, which may yield the Coulomb blockade effect should those strongly localized discrete levels become occupied. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

A linear-scaling electron-correlation method based on a truncated many-body expansion of the energies of molecular crystals has been applied to solid hydrogen fluoride. The energies, structures, harmonic, and anharmonic frequencies of the infrared- and/or Raman-active vibrations, phonon dispersions, and inelastic neutron scattering (INS) of the solid have been simulated employing an infinite, periodic, one- dimensional zigzag hydrogen-bonded chain model. The Hartree-Fock, second-order Moller-Plesset (MP2), coupled-cluster singles and doubles (CCSD), and CCSD with a noniterative triples correction (CCSD(T)) methods have been combined with the aug-cc- pVDZ and aug-cc-pVTZ basis sets and, in some instances, the counterpoise corrections of the basis-set superposition errors. The computed structural parameters agree with the observed within 0.1- 0.2 A and a few degrees, and the anharmonic frequencies obtained by vibrational MP2 allowing two-phonon couplings reproduce the observed frequencies

A previous self-consistent field molecular orbital method, able to describe systems having a large number of unpaired electrons, n, is reviewed and improved. This method is applied to the study of paramagnetism in large (1,000–16,000 atoms) zigzag carbon nanotubes, represented by their n values. The computational scheme is based on the Huckel neglect differential overlap approach. It is shown that dependence of n on the semiempirical parameters is very small, and so they can be removed from the calculation. Enhancement of the paramagnetism (increase of n), by use of a strong external magnetic field, is also studied. Finally, the dependence of the Fermi one-electron potential energies and the spin atomic densities on both the parameters and the shape of the nanotubes is analyzed. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

We investigate the effect of single lattice vacancies on the electron transport of graphene nanoribbon devices using the Landauer formalism within a tight binding approach. For a zigzag nanoribbon, a single lattice vacancy creates conductance dips in the low energy region, due to quasi bound states around the vacancy site. The energy of the bound state is related to the position of the lattice vacancy relative to the edge of the ribbon. We carried out calculations of electron transport properties in a T-junction device with lattice vacancies. We find that the effect of the vacancies depends on how energetically favorable the lattice vacancy is, which can be studied in terms of the alternate atomic structure of the graphene lattice. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Calculations of electron energetic characteristics of atomic hydrogen adsorption processes on internal and external surfaces of single-walled carbon nanotubes (tubulenes) of (6,6) “armchair” type having cylindrical symmetry have been carried out. This article reports also on the results of calculations of the atomic adsorption of oxygen on the surface of armchair and zigzag single-walled carbon nanotubes. Ionic-embedded covalent-cyclic cluster (IECCC) and molecular cluster models within the framework of semiempirical quantum chemical scheme MNDO well shown in the theoretical researches of electronic molecular and periodic solid-state structures have been used. The electronic and energy characteristics of the oxidation processes are analyzed, and the most energetically favorable oxide structure of the (6,6) nanotube is determined. It is found that narrow-gap nanotubes show a tendency to metallic behavior as their surface is saturated with oxygen atoms. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

First principles calculations of the electronic structure and total energy of narrow zigzag carbon nanotubes and their corresponding flat graphene strips have been carried out to assess the relative stability of the tube form. The results indicate that the smallest energetically stable carbon nanotube has a radius of about 0.2 nm. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

Using density functional theory, the ground-state structural, electronic, and magnetic properties of monoatomic lithium and sodium chains with low average density are investigated. A metallic, zigzag ground-state structure is predicted, but, most interestingly, stable equilibria for chains under tension are predicted to be ferromagnetic, which can be traced to exchange effects arising from occupation of the second subband as a function of the interatomic distance. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

Ab initio RHF/STO-3G, 3-21~, and 6-31~ band structure calculations were carried out on an extended regular chain of beryllium atoms (--Bez-)= to study its stability in comparison with corresponding data on finite chains and previously reported results on small elemental beryllium clusters. It is found that, starting from a linear distribution of atoms along the chain, the system evolves toward a regular zigzag planar structure. The total RHF energy per constituent atom is higher for the chains than for the clusters and is thereby suggestive of that isolated chains are unlikely to exist except in constraining channels of host structures. In the limit of the infinite chain, there are signs indicating that the RHF model is no longer an adequate description. 0 1996 John Wiley & Sons, Inc.

The basic problem of weak interaction between odd electrons in graphene is considered within the framework of broken spin-symmetry single-determinant approach. The modern implementations of the approach in the form of either unrestricted Hartree-Fock scheme (UBS HF) or spin-polarized DFT (UBS DFT) were discussed with particular attention to the applicability of spin-contaminated solutions of both techniques for the description of electronic properties of graphene. The UBS DFT applications generally reveal the open-shell character of the singlet state of the object and manifest an extra spin density concentrated on zigzag edge atoms. The UBS HF approach supports these findings but exhibits the extra spin density not only on zigzag edge atoms but on all atoms of the sheet. This very peculiarity permits to quantitatively describe the odd electron behavior via both enhanced chemical reactivity and magnetism. The former is presented in terms of a quantified atomic chemical susceptibility. The magnetic response of graphene sheets is provided by a collective action of all odd electrons and is size dependent. The relative magnetic coupling constant J decreases when the sheet size increases and J approaches the limit value of 10−2 to 10−3 kcal/mol when the sheet size is of a few nanometers. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010.

A new electron structural mechanism is proposed for interpretations in molecular biology, in addition to the already existing theories of Szent-Gyorgyi (semiconductivity) and Frohlich (long-range coherence). The hypothetical “intermediate-” or “zigzag states” (ZZS) of solids are investigated by the recursion (transfer matrix) method. The physical reality of the ZZS is discussed up to an SCFDODS-type theory and the necessity of additional less approximate investigations is emphasized. The possible role of ZZS in the explanation of: (i) translation in protein synthesis, (ii) energy and charge transfer processes, as well as (iii) initiation of protein formation is outlined.