With a view to the design of hard magnets without rare earths we explore the possibility of large magnetocrystalline anisotropy energies in Heusler compounds that are unstable with respect to a tetragonal distortion. We consider the Heusler compounds Fe2YZ with Y = (Ni, Co, Pt), and Co2YZ with Y = (Ni, Fe, Pt) where, in both cases, Z = (Al, Ga, Ge, In, Sn). We find that for the Co2NiZ, Co2PtZ, and Fe2PtZ families the cubic phase is always, at T = 0, unstable with respect to a tetragonal distortion, while, in contrast, for the Fe2NiZ and Fe2CoZ families this is the case for only 2 compounds -Fe2CoGe and Fe2CoSn. For all compounds in which a tetragonal distortion occurs we calculate the MAE finding remarkably large values for the Pt containing Heuslers, but also large values for a number of the other compounds (e.g. Co2NiGa has an MAE of -2.11 MJ/m 3 ). The tendency to a tetragonal distortion we find to be strongly correlated with a high density of states at the Fermi level in the cubic phase. As a corollary to this fact we observe that upon doping compounds for which the cubic structure is stable such that the Fermi level enters a region of high DOS, a tetragonal distortion is induced and a correspondingly large value of the MAE is then observed.PACS numbers:

The energy spectra of three classes of polybenzenoid hydrocarbons with a large number N (N ≈ 10 3 ) of carbon atoms have been studied theoretically. It is shown that in the asymptotic case N f ∞ the energy gap (EG) ∆E(Nf∞) is different from zero if the electron correlation is taken into account; that is, the π systems calculated should possess semiconductor properties. The results for the ∆E(Nf∞) * 0 of the hydrocarbons are in qualitative agreement with the results calculated for the EG of three classes of 1-D ladder polymers, which can be considered as models of quasi-1-D graphite. With increasing width (L) of the polymers, the band gap ∆E(Lf∞) approaches a value different from zero. The problem of the existence of defect states of hydrocarbons with vacancies is briefly discussed.

We have performed first-principles calculations in order to understand the binding mechanism of Li atoms in disordered carbon materials that are used for negative electrodes of rechargeable lithium batteries. We used pyrene, anthracene, and phenanthrene molecules as parts of disordered carbon. We examined several binding sites for two Li atoms in these aromatics and found that they are bound with substantial negative binding energies. The most negative one was -142.8 kJ/mol for Li-containing pyrenes, -211.0 kJ/mol for anthracenes, and -146.2 kJ/mol for phenanthrenes at the B3LYP/6-31G*//HF/6-31G* level of calculation. Li atoms are bound to interstitial (ring-over) and edge sites. In addition to these binding mechanisms, we found that Li atoms could be bound, forming a Li dimer in anthracene and phenanthrene. Their binding energies are -200.5 and -146.2 kJ/mol, respectively, being larger in magnitude than Li 2 dissociation energy. These aromatics lose their planarity when they accommodate Li atoms. We found that larger distortion brings more strong interaction between the aromatics and Li atoms. The amount of energy required for the distortion increases in the order the interstitial, edge, and Li-dimerized sites. The highest occupied molecular orbital energy, which is closely related to the electrode potential during discharge process, decreases in that order. This energy lowering may be related to the origin of the hysteresis observed during the charge/discharge cycles.

Classical molecular dynamics simulations are employed to monitor the aggregation behavior of six bile salts (nonconjugated and glycine- and taurine-conjugated sodium cholate and sodium deoxycholate) with concentration of 10 mM in aqueous solution in the presence of 120 mM NaCl. There are 150 ns trajectories generated to characterize the systems. The largest stable aggregates are analyzed to determine their shape, size, and stabilizing forces. It is found that the aggregation is a hierarchical process and that its kinetics depends both on the number of hydroxyl groups in the steroid part of the molecules and on the type of conjugation. The micelles of all salts are similar in shape-deformed spheres or ellipsoids, which are stabilized by hydrophobic forces, acting between the steroid rings. The differences in the aggregation kinetics of the various conjugates are rationalized by the affinity for hydrogen bond formation for the glycine-modified salts or by the longer time needed to achieve optimum packing for the tauro derivatives. Evidence is provided for the hypothesis from the literature that the entirely hydrophobic core of all aggregates and the enhanced dynamics of the molecules therein should be among the prerequisites for their pronounced solubilization capacity for hydrophobic substances in vivo.

Calculations for model oligomers of the emeraldine salt with UBLYP/6-31G*/PCM are performed. The models differ in number of monomers, in the position of the counterions (Cl(-)), and in multiplicity. The molecular features affected most prominently by the protonation, namely, structure, energetics, and electron and spin density partitioning are analyzed. The results show unequivocally that the studied molecular characteristics are essentially size independent. The octamer profiles of all parameters are repeated in the dodecamer and the hexadecamer. The bipolaronic forms are energetically more favorable than the polaronic ones within the chosen protocol. The electronic structure in the intermediate multiplicities differs from the bipolaronic and polaronic periodicity. The geometrical changes and electron density redistribution upon increase of multiplicity illustrate the pathway of intramolecular bipolaron-polaron conversion. The orbital analysis rationalizes the observed behavior of the oligomers.

The structure and energy spectra of four classes of polybenzenoid hydrocarbons with different edge structures, a large number N (N ∼ 10 4 ) of carbon atoms, and different types of defect states (Tamm, Schottky and chemisorption states) have been studied theoretically. Several types of (probably) stable monoradicals and triplet biradicals with singly occupied MOs in the energy gap (molecular analogues of semiconductors with defect states) are characterized which could be used in the molecular electronics. A new concept for viewing high-spin π systems with ferromagnetically coupled electrons within the half-filled band of NBMOs in term of point defects (vacancies) was developed. The NBMOs can be considered as surface states (Tamm or Schottky).

Polyaniline (PANI) has attracted lasting interest due to its unconventional electronic, optical, and electro-optical properties. A rigid backbone polymer, PANI exhibits very strong intra-and intermolecular interactions governing its overall behavior. The purpose of this study is to investigate theoretically the contribution of the various inter-and intramolecular interactions to the process of organization of PANI macromolecules in aqueous environment. The structure and spectral conduct of oligoanilines (tetramers) in different oxidation states have been calculated at the Monte Carlo/Molecular Mechanics (MC/MM) and quantum chemical (QC) semiempirical level. The effect of protonation on the optical properties of partially and fully oxidized oligomers is discussed. The role of intermolecular interactions and the influence of water as a solvent are estimated for a number of hydrated clusters. Computed results and experimental spectra are compared.

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