229 research outputs found
A DFT study of structural, dynamical properties and quasiparticle band structure of solid nitromethane
We report a detailed theoretical study of the structural, vibrational, and
optical properties of solid nitromethane using first principles density
functional calculations. The ground state properties were calculated using a
plane wave pseudopotential code with either the local density approximation
(LDA), the generalized gradient approximation (GGA), or with a correction to
include van derWaals interactions. Our calculated equilibrium lattice
parameters and volume using a dispersion correction are found to be in
reasonable agreement with the experimental results. Also, our calculations
reproduce the experimental trends in the structural properties at high
pressure. It was found to be a discontinuity in the bond length, bond angles
and also a weaking of hydrogen bond strength in the pressure range from 10 to
12 GPa, picturing the structural transition from phase I to Phase II. Moreover,
we predict the elastic constants of solid nitromethane and found that the
corresponding bulk modulus is in good agreement with experiments. The
calculated elastic constants are showing an order of C11> C22 > C33, indicating
that the material is more compressible along the c-axis. We also calculated the
zone center vibrational frequencies and discuss the internal and external modes
of this material under pressure. From this, we found the softing of lattice
modes around 8 to 12 GPa. We have also attempt the quasiparticle band structure
of solid nitromethane with the G0W0 approximation and found that nitromethane
is an indirect band gap insulator with a value of the band gap of about 7.8 eV
with G0W0 approximation. Finally, the optical properties of this material,
namely the absorptive and dispersive part of the dielectric function, and the
refractive index and absorption spectra are calculated and the contribution of
different transition peaks of the absorption spectra are analyzed.Comment: 12 pages, 9 figure
Phase Stability and Thermoelectric Properties of the Mineral FeS2: An Ab Initio Study
First principles calculations were carried out to study the phase stability
and thermoelectric properties of the naturally occurring marcasite phase of
FeS at ambient condition as well as under pressure. Two distinct density
functional approaches has been used to investigate the above mentioned
properties. The plane wave pseudopotential approach was used to study the phase
stability and structural, elastic, and vibrational properties. The full
potential linear augment plane wave method has been used to study the
electronic structure and thermoelectric properties. From the total energy
calculations, it is clearly seen that marcasite FeS is stable at ambient
conditions, and it undergoes a first order phase transition to pyrite FeS
at around 3.7 GPa with a volume collapse of about 3. The calculated ground
state properties such as lattice parameters, bond lengths and bulk modulus of
marcasite FeS agree quite well with the experiment. Apart from the above
studies, phonon dispersion curves unambiguously indicate that marcasite phase
is stable under ambient conditions. Further, we do not observe any phonon
softening across the marcasite to pyrite transition and the possible reason
driving the transition is also analyzed in the present study, which has not
been attempted earlier. In addition, we have also calculated the electronic
structure and thermoelectric properties of the both marcasite and pyrite
FeS. We find a high thermopower for both the phases, especially with p-type
doping, which enables us to predict that FeS might find promising
applications as good thermoelectric materials.Comment: 10 Figure
Water-mediated interactions between hydrophobic and ionic species in cylindrical nanopores
We use Metropolis Monte Carlo and umbrella sampling to calculate the free
energies of interaction of two methane molecules and their charged derivatives
in cylindrical water-filled pores. Confinement strongly alters the interactions
between the nonpolar solutes, and completely eliminates the solvent separated
minimum (SSM) that is seen in bulk water. The free energy profiles show that
the methane molecules are either in contact or at separations corresponding to
the diameter and the length of the cylindrical pore. Analytic calculations that
estimate the entropy of the solutes, which are solvated at the pore surface,
qualitatively explain the shape of the free energy profiles. Adding charges of
opposite sign and magnitude or (where is the electronic charge)
to the methane molecules decreases their tendency for surface solvation and
restores the SSM. We show that confinement induced ion-pair formation occurs
whenever , where is the Bjerrum length, and is the
pore diameter. The extent of stabilization of the SSM increases with ion charge
density as long as . In pores with nm, in which the
water is strongly layered, increasing the charge magnitude from to
reduces the stability of the SSM. As a result, ion-pair formation which occurs
with negligible probability in the bulk, is promoted. In larger diameter pores
that can accomodate a complete hydration layer around the solutes, the
stability of the SSM is enhanced.Comment: 23 pages, 8 figures. To be published in The Journal of Chemical
Physic
- …