Influence of domain walls thickness, density and alignment on Barkhausen noise emission in low alloyed steels.

This study deals with the characterization of low alloyed steels of different yield strengths (varying in the range of 235-1100 MPa) via Barkhausen noise emission. The study investigates the potential of this technique to distinguish among the low alloyed steels and all significant aspects contributing to Barkhausen noise, such as the residual stress state, microstructure expressed in terms of dislocation density, grain size, prevailing phase, as well as associated aspects of the domain wall substructure (domain wall thickness, energy, their spacing and density in the matrix). Barkhausen noise in the rolling as well as transversal direction grows along with the yield strength (up to 500 MPa) and the corresponding grain refinement of ferrite. As soon as the martensite transformation occurs in a high strength matrix, this evolution saturates, and remarkable magnetic anisotropy is developed when Barkhausen noise in the transversal direction grows at the expense of the rolling direction. The contribution of residual stresses as well as the domain wall thickness is only minor, and the evolution of Barkhausen noise is driven by the density of the domain walls and their realignment.

Three- and four-body nonadditivities in nucleic acid tetramers: a CCSD(T) study.

Three- and four-body nonadditivities in the uracil tetramer (in DNA-like geometry) and the GC step (in crystal geometry) were investigated at various levels of the wave-function theory: HF, MP2, MP3, L-CCD, CCSD and CCSD(T). All of the calculations were performed using the 6-31G**(0.25,0.15) basis set, whereas the HF, MP2 and the MP3 nonadditivities were, for the sake of comparison, also determined with the much larger aug-cc-pVDZ basis set. The HF and MP2 levels do not provide reliable values for many-body terms, making it necessary to go beyond the MP2 level. The benchmark CCSD(T) three- and four-body nonadditivities are reasonably well reproduced at the MP3 level, and almost quantitative agreement is obtained (fortuitously) either on the L-CCD level or as an average of the MP3 and the CCSD results. Reliable values of many-body terms (especially their higher-order correlation contributions) are obtained already when the rather small 6-31G**(0.25,0.15) basis set is used. The four-body term is much smaller when compared to the three-body terms, but it is definitely not negligible, e.g. in the case of the GC step it represents about 16% of all of the three- and four-body terms. While investigating the geometry dependence of many-body terms for the GG step at the MP3/6-31G**(0.25,0.15) level, we found that it is necessary to include at least three-body terms in the determination of optimal geometry parameters.

Convergence of the CCSD(T) Correction Term for the Stacked Complex Methyl Adenine-Methyl Thymine: Comparison with Lower-Cost Alternatives.

We have performed large-scale calculations for the interaction energy of the stacked methyl adenine-methyl thymine complex at the CCSD(T)/aug-ccpVXZ (X = D,T) levels. The results can serve as benchmarks for the evaluation of two methods, MP2.5, introduced recently, and the widely used ΔCCSD(T) correction defined as the difference between the CCSD(T) and MP2 energies. Our results confirm that the ΔCCSD(T) correction converges much faster toward the complete basis set (CBS) limit than toward the MP2 or CCSD(T) energies. This justifies approximating the CBS energy by adding the ΔCCSD(T) correction calculated with a modest basis set to a large basis MP2 energy. The fast convergence of the ΔCCSD(T) correction is not obvious, as the individual CCSD and (T) contributions converge less rapidly than their sum. The MP2.5 method performs very well for this system, with results very close to CCSD(T). It is conjectured that using a ΔMP2.5 correction, defined analogously to ΔCCSD(T), with large basis sets may yield more reliable nonbonded interaction energies than using ΔCCSD(T) with a smaller basis set. This would result in important computational savings as the MP3 scales computationally much less steep than CCSD(T), although higher than SCS-MP2, a similar approximation.

Benzene Dimer: High-Level Wave Function and Density Functional Theory Calculations.

High-level OVOS (optimized virtual orbital space) CCSD(T) interaction energy calculations (up to the aug-cc-pVQZ basis set) and various extrapolations toward the complete basis set (CBS) limit are presented for the most important structures on the benzene dimer potential energy surface. The geometries of these structures were obtained via an all-coordinate gradient geometry optimization using the DFT-D/BLYP method, covering the empirical dispersion correction fitted exclusively for this system. The fit was carried out against two estimated CCSD(T)/CBS potential energy curves corresponding to the distance variation between two benzene rings for the parallel-displaced (PD) and T-shaped (T) structures. The effect of the connected quadruple excitations on the interaction energy was estimated using the CCSD(TQf) method in a 6-31G*(0.25) basis set, destabilizing the T and T-shaped tilted (TT) structures by ≈0.02 kcal/mol and the PD structure by ≈0.04 kcal/mol. Our best CCSD(T)/CBS results show, within the error bars of the applied methodology, that the energetically lowest-lying structure is the TT structure, which is nearly 0.1 kcal/mol more stable than the almost isoenergetic PD and T structures. The specifically parametrized DFT-D/BLYP method leads to a correct energy ordering of the structures, with the errors being smaller by 0.2 kcal/mol with respect to the most accurate CCSD(T) values.