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We study the phenomenon of unconventional superconductivity in iron pnictides on the basis of localized-itinerant model. In this proposed model, superconductivity arises from the itinerant part of electrons, whereas antiferromagnetism arises from the localized part. The itinerant electrons move over the sea of localized electrons in antiferromagnetic alignment and interact with them resulting in excitation of magnons. We find that triplet pairing of itinerant electrons via magnons is possible in checkerboard antiferromagnetic spin configuration of the substances CaFe2As2 and BaFe2As2 in pure form for umklapp scattering with scattering wave vector Q = ( 1 , 1 ) , in the unit of π/a where a being one orthorhombic crystal parameter, which is the nesting vector between two Fermi surfaces. The interaction potential figured out in this way, increases with the decrease in nearest neighbour (NN) exchange couplings. Under ambient pressure, with stripe antiferromagnetic spin configuration, a very small value of coupling constant is obtained which does not give rise to superconductivity. The critical temperature of superconductivity of the substances CaFe2As2 and BaFe2As2 in higher pressure checkerboard antiferromagnetic spin configuration are found to be 12.12 K and 29.95 K respectively which are in agreement with the experimental results.

In this letter, a possible reason behind selective host-guest organization in the initial stage of sI methane hydrate nucleation is provided, through density functional theory based calculations. In doing so, we have connected earlier experimental and theoretical observations on the structure and energetics of sI methane hydrate to our findings. Geometry and relative stability of small (H 2 O) 5 and (H 2 O) 6 clusters, presence of CH 4 guest, integrity and cavity radius of (H 2 O) 20 and (H 2 O) 24 , as well as the weak van der Waals type of forces, particularly dispersion interaction, are major factors responsible for initial formation of methane encapsulated dodecahedron cavity over tetrakaidecahedron.

We have investigated four different already synthesized meta- and para-connected diradicals to get their magnetic exchange coupling constants (J) through generalized valence bond (GVB) approach and also with Hartree-Fock (HF) theory with a view to establish the superiority of GVB method over HF theory for the prediction of J. In doing so, at first optimization of these molecules are done at B3LYP level of density functional theory using the 6-31++g(d) basis set. With these optimized geometries, J values are computed at GVB method along with TZV basis set and these values are compared with those of the J values obtained using HF/TZV level of theory. Nonetheless, J values found in both the theoretical processes are also compared with the experimental findings. Thus, the supremacy of the wavefunction based GVB method in predicting J is established in comparison with the HF method.

This work delves into the conductance property of transition metal (Ti, V, Cr, Mn, Fe, Co, Ni) encapsulated C20 endohedral metallofullerenes (EMFs), sandwiched between non-magnetic gold electrodes. DFT optimization of the designed systems casts the encapsulated metal at the center of the C20 cage irrespective of the system and spin states. Application of density functional theory coupled with non-equilibrium green function techniques on these EMFs results variant transport characteristics for different metal encapsulation, represented through transmission spectra at zero bias and current vs. voltage (I-V) plots. Spin-polarized electrical transport has been observed for the Mn, Fe, Co and Ni encapsulated C20, while no such spin polarization could be traced for Ti, V and Cr both in absence and presence of applied bias. The EMFs with Mn, Fe, Co and Ni display half-metallic behavior and significant spin-filtering efficiency, which reaches maximum value for the Fe encapsulated C20. Spin-resolved electrical conductance in the Mn/Fe/Co/Ni-EMFs are explained by density of states, projected density of states at zero bias and molecular orbital analyses, which reveal the contribution of C20 electrons towards the down-spin-polarized transmission for Fe/Co/Ni-EMFs, while for the Mn-EMF, the metal d-electrons mostly participate in the up-spin-polarized transmission. Taken these observations together, the present computational study showcases the importance of encapsulating transition metal inside carbon cages in inducing spin polarization and thus provides an aid to tailor suitable molecules for spintronic applications.

In this note, we have theoretically evaluated second-order nonlinear optical (NLO) responses of two structural isomeric forms of three different green fluorescent protein chromophores (viz. cyan fluorescent protein (CFP), orange variant of Discosoma coral (DsRed) and Zoanthus sp. (zFP)) coupled bis-TEMPO based photomagnetic high-spin diradicals by measuring the second-order effect of first molecular hyperpolarizabilities. The magnetic character and certain biological applicabilities of these diradicals are investigated theoretically in one of our recent papers (Bhattacharya et al., 2013). The second-order total molecular hyperpolarizabilities of these molecules are correlated with their respective HOMO–LUMO energy gaps (ΔEH-L) and also with the extent of the π-network of the couplers. Relations between ΔEH-L, chemical hardness, NLO response and exchange energy (Bhattacharya et al., 2013) are made. The applicability of these diradicals in the light of NLO properties are also highlighted in this note. Here these resulting fluoroprotein coupled diradicals can switch back and forth between two isomeric states of different colors upon irradiation by light of appropriate wavelength. Nonetheless, the magnitude of second-order NLO responses is different for different isomeric forms; hence we made a correlation between the color codes with the magnitude of second-order NLO responses. Thence with just simple visual inspection one can understand the extent of second-order of NLO response for these diradicals.

Three different pairs of isomeric trans and corresponding cis forms of fluoroprotein chromophore (FPC) fragments coupled bis-2,2′,6,6′-tetramethylpiperidinyloxy (TEMPO) based diradicals have been designed, and subsequently the magnetic properties of these diradicals have been investigated. We have used blue shifted variants of green fluorescent protein (cyan fluorescent protein or CFP), orange variant of Discosoma coral (DsRed) and yellow mutant of Zoanthus sp. (zFP) chromophore fragments as spin couplers. Along with the gas phase magnetic exchange coupling constant ( J ), the zero field splitting parameters (ZFSs) of all the diradicals in water and also in blood plasma medium have been calculated. We have found the all molecules to be ferromagnetic in nature with trans diradicals having lower spin–spin ZFS parameter (| D |) and the higher magnetic exchange coupling constants values than their respective fluorescent cis isomers. This has also been noted that due to the presence of the fluoroprotein chromophores, the resulting diradicals can switch back and forth between the isomeric trans and cis forms upon irradiation of the proper wavelength of light. The time dependent density functional (TDDFT) technique is used to measure the actual wavelength of this conversion for these diradicals. A comparison between the evaluated ZFS values with other nitroxide based synthesized molecular systems which are in use as magnetic resonance imaging (MRI) contrast agents (MRICAs) indicates that if these diradicals are synthesized, they can be useful as MRICAs. A relation between the distance and the spin–spin ZFS parameter is also made. To support this fact, we have computed the solvent accessible surface area, molecular volume, the surface to volume ratio, and molecular diameter of each diradical species to address their potentiality to act as contrast agents in MRI. Finally the ability of these diradicals to act as MRICAs is further justified with the help of molecular electrostatic potential surfaces.

We present density functional theory based calculations on six diradicals in which imino nitroxide (IN) and IN or oxo- or phospha-verdazyl radical centres are linked by fragments of structures called cyan, blue and green fluorescent protein, respectively. In the latter two variants the indolyl fragment of cyan fluorescent protein (CFP) is replaced by phenyl and phenol moiety to obtain blue fluorescent protein Y66F GFP (BFPF) and green fluorescent protein (GFP). The photoinduced cis–trans isomerization of these diradical substituted chromophores is accompanied by changes from antiferro- to ferro-magnetic spin coupling on the radical centres. The calculations in this work provide predictions of the change of magnetic response to photo excitation. The estimated magnetic exchange coupling constants associated with cis–trans isomerization of the chromophores can be useful parameters in designing radical substituted isomeric fluorescent chromophores. Upon irradiation with light of appropriate wavelength, the dark trans diradicals turn into their fluorescent cis isomers. Therefore, photoinduced magnetic crossover from antiferromagnetic to ferromagnetic regime associated with the change in color would be noticed in all three cases. This is a novel observation in case of the systems with GFP chromophore and its variants. These diradicals are potentially very useful in different applications and biocompatibility of such systems makes them prospective in different in vivo treatments. Moreover, change in color associated with magnetic crossover for these diradicals increases their suitability as biological taggers.

If one applies an rf magnetic field parallel to a strong static field, temperature of a ferromagnetic substance rises due to ferromagnetic resonance. This phenomenon finds immediate application in the field of hyperthermic oncology. In this work, we have found expression for the increase in temperature when a ferromagnetic material is placed under a static and a varying magnetic field of high frequency through spin wave approach. The numerical value of this increment of local temperature has been estimated for yttrium iron garnet (YIG). We also have examined the possibility of enhancing the temperature of a ferromagnetic material only by applying a strong static field.

We have investigated mechanistic pathways for four different electrophilic addition processes with benzene (monoprotonation, diprotonation, monomethylation and dimethylation) in DFT framework. In all the cases, transition states have been isolated and characterized through intrinsic reaction coordinate calculations. Fukui functions, local softness values, local charge densities, different electrophile affinities, thermochemical parameters have been calculated and correlated. Fukui functions and local softness values have been found to be reliable descriptors for kinetically controlled products; whereas, prediction of thermodynamically controlled products can be made through local charge density values. Electrophile affinities are found to be additive and correlate well with thermochemical parameters.

Manganese atom having number of unpaired electrons appears as a strong building block for molecular magnets. This prompts the investigation of the magnetic behavior of Mn based clusters. In this work, we have studied the magnetic characteristics of neutral Mn dimer, charged Mn dimer and its oxide. The magnetic characteristics of the chosen systems have been quantified in terms of intramolecular exchange coupling constant. The computation is performed in the framework of density functional theory using broken symmetry approach. We obtain positive values of magnetic exchange coupling constants for all these three species and hence their ferromagnetic ground states are recognized. Direct exchange of spins between the sets of d-electrons in the metals is found responsible for the ferromagnetic behavior of these clusters. In case of Mn 2 O − , super exchange, another possible exchange mechanism, is barred due to unavailability of vacant metal d-orbitals. Dependence of exchange coupling constant on intermetallic distance is also studied taking these Mn clusters as reference, which shows a maximum value of coupling constant in the proximity of equilibrium bond distance.