12,294 research outputs found
Basis set effects on the hyperpolarizability of CHCl_3: Gaussian-type orbitals, numerical basis sets and real-space grids
Calculations of the hyperpolarizability are typically much more difficult to
converge with basis set size than the linear polarizability. In order to
understand these convergence issues and hence obtain accurate ab initio values,
we compare calculations of the static hyperpolarizability of the gas-phase
chloroform molecule (CHCl_3) using three different kinds of basis sets:
Gaussian-type orbitals, numerical basis sets, and real-space grids. Although
all of these methods can yield similar results, surprisingly large, diffuse
basis sets are needed to achieve convergence to comparable values. These
results are interpreted in terms of local polarizability and
hyperpolarizability densities. We find that the hyperpolarizability is very
sensitive to the molecular structure, and we also assess the significance of
vibrational contributions and frequency dispersion
Magnetic patterning of (Ga,Mn)As by hydrogen passivation
We present an original method to magnetically pattern thin layers of
(Ga,Mn)As. It relies on local hydrogen passivation to significantly lower the
hole density, and thereby locally suppress the carrier-mediated ferromagnetic
phase. The sample surface is thus maintained continuous, and the minimal
structure size is of about 200 nm. In micron-sized ferromagnetic dots
fabricated by hydrogen passivation on perpendicularly magnetized layers, the
switching fields can be maintained closer to the continuous film coercivity,
compared to dots made by usual dry etch techniques
Accuracy of generalized gradient approximation functionals for density functional perturbation theory calculations
We assess the validity of various exchange-correlation functionals for
computing the structural, vibrational, dielectric, and thermodynamical
properties of materials in the framework of density-functional perturbation
theory (DFPT). We consider five generalized-gradient approximation (GGA)
functionals (PBE, PBEsol, WC, AM05, and HTBS) as well as the local density
approximation (LDA) functional. We investigate a wide variety of materials
including a semiconductor (silicon), a metal (copper), and various insulators
(SiO -quartz and stishovite, ZrSiO zircon, and MgO periclase).
For the structural properties, we find that PBEsol and WC are the closest to
the experiments and AM05 performs only slightly worse. All three functionals
actually improve over LDA and PBE in contrast with HTBS, which is shown to fail
dramatically for -quartz. For the vibrational and thermodynamical
properties, LDA performs surprisingly very good. In the majority of the test
cases, it outperforms PBE significantly and also the WC, PBEsol and AM05
functionals though by a smaller margin (and to the detriment of structural
parameters). On the other hand, HTBS performs also poorly for vibrational
quantities. For the dielectric properties, none of the functionals can be put
forward. They all (i) fail to reproduce the electronic dielectric constant due
to the well-known band gap problem and (ii) tend to overestimate the oscillator
strengths (and hence the static dielectric constant)
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