Modified Point Charge Research Articles

Modified Interaction-Strength Interpolation Method as an Important Step toward Self-Consistent Calculations

The modified point charge plus continuum (mPC) model [ConstantinL. A.; Phys. Rev. B2019, 99, 085117] solves the important failures of the original counterpart, namely, the divergences when the reduced gradient of the density is large, such as in the tail of the density and in quasi-dimensional density regimes. The mPC allows us to define a modified interaction-strength interpolation (mISI) method inheriting these good features, which are important steps toward the full self-consistent treatment. Here, we provide an assessment of mISI for molecular systems (i.e., considering thermochemistry properties, correlation energies, vertical ionization potentials, and several noncovalent interactions), harmonium atoms, and functional derivatives in the strong-interaction limit. For all our tests, mISI provides a systematic improvement over the original ISI method. Semilocal approximations of the second-order Görling–Levy (GL2) perturbation theory are also considered in the mISI method, showing considerable worsening of the results. Possible further development of mISI is briefly discussed.

The effective point charge of probe tip in piezoresponse force microscopy

Abrasions of the probe apexes always happen during piezoresponse force microscopy (PFM) experiments, resulting in variation of distribution of the electroelastic field in piezoelectric materials, which finally influences the spatial resolution of PFM. In this paper, we find the effective point charges coupled with the piezoelectric coefficients for three probe models including the modified point charge model, the sphere-plane model, and the disk-plane model, through the fully-coupled electromechanical method. It is proved that the wear of the probe apex induces spreading of electroelastic field from the contact area to the surrounding area, and the electroelastic fields computed using the coupled method are much more localized than that computed by the decoupled method. The piezoresponses underneath the probe apexes have no correlation with the geometries of the probes, yet strongly depend on the choices of calculation methods. This analysis paves new ways for studies of the piezoresponses in complicated domain structures in ferroelectric materials.

Domain nucleation and hysteresis loop shape in piezoresponse force spectroscopy

Electromechanical hysteresis loop measurements in piezoresponse force microscopy (PFM) [piezoresponse force spectroscopy (PFS)] have emerged as a powerful technique for probing ferroelectric switching behavior on the nanoscale. Interpretation of PFS data requires the relationship between the domain parameters and PFM signal to be established. Here, the authors analyze the switching process using modified point charge model. The charge parameters are selected to reproduce tip-induced surface potential and tip radius of curvature. The relationship between geometric parameters of semiellipsoidal domain and PFM signal is derived using linear Green’s function theory. The role of domain nucleation on hysteresis loop is established.

Simulation of the spectroscopic and magnetic properties of RE(III) ions in RE oxychlorides based on exact crystal structure from Rietveld refinements

The procedures to calculate the 4f N energy level schemes — together with the related properties such as the paramagnetic susceptibility — of the trivalent rare earth ions (RE 3+) in the tetragonal RE oxychlorides (REOCl, RE=La–Nd, Sm–Ho and Y) from structural data are presented. The methods include the determination of coherent and reliable structural parameters from X-ray powder diffraction measurements using Rietveld analyses. The measurement of the absorption and luminescence spectra of the RE 3+ ions from the IR to UV region yield the experimental 4f N energy level schemes which are then simulated with a phenomenological model involving the parametrization of the inter- and intra-ionic interactions. Parallel calculations — already proven to be successful for ionic systems such as RE oxyfluorides — are based on the modified point charge model (PCEM). The more covalent simple overlap (SOM) model is also applied and compared to the other results. Finally, the paramagnetic susceptibility of the pure oxychlorides as a function of temperature was calculated using the eigenfunctions and eigenvalues from the phenomenological simulation of the experimental 4f N energy level schemes.

Study of the crystal field effect in REOF:Eu 3+ (RELa, Gd and Y) using optical spectroscopy and X-ray powder diffraction

The luminescence spectra of the Eu 3+-doped rare earth oxyfluorides REOF:Eu 3+ (RELa, Gd and Y) were measured at room and liquid nitrogen temperatures. The interpretation of the spectra and the simulation of the 7F J ( J = 0–5) Stark levels were carried out with the aid of phenomenological crystal field (CF) theory for the C 3v symmetry of the RE 3+ site. The best-fit set of the six non-zero CF parameters yielded the r.m.s. deviations 8, 11 and 10 cm −1 between the experimental and calculated energy levels for YOF, GdOF and LaOF respectively. The strength of the CF effect increases from the LaOF to the YOF host. The X-ray powder diffraction data for LaOF and GdOF were analysed by the Rietveld profile refinement method. From these results the CF parameters were calculated with a modified point charge model. The parameters of ranks four and six were reproduced quite close to the experimental values. The calculated | B 0 2| values were too large, evidently owing to short-range covalency effects.

Crystalline-field effect in the intermetallic compound SmCu 2

The anisotropic temperature dependence of magnetic susceptibility and electrical resistivity and the low temperature specific heat have been measured in the orthorhombic intermetallic compound SmCu 2. The anisotropic temperature dependence of the susceptibility is explained well by the theory on the basis of the modified point charge model of the trivalent Sm ions.

Studies with retinal pigments: modified point charge model for bacteriorhodopsin and difference FTIR (Fourier transform infrared) studies

Abstract

π-Bonding approach in the modified charge model

Originating from the modified point charge model (MPCM) showed recently to be successful in calculations of crystal field parameters (CFP) in the σ-bonding limit (I), a new π-bonding approach (II) is proposed. Unlike (I) which only defines R eff and q eff the π-bonding approach requires extra adjustable parameters: one ξ for cubic and at least two parameters for lower crystal field symmetries. These adjustable parameters reflect a displacement of the electric charges whenever instead of the σ, the π-bonding occurs. It has been shown for the cubic symmetry case that predicted by (I) signs of A 4〈 r 4〉 and A 6〈 r 6〉 crystal field parameters may be reversed if (II) is applied. Since in many ƒ-electron intermetallics the CFP signs determined experimentally are different from those expected in the framework of (I), the use of (II) seems to be justified. Setting up ξ = ϵ L /2 ϵ M the (II) gives an excellent agreement with the experimental CFP values for many RE-intermetallics and is considered to be a further extension of MPCM over the non-semiconducting ƒ-electron systems.

Crystal field parameters in UCI 4: Experiment versus theory

Crystal field effect on U 4+ ion with the 3H 4 ground term in tetragonal ligand field of UCl 4 has been studied in detail. Crystal field parameters determined experimentally from optical spectroscopy and magnetic susceptibility are in good agreement with CFP sets derived from the modified point charge model and the ab initio method. Theoretical calculations lead to overestimating the A 4 4〈 r 4〉 and lowering the A 0 2〈 r 2〉 values in comparison to those found in the experiments. The discrepancies are, however, within an accuracy of calculations. A large reduction of expectation values of the magnetic moment operator for the eigenvectors of lowest CF levels (17.8%), determined from magnetic susceptibility, cannot be attributed to the overlap and covalency effects only. The detailed calculations have shown that the latter effects provide about 4.6% reduction of respective matrix elements, and the applied J-J mixing procedure increases this factor up to 6.5%. Since similar, as in UCl 4, reduction factor(≈15%) has already been observed in a number of different uranium compounds, it seems likely that this feature is involved in the intrinsic properties of the U 4+ ion. We endeavor to explain this effect in terms of configuration interaction mechanisms.

Effects of small distortions on the EPR of the Γ6 state (T d symmetry) of Np(BH3CH3)4 diluted in Zr(BH3CH3)4

The single crystal and powder EPR spectra of 237Np(BH3CH3)4 diluted in Zr(BH3CH3)4 are reported and interpreted in terms of an orthorhombic spin Hamiltonian with gx=1.7739(4), gy=1.8292(4), gz=1.7961(5), Ax=0.1079(2) cm−1, Ay=0.1153(2) cm−1, Az=0.1135(2) cm−1. A modified point charge calculation indicates qualitatively that very small deviations of the site symmetry from Td are enough to account for the observed splitting of the g values.