Chemical Physics Letters (v.461, #1-3)

Contents (iii-xi).

Downhill protein folding allows protein populations to be probed everywhere along the reaction coordinate.Proteins are held together by many weak contacts, each corresponding to a local reaction coordinate. The activation barrier for folding is distributed along a resultant global folding coordinate. Hence folding barriers are low, and could even become comparable to the thermal energy kT. In that case, proteins become downhill folders, with folding times in the microsecond region. Small barriers allow the diffusion of population along the reaction coordinate – the molecular rate – to be observed directly. Five simple free energy building blocks can explain all experimentally observed fast folding data, revealing a range of behaviors from low barrier crossings to completely downhill folding.

The one- and two-photon absorption properties of a series of novel spiro-linked oligofluorenes and their derivatives are predicted theoretically.A series of spiro-linked oligofluorenes and derivatives have been theoretically studied with DFT/B3LYP/6-31 G(d) associated with Zindo/SOS method. The increase of two-photon absorption (TPA) cross-sections of spiro-linked oligofluorenes with chain length can be expressed by formula lgδ TPA(N) =  αlgN  + lgδ TPA(N  = 1). (N is the number of fluorene rings in the molecule). The effects of Si center and interconnection pattern of fluorene rings on TPA response are discussed. The introduction of ethyne and ethene conjugated-bridges can further increase TPA cross-sections in longer wavelength range. The maximum one-photon absorption wavelengths of all molecules are less than 400 nm, which demonstrate spiro-linked oligofluorenes and derivatives exhibit at the same time strong two-photon absorption and good transparency.

Reactivity of nitrobenzofurazan towards nucleophiles: Insights from DFT by N. Latelli; S. Zeroual; N. Ouddai; M. Mokhtari; I. Ciofini (16-20).
The reactivity of 4-chloro-7-nitrobenzofurazan with two classes of nucleophiles has been investigated using DFT and interpreted using chemical reactivity descriptors.The reactivity of 4-chloro-7-nitrobenzofurazan (NBD-Cl) with two different classes of nucleophiles (namely 4-substituted anilines and thiophenols) has been investigated using density functional theory (DFT) and interpreted using chemical reactivity descriptors. A good correlation between chemical descriptors, such as ionisation potential and nucleophilicity, and reaction rates was found for 4-substituted anilines. On this basis, predictions on the reactivity of 4-substituted thiophenols were drawn. To get further insights on the mechanism, the transition state corresponding to the rate determining step was characterized for each reaction and the barrier heights related to the experimental reaction rates and the computed chemical descriptors.

Gas-phase ion chemistry of BF3/CH4 mixtures: Activation of methane by BF 2 + ions by Federico Pepi; Alessandra Tata; Stefania Garzoli; Marzio Rosi (21-27).
The reaction of BFCH 3 + ions with NH3 leads to cyclic FBCH4N+ ion, an interesting new species characterized by the formation of a carbon–nitrogen bond.The gas-phase ion chemistry of BF3/CH4 mixtures was investigated by the joint application of mass-spectrometric and ab initio theoretical methods. The addition of BF 2 + to CH4 led to the observation of BF 2 CH 4 + and BFCH 3 + ions in the gas phase. Collisionally activated dissociation (CAD) mass-spectrometric results and theoretical calculations performed at the B3LYP and CCSD(T) levels identified the F 2 B – CH 4 + and F – B – CH 3 + ions as the most stable isomers on the [BF n CH n +2]+ (n  = 1, 2) potential energy surfaces. It is worth noting that C–N bond formation was observed when BFCH 3 + ions were allowed to react with NH3.

The square planar structure suggested from the infrared spectrum of Pt+(CO)4.Platinum carbonyl complexes, Pt+(CO) n , are generated using laser vaporization and probed with infrared photodissociation spectroscopy in the C–O stretching region. No fragmentation is observed for the n  = 1–3 complexes, indicative of their high binding energies. Pt+(CO)4 has enhanced abundance in cluster growth and is difficult to fragment, producing a noisy spectrum. The Pt+(CO)5,6 complexes fragment efficiently, producing the n  = 4 ion. Their spectra, with carbonyl absorptions blue-shifted ∼10 cm−1 with respect to free CO, represent the resonances of Pt+(CO)4 detected by elimination of weakly bound external ligands. Pt+(CO)4 is indicated to have filled coordination and a high symmetry structure.

Based on recently B3LYP calculated spin–spin coupling constants (SSCC) of several small molecules with cc-pVxZ, cc-pCVxZ, cc-pCVxZ-sd and cc-pCVxZ-sd+t basis sets, a reasonably fit, using the two-parameter formula, to the complete basis set limit (CBS) is shown.Calculation of water’s 1 J(OH) using B3LYP/cc-pCVxZ and B3LYP/pcJ-n significantly improved the FC term and its total convergence.Based on B3LYP spin–spin coupling constants (SSCC) of several molecules calculated with cc-pVxZ, cc-pCVxZ, cc-pCVxZ-sd and cc-pCVxZ-sd+t basis sets, a reasonably fit, using the two-parameter formula, to the Kohn–Sham complete basis set limit (CBS) is shown. Improvement in the CBS values going from cc-pVxZ to the most elaborated cc-pCVxZ-sd+t basis set family is observed: standard deviation for all data drops from 33.7 to 23.1, and from 6.0 to 4.8 Hz after excluding problematic 1 J(F,H) and 1 J(F,C). Calculation of water’s 1 J(OH) using B3LYP/cc-pCVxZ and B3LYP/pcJ-n significantly improved the FC term convergence.

The vibrational frequencies of pyrite-type compounds were calculated within the LCAO-DFT scheme. They are directly related to the bonding within the Pn–Pn dumbbells and the bonding of the dumbbells to the neighbouring A atoms that forma an fcc type sublattice.DFT calculations on pyrite-type SiP2, PtN2, PtP2 and PtAs2 have been performed using the Crystal06 code for the simulation of fundamental vibrational frequencies in periodic systems. The obtained spectra of SiP2, PtP2 and PtAs2 are in very good agreement with available Raman and IR studies. The vibrational spectra of the novel PtN2 is calculated including Raman and IR data. The results are discussed with respect to the dumbbell structural units and related compounds.

Snapshots of toroid (a) and core–shell (b), for long polymer chains.The folding transition of single, long semiflexible polymers was studied with special emphasis on the chain length effect using Monte Carlo simulations. While a relatively short chain (10–25 Kuhn segments) undergoes a large discrete transition between swollen coil and compact toroid conformations, a long chain (50 Kuhn segments) exhibits an intrachain segregated state between the disordered coil and ordered toroid.

CH2I2 adsorption and dissociation on Ag(1 1 1) surface using density functional theory study by Bo-Tao Teng; Wei-Xin Huang; Feng-Min Wu; Xiao-Dong Wen; Shi-Yu Jiang (47-52).
The potential energy surfaces for iodine atom dissociation reaction of CH2I2 at different coverages indicate that CH2I2(a) on Ag(1 1 1) at low coverage is much more prone to dissociate and generate CH2(a) species than those at high coverage.Density functional theory (DFT) calculation has been performed to study the adsorption and dissociation of CH2I2 on Ag(1 1 1) surface at different coverages. CH2I2(a) with two iodine atoms bonded to Ag(1 1 1) is the main stable adsorbed species at low coverage, while CH2I2(a) with one iodine atom bonded to Ag(1 1 1) will dominate on the surface at high coverage. The dissociation barriers of CH2I2 to generate CH2(a) species on Ag(1 1 1) also increase with the increase of coverage. Analysis of density of states shows that relatively strong interactions between CH2I2(a) and Ag(1 1 1) surface exist.

IR absorption spectrum of radical C2H in solid Ne.Irradiation of a sample near 3.0 K of ethyne, C2H2, in a Ne matrix with vacuum ultraviolet radiation from a synchrotron produced various photoproducts that were analyzed through their infrared absorption spectra. Photolysis with light at 171 nm yielded as major product ethynyl radical, C2H. Based on observed D-isotopic shifts and comparison of observed vibrational wavenumbers and isotopic ratios with values predicted with quantum-chemical calculations, we identified the ν 1 and ν 3 modes of C2H to be 3293.3 and 1835.7 cm−1, respectively. The fundamental C–H stretching ν 1 mode of C2H was determined for the first time in solid Ne.

Double-diamond structure with space group Pn 3 ¯ m formed in the melt of amphiphilic copolymers.Using dissipative particle dynamics, we study microphase separation in the melt of amphiphilic/nonpolar diblock copolymers in the strong segregation regime. We show that the phase diagram for the copolymers with an amphiphilic block can be significantly different from that known for the conventional diblocks. In the limit of significant amphiphilicity (surface activity), the resulting morphology corresponds to thin channels and slits of amphiphilic units penetrating through the matrix of a majority nonpolar component. The physical reason behind this is connected with the surface activity of amphiphilic monomers, which forces them to be located in the regions of maximum concentration gradient.

Silk-inspired ‘molecular chimeras’: Atomistic simulation of nanoarchitectures based on thiophene–peptide copolymers by Olga A. Gus’kova; Pavel G. Khalatur; Peter Bäuerle; Alexei R. Khokhlov (64-70).
Snapshot from MD simulation of triblock oligopeptide–oligothophene–oligopeptide ‘molecular chimeras’ adsorbed on a graphite surface.We present the results of an extensive molecular dynamics simulation aimed at investigating the structural organization of bioinspired copolymers consisting of thiophene and silk-forming peptide blocks. These ‘molecular chimeras’ are studied, apparently for the first time, both in adsorbed state (on the surface of graphite) and in dilute solution. It is shown that the attachment of the terminal peptide blocks to the oligothiophene fragments can lead to the formation of various stable supramolecular structures, including bilayers and fibrils, which can be promising for nanoelectronic applications.

Different structures of the decahedral Pd104Pt43 cluster at 600 and 860 KWe use canonical Monte Carlo simulations to study the melting of icosahedral and decahedral onion-ring Pd–Pt bimetallic clusters consisting of 147 atoms. Structural transition from decahedron to icosahedron-like is found for the decahedral onion-ring 147-atom Pd–Pt cluster before melting. Also, the melting point of the decahedral onion-ring 147-atom Pd–Pt cluster is lower than the corresponding icosahedral cluster. In addition, at the higher temperatures after melting, the four-shell onion-ring structure becomes unstable, and can be transformed into the three-shell onion-like structure.

Time-resolved morphological study of organic thin film solar cells based on calcium/aluminium cathode material by B. Paci; A. Generosi; V. Rossi Albertini; P. Perfetti; R. de Bettignies; C. Sentein (77-81).
In situ time-resolved energy dispersive X-ray reflectometry measurements performed on calcium/aluminium cathode organic solar cells, under working conditions.The stability and degradation of calcium/aluminium cathode organic solar cells are investigated in situ by time-resolved energy dispersive X-ray reflectometry. They combine the good charge carrier separation and transport properties of the poly(3-hexylthiophene-2,5-diyl):C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction blend and the capability of the calcium/aluminium cathode to improve the fill factor and the open circuit voltage, with respect to aluminium cathodes cells. The study focuses on the crucial problem of the device structural/morphological stability in working condition. It aims to detect and control possible morphological variations at the various interfaces and to correlate these changes to the device aging.

Zirconium oxide anodic films: Optical and structural properties by Francisco Trivinho-Strixino; Francisco E.G. Guimarães; Ernesto C. Pereira (82-86).
Anodic ZrO2 films exhibit an important photoluminescence behavior in the visible spectral range. The origin of this emission is proposed to be associated with defect centers (F-center) generated during the film preparation.The photoluminescence of anodically prepared ZrO2 films was investigated. Morphological and microstructural analyses reveal an oxide surface covered by a high distribution of blisters and the major crystallographic structure is the monoclinic phase with small quantities of tetragonal phase. The photoluminescence reveals a broad emission band in the range between 350 and 650 nm. It was proposed that the photoluminescence mechanism is originated from the emission of a recombination center related to defects (oxygen vacancies) formed during Zr anodization.

Host–guest interaction in endohedral fullerenes by C.N. Ramachandran; Debmalya Roy; N. Sathyamurthy (87-92).
Ab initio calculations at the Hartree–Fock and second order Møller–Plesset Perturbation level using the 6-31G basis set show significant interaction between the C60 fullerene and species like H+ and H, but much less interaction with Li+, He and H2. The influence of the guest species on the HOMO–LUMO energy gap and the electrostatic potential of the host cage is also discussed.Ab initio calculations using Hartree–Fock (HF) and second order Møller–Plesset perturbation (MP2) theoretic methods using the 6-31G basis set have been used to study the interaction between H+, H, He, Li+ and H2 with C60 fullerene. The barrier for penetration of the guest species through the center of the hexagon of the cage is reported. There is a substantial change in the HOMO–LUMO energy gap for the endohedral complex of C60 fullerene when the proton or hydride ion is encapsulated. The calculated HOMO–LUMO energy gap for the endohedral complex is correlated with the orbital energy of the guest species. The interaction of the guest species with the host is examined by a critical point analysis using Bader’s theory of atoms in molecules. The effect of the guest species on the electrostatic potential inside and outside of the C60 cage is also discussed.

Charge carrier dynamics in TiO2 nanoparticles at various temperatures by Tom J. Savenije; Annemarie Huijser; Martien J.W. Vermeulen; Ryuzi Katoh (93-96).
Time-resolved microwave photo-conductance measurements at various temperatures were carried out to characterize the charge carrier dynamics in TiO2 nanoparticles.In order to characterize electron traps in TiO2 nanoparticles, we performed time-resolved microwave photoconductance measurements at temperatures ranging from 93 to 373 K. It was found that the initial photoconductance was almost temperature independent, while the photoconductance decay was thermally activated. From the results it is inferred that on pulsed UV illumination, part of the conduction band electrons decay on a picosecond timescale by deep trapping or by recombination. Thermally activated interparticle hopping towards the bulk of the TiO2 film, results in deep trapping of remaining conduction band electrons, which explains the photo-conductance decay on the microsecond timescale.

Effect of vibrations on the formation of gold nanoparticle aggregates at the toluene–water interface by M.K. Bera; M.K. Sanyal; R. Banerjee; K.P. Kalyanikutty; C.N.R. Rao (97-101).
We have shown profound effect of vibration on the formation of aggregates of gold nanoparticles using UHV-AFM measurements.We report on the profound effect of vibrations on the formation and ordering of aggregates of gold nanoparticles formed at the toluene–water interface using high-resolution atomic force microscopy. We obtain 2.3 nm thick monolayer films composed of gold nanoparticles of 1.2 nm diameter, with an organic capping of 1.1 nm, at the early stages of the reaction carried out on an anti-vibration table. Reaction carried out on a table without anti-vibration fixtures produces thicker films composed of 1.2 nm nanoparticles. The results demonstrate that the reduction reaction carried out at the toluene–water interface on an anti-vibration table provides a suitable means for large scale synthesis of Au-55 like nanoparticles.

Investigate the energy band positions of Aurivillius phase Bi2MoO6 by DFT calculation and photoelectrochemical measurement.The pressed Bi2MoO6 film electrode was prepared from hydrothermal synthesized Aurivillius phase Bi2MoO6 powders. The physical properties of the powder were characterized by X-ray diffraction and UV–vis diffuse reflectance spectra. The band structure of Bi2MoO6 was investigated by DFT calculations, and the band potentials were measured by slurry method. The flatband potential was determined as −0.32 V vs. NHE at pH 7. Potential and wavelength dependence of photocurrent were investigated, and the effects of reductants on incident-photon-to-current efficiencies were discussed.

Liquid crystal diffusion in thin films studied by magnetic resonance imaging.NMR is a well-established technique to study orientational order and dynamics of liquid crystals. We present a study of liquid crystal MBBA properties in both bulk and constrained thin film capillaries (thickness ∼200 μm) at different temperatures. Translational self-diffusion measurements have been performed in the nematic and isotropic phases. For constrained samples in the isotropic phase the diffusion coefficient has orientational dependence (D /D  ≈ 7). To characterize the diffusion distribution of liquid crystals within the thin film, a diffusion mapping experiment was performed. The method provides accurate diffusion coefficients (D and D ) measurements with a pixel resolution of 4 μm.

Selection rules for Brillouin light scattering from eigenvibrations of a sphere by Y. Li; H.S. Lim; S.C. Ng; Z.K. Wang; M.H. Kuok (111-113).
Selection rules governing inelastic light scattering from the eigenvibrations of a submicron sphere have been derived from group theory, and have been experimentally verified by well-resolved Brillouin spectra.Selection rules governing Brillouin light scattering from the vibrational eigenmodes of a homogeneous, free-surface submicron sphere have been derived using group theory. The derivation is for the condition where the sphere diameter is of the order of the excitation light wavelength. Well-resolved spectral data obtained from Brillouin light scattering from submicron silica spheres provide experimental verification of the selection rules.

Double-layer silica core-shell nanospheres with superparamagnetic and fluorescent functionalities by Ling Li; Eugene Shi Guang Choo; Zhaoyang Liu; Jun Ding; Junmin Xue (114-117).
Double-layer silica core-shell nanospheres with superparamagnetic and fluorescent functionalities were synthesized through a water-in-oil reverse microemulsion route.Superparamagnetic nanocrystals and QDs have attracted much attention due to their great potential in biomedical applications. In this Letter, the interest arises from the possibility of positioning magnetic and optical particles at different layers in a silica core-shell structure. In this double layered core-shell structure, QDs are sandwiched between the core and shell silica layers, whereas Fe3O4 nanoparticles are entrapped in the core. This structure was designed with the idea of preventing direct contact between QDs and Fe3O4 nanoparticles, thus minimizing the ‘quenching effect’ and achieving high quality magnetic-fluorescent dual functionality. The obtained double-layer silica nanospheres are promising candidates for applications in simultaneous biolabeling, imaging, cell sorting and separation.

The dynamic second-order hyperpolarizabilities of Si3 and Si4 clusters have been calculated using the highly accurate coupled cluster cubic response theory at CCSD/aug-cc-pVTZ level.The dynamic second-order hyperpolarizabilities γ of Si3 and Si4 clusters have been first calculated using the highly accurate coupled cluster cubic response theory. At ω  = 0.43 au (1064 nm), the dynamic γ values of the Si3 and Si4 clusters are 46.26 × 10−36 and 92.50 × 10−36  esu, respectively. The dynamic γ values of the Si3 and Si4 clusters exhibit wide non-resonant optical region (ω  < 0.04 au). The magnitude of macroscopic second-order hyperpolarizabilities χ (3)(−3ω; ω, ω, ω) is of the order of 10−12  esu at ω  < 0.04 au. Small Si clusters might be good candidates for nonlinear optical materials in infrared range.

The contact line tension changed sign, from negative to positive, with a decrease in the bubble size.Small bubbles with average diameters of 10 and 100 nm were spontaneously formed on the surface of Au(1 1 1) in contact with air–saturated water and ethanol–water 1/4 solution, respectively. Analyses of topographic images of the bubbles demonstrated that the contact line tension changed sign, from negative to positive, with a decrease in the bubble size. Despite the apparent stability and long lifetime of the bubbles, both of these bubble systems were found to be thermodynamically unstable, as estimated from the free energies of bubble formation by combination of their surface and line energies.

Dimensional scaling for stability of two particles in a dipole field by Alejandro Ferrón; Pablo Serra; Sabre Kais (127-130).
We find that calculations at the large-D limit are much simpler that D  = 3, yet yield similar results for stability diagrams.We present dimensional scaling calculations for the critical parameters needed to bind one and two-electrons to a finite linear dipole field and the stability diagram for the hydrogen–antihydrogen like molecules. We find that calculations at the large-D limit are much simpler that D  = 3, yet yield similar results for the critical parameters and the stability diagrams.

Intracellular quantification by surface enhanced Raman spectroscopy by Ali Shamsaie; Jordan Heim; Ahmet Ali Yanik; Joseph Irudayaraj (131-135).
An intracellular quantification procedure based on the pasmon or back scattering peak of gold nanoparticles is presented. This concept was then used to estimate the lysosomal concentration of DAMP using surface enhanced Raman spectroscopy.Metallic nanoparticles in a cellular environment have a tendency to aggregate which poses a major obstacle in extending in vivo surface enhanced Raman spectroscopy (SERS) applications beyond the qualification and into quantification domain. We introduce and demonstrate a novel SERS technique that will enable precise quantification of exogenous chemicals in living human cells. Effective quantification of the local concentrations of a dinitrophenol derivative (DAMP) based on a normalizing technique is shown by utilizing gold nanoparticle entrapment in the lysosomal compartments in human cells. We believe that the quantification technique developed here is general and can be extended to different environments utilizing different types of nanoparticles beyond the intracellular scheme proposed.

The ozone ring closure as a test for multi-state multi-configurational second order perturbation theory (MS-CASPT2) by Luca De Vico; Luis Pegado; Jimmy Heimdal; Pär Söderhjelm; Björn O. Roos (136-141).
Ozone ring closure potential energy surfaces are explored at MS-CASPT2 level of theory. Their notable structures are localized and discussed.The open and closed forms of ozone and the path connecting them through a transition state, in C 2v symmetry, are explored using the multi-state, multi-configurational second order perturbative method, MS-CASPT2. It is demonstrated that, by using an ANO-L triple-ζ basis set, it is possible to set up an active space able to describe the otherwise troublesome transition state region. Both a conical intersection and a near degeneracy region between the 11A1 and 21A1 states are located in the vicinity of the transition state. The relative position of the intersection and the transition state are discussed.

For the first time, cartesian coordinate grid is employed to perform density functional calculations for atoms and molecules. Detailed results are presented for local and nonlocal exchange-correlation functionals.This is a follow-up of our recently proposed work on pseudopotential calculation [A.K. Roy, Int. J. Quant. Chem. 108 (2008) 837] of atoms and molecules within DFT framework, using cartesian coordinate grid. Detailed results are presented to demonstrate the usefulness, applicability of the same for a larger set of species (5 atoms; 53 molecules) and exchange-correlation functionals (local, nonlocal). A thorough comparison on total, component, ionization, atomization energies, eigenvalues, potential energy curves with available literature data shows excellent agreement. Additionally, HOMO energies for a series of molecules show significant improvements by using the Leeuwen–Baerends exchange potential, compared to other functionals considered. Comparison with experiments has been made, wherever possible.

We propose a practical approach, independent of the dimensionality of the problem, to model tunnelling effects in quasiclassical trajectories.A new practical approach to model tunnelling effects in quasiclassical trajectories is presented. The proposed method only requires the knowledge of the minimum energy pathway and is independent of the dimensionality of the problem. This approach uses the quasiclassical trajectory to locate the starting tunnelling point and avoids the definition of a tunnelling path. The tunnelling probability is computed as an average over WKB semiclassical probabilities for different possible pathways. Test results on the O(3P) + H2 reaction present a good agreement with quantum results on the same potential energy surface.

Complex quantum potential for 1D Eckart scattering; regions of high potential correspond to wave packet nodes when transiting the real-axis.In this study, we utilize numerical analytic continuation (NAC) to compute the complex quantum potential from the complex-extension of wave packets computed on the real-axis. The Cauchy method is utilized due to its robustness and flexibility. The Bohm and complex quantum potentials are compared; both potentials display complicated (but dissimilar) structure on the real-axis. We focus on generating numerically accurate maps of the evolving complex quantum potential for the Eckart barrier scattering problem. Although the complex quantum potential is initially constant, it later develops structures for the reflected wave packet that make it just as complicated as Bohm’s quantum potential, at least on the real-axis.

The local structures for Fe3+ and Mn2+ doped in TiO2 exist expansion distortions with the temperature increasing.Based on the 252 × 252 complete energy matrices for a d5 configuration ion in tetragonal ligand–field, the relationship between the defect structure and the EPR parameters b 2 0 , b 4 0 has been investigated. From our calculation, the local structure around impurity ion has been determined. Meanwhile, the temperature influence which varies with the kinds of doping ions on distortion has been discussed and the change curves of the zero-field splitting energies ΔE 1, ΔE 2 as functions of the local structure distortion parameter ΔR for TiO2:Fe3+ and TiO2:Mn2+ systems have been plotted.

Current measurements across a nematic cell submitted to an external voltage and its equivalent electrical circuit by R. Atasiei; A.L. Alexe-Ionescu; J.C. Dias; L.R. Evangelista; G. Barbero (164-169).
Parametric plot of the electrical current versus the voltage in an equivalent circuit representing the nematic cell, showing the role of the external resistance over which the potential drop is measured.The dielectric characterization of a liquid crystal by means of measurements of electric current flowing in the circuit is discussed. Few equivalent circuits are analyzed focusing the attention on the role of the resistance over which the potential drop is measured to determine the total current in the circuit. A comparison of our model with experimental data is presented, showing that the model works well when the maximum applied voltage is below the threshold voltage for Fréedericksz’s transition. In this case, the description of the nematic cell using a Maxwell–Wagner model is possible.