Density Functional Theory

Three semi-empirical force field FeCr potentials, two within the formalism of the two-band model and one within the formalism of the concentration dependent model, have been benchmarked against a wide variety of density functional theory (DFT) structures. The benchmarking allows an assessment of how reliable empirical potential results are in different areas relevant to radiation damage modelling. The DFT data consist of defect-free structures, structures with single interstitials and structures with small di- and tri-interstitial clusters. All three potentials reproduce the general trend of the heat of formation (h.o.f.) quite well. The most important shortcomings of the original two-band model potential are the low or even negative h.o.f. for Cr-rich structures and the lack of a strong repulsion when moving two solute Cr atoms from being second-nearest neighbours to nearest neighbours. The newer two-band model potential partly solves the first problem. The most important shortcoming in the concentration dependent model potential is the magnitude of the Cr-Cr repulsion, being too strong at short distances and mostly absent at longer distances. Both two-band model potentials do reproduce long-range Cr-Cr repulsion. For interstitials the two-band model potentials reproduce a number of Cr-interstitial binding energies surprisingly well, in contrast to the concentration dependent model potential. For Cr interacting with clusters, the result can sometimes be directly extrapolated from Cr interacting with single interstitials, both according to DFT and the three empirical potentials.

The solid phase FT-IR and FT-Raman, solution phase linear dichroism IR (in nematic liquid crystal), and vapor phase GC/IR spectra of 2-(methylthio)benzonitrile have been recorded in the regions 4000–50, 3500–100, 4000–400, and 4000–650 cm−1, respectively. The spectra were interpreted with the aid of normal coordinate analysis following full structure optimizations and force field calculations based on density functional theory (DFT) using standard B3LYP/6-31G* and B3LYP/6-311+G** method and basis set combinations. Normal coordinate calculations were performed with the DFT force field corrected by a recommended set of scaling factors yielding fairly good agreement between observed and calculated frequencies. IR dichroism data revealed an error in band assignment associated with a νCS vibration, which could be eliminated only by introducing independent scaling factors for sulfur, whereas the overall frequency fit was further improved. Simulation of infrared and Raman spectra utilizing...

In this paper, we explore the electric field gradients (EFGs) at 238U sites for antiferromagnetic UX2 (X ¼ P, As,
Sb, Bi) using LDA, LDA + U, GGA, GGA + U, and the exact exchange for correlated electrons schemes by
considering the diagonalization of the spin–orbit coupling Hamiltonian in the space of the scalar
relativistic eigenstates using the second-order variational procedure. The electronic structures and
magnetic properties of the compounds are also investigated. It is found that the density functional
theory approaches except exact exchange for correlated electrons are not successful in reproducing the
experimental zero electric field gradient value in UBi2, even LDA + U and GGA + U within their default 4f
density matrices by varying the U parameter in an energy interval of [0; 4 eV], though these techniques
with no need to manually adopt their initial conditions (elements of the occupation matrix) are effective
in the calculation of the nonzero electric field gradients for the other compounds. The exact exchange
for correlated electrons has efficiently provided a null electric field gradient in UBi2 and nonzero electric
field gradients for the other compounds by adjusting its dimensionless parameter a to 0.4. The physics
of the null electric field gradient in UBi2 is revealed in this article and it is discussed that the source of the
ignorable electric field gradient originates from the antiferromagnetic ordering of [Y as compared to the
long-range antiferromagnetic ordering of [[YY in the other compounds. Furthermore, our calculated
magnetic moments for the uranium atoms in these compounds are consistent with the available
experimentally measured values as compared to the severely underestimated theoretical results.