arXiv subjects

David C. Langreth

Publications and source records attributed to David C. Langreth.

At least 19 recordsLinked to original sources

arXiv2012

Structure and energetics of a ferroelectric organic crystal of phenazine and chloranilic acid

We report first-principles calculations for a ferroelectric organic crystal of phenazine and chloranilic acid molecules. Weak intermolecular interactions are properly treated by using a second version of van der Waals density functional known as vdW-DF2 [K. Lee et al., Phys. Rev. B 82, 081101 (2010)]. Lattice constants, total energies, spontaneous electric polarizations, phonon modes and frequencies, and the energy barrier of proton transfer are calculated and compared with PBE and experiments whenever possible. We show that the donation of one proton from a chloranilic acid molecule to a neighboring phenazine molecule is energetically favorable. This proton transfer is the key structural change that breaks the centrosymmetry and leads to the ferroelectric structure. However, there is no unstable phonon associated with the proton transfer, and an energy barrier of 8 meV is found between the paraelectric and ferroelectric states.

cond-mat.mtrl-sci↗

arXiv2012

Analyzing the frequency shift of physiadsorbed CO2 in metal organic framework materials

Combining first-principles density functional theory simulations with IR and Raman experiments, we determine the frequency shift of vibrational modes of CO2 when physiadsorbed in the iso-structural metal organic framework materials Mg-MOF74 and Zn-MOF74. Surprisingly, we find that the resulting change in shift is rather different for these two systems and we elucidate possible reasons. We explicitly consider three factors responsible for the frequency shift through physiabsorption, namely (i) the change in the molecule length, (ii) the asymmetric distortion of the CO$_2$ molecule, and (iii) the direct influence of the metal center. The influence of each factor is evaluated separately through different geometry considerations, providing a fundamental understanding of the frequency shifts observed experimentally.

cond-mat.mtrl-sci↗

arXiv2010

First-principles approach to rotational-vibrational frequencies and infrared intensity for H$_2$ adsorbed in nanoporous materials

The absorption sites and the low-lying rotational and vibrational (RV) energy states for H$_2$ adsorbed within a metal-organic framework are calculated via van der Waals density functional theory. The induced dipole due to bond stretching is found to be accurately given by a first-principles driven approximation using maximally-localized-Wannier-function analysis. The strengths and positions of lines in the complex spectra of RV transitions are in reasonable agreement with experiment, and in particular explain the experimentally mysteriously missing primary line for para hydrogen.

cond-mat.other↗

arXiv2010

A van der Waals density functional study of adenine on graphite: Single molecular adsorption and overlayer binding

The adsorption of an adenine molecule on graphene is studied using a first-principles van der Waals functional (vdW-DF) [Dion et al., Phys. Rev. Lett. 92, 246401 (2004)]. The cohesive energy of an ordered adenine overlayer is also estimated. For the adsorption of a single molecule, we determine the optimal binding configuration and adsorption energy by translating and rotating the molecule. The adsorption energy for a single molecule of adenine is found to be 711 meV, which is close to the calculated adsorption energy of the similar-sized naphthalene. Based on the single molecular binding configuration, we estimate the cohesive energy of a two-dimensional ordered overlayer. We find a significantly stronger binding energy for the ordered overlayer than for single-molecule adsorption.

cond-mat.soft↗

arXiv2010

A Higher-Accuracy van der Waals Density Functional

We propose a second version of the van der Waals density functional (vdW-DF2) of Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)], employing a more accurate semilocal exchange functional and the use of a large-N asymptote gradient correction in determining the vdW kernel. The predicted binding energy, equilibrium separation, and potential-energy curve shape are close to those of accurate quantum chemical calculations on 22 duplexes. We anticipate the enabling of chemically accurate calculations in sparse materials of importance for condensed-matter, surface, chemical, and biological physics.

cond-mat.mtrl-sci↗

arXiv2009

Energetics and dynamics of H$_2$ adsorbed in a nanoporous material at low temperature

Molecular hydrogen adsorption in a nanoporous metal organic framework structure (MOF-74) was studied via van der Waals density-functional calculations. The primary and secondary binding sites for H$_2$ were confirmed. The low-lying rotational and translational energy levels were calculated, based on the orientation and position dependent potential energy surface at the two binding sites. A consistent picture is obtained between the calculated rotational-translational transitions for different H$_2$ loadings and those measured by inelastic neutron scattering exciting the singlet to triplet (para to ortho) transition in H$_2$. The H$_2$ binding energy after zero point energy correction due to the rotational and translational motions is predicted to be $\sim$100 meV in good agreement with the experimental value of $\sim$90 meV.

cond-mat.mes-hall↗

arXiv2008

Theoretical and experimental analysis of H2 binding in a prototype metal organic framework material

Hydrogen adsorption by the metal organic framework (MOF) structure Zn2(BDC)2(TED) is investigated using a combination of experimental and theoretical methods. By use of the nonempirical van der Waals density-functional (vdW-DF) approach, it is found that the locus of deepest H2 binding positions lies within two types of narrow channel. The energies of the most stable binding sites, as well as the number of such binding sites, are consistent with the values obtained from experimental adsorption isotherms and heat of adsorption data. Calculations of the shift of the H-H stretch frequency when adsorbed in the MOF give a value of approximately -30 cm-1 at the strongest binding point in each of the two channels. Ambient temperature infrared absorption spectroscopy measurements give a hydrogen peak centered at 4120 cm-1, implying a shift consistent with the theoretical calculations.

cond-mat.mtrl-sci↗

arXiv2007

Predicting C-H/$π$ interactions with nonlocal density functional theory

We examine the performance of a recently developed nonlocal density functional in predicting a model noncovalent interaction, the weak bond between an aromatic $π$ system and an aliphatic C-H group. The new functional is a significant improvement over traditional density functionals, providing results which compare favorably to high-level quantum-chemistry techniques but at considerably lower computational cost. Interaction energies in several model C-H/$π$ systems are in generally good agreement with coupled-cluster calculations, though equilibrium distances are consistently overpredicted when using the revPBE functional for exchange. The new functional correctly predicts changes in energy upon addition of halogen substituents.

cond-mat.mtrl-sci↗

arXiv2007

Van der Waals density functional: Self-consistent potential and the nature of the van der Waals bond

We derive the exchange-correlation potential corresponding to the nonlocal van der Waals density functional [M. Dion, H. Rydberg, E. Schroder, D. C. Langreth, and B. I. Lundqvist, Phys. Rev. Lett. 92, 246401 (2004)]. We use this potential for a self-consistent calculation of the ground state properties of a number of van der Waals complexes as well as crystalline silicon. For the latter, where little or no van der Waals interaction is expected, we find that the results are mostly determined by semilocal exchange and correlation as in standard generalized gradient approximations (GGA), with the fully nonlocal term giving little effect. On the other hand, our results for the van der Waals complexes show that the self-consistency has little effect at equilibrium separations. This finding validates previous calculations with the same functional that treated the fully nonlocal term as a post GGA perturbation. A comparison of our results with wave-function calculations demonstrates the usefulness of our approach. The exchange-correlation potential also allows us to calculate Hellmann-Feynman forces, hence providing the means for efficient geometry relaxations as well as unleashing the potential use of other standard techniques that depend on the self-consistent charge distribution. The nature of the van der Waals bond is discussed in terms of the self-consistent bonding charge.

cond-mat.mtrl-sci↗

arXiv2006

Towards a working density-functional theory for polymers: First-principles determination of the polyethylene crystal structure

Equilibrium polyethylene crystal structure, cohesive energy, and elastic constants are calculated by density-functional theory applied with a recently proposed density functional (vdW-DF) for general geometries [Phys. Rev. Lett. 92, 246401 (2004)] and with a pseudopotential-planewave scheme. The vdW-DF with its account for the long-ranged van der Waals interactions gives not only a stabilized crystal structure but also values of the calculated lattice parameters and elastic constants in quite good agreement with experimental data, giving promise for successful application to a wider range of polymers.

cond-mat.mtrl-sci↗

arXiv2006

Application of van der Waals Density Functional to an Extended System: Adsorption of Benzene and Naphthalene on Graphite

It is shown that it is now possible to include van der Waals interactions via a nonempirical implementation of density functional theory to describe the correlation energy in electronic structure calculations on infinite systems of no particular symmetry. The vdW-DF functional [Phys. Rev. Lett. 92, 246401 (2004)] is applied to the adsorption of benzene and naphthalene on an infinite sheet of graphite, as well as the binding between two graphite sheets. Comparison with recent thermal desorption data [Phys. Rev. B 69, 535406 (2004)] shows great promise for the vdW-DF method.

cond-mat.mtrl-sci↗

arXiv2005

Binding Energies in Benzene Dimers: Nonlocal Density Functional Calculations

The interaction energy and minimum energy structure for different geometries of the benzene dimer has been calculated using the recently developed nonlocal correlation energy functional for calculating dispersion interactions. The comparison of this straightforward and relatively quick density functional based method with recent calculations can elucidate how the former, quicker method might be exploited in larger more complicated biological, organic, aromatic, and even infinite systems such as molecules physisorbed on surfaces, and van der Waals crystals.

cond-mat.soft↗

arXiv2005

Interaction energies of monosubstituted benzene dimers via nonlocal density functional theory

We present density-functional calculations for the interaction energy of monosubstituted benzene dimers. Our approach utilizes a recently developed fully nonlocal correlation energy functional, which has been applied to the pure benzene dimer and several other systems with promising results. The interaction energy as a function of monomer distance was calculated for four different substituents in a sandwich and two T-shaped configurations. In addition, we considered two methods for dealing with exchange, namely using the revPBE generalized gradient functional as well as full Hartree-Fock. Our results are compared with other methods, such as Moller-Plesset and coupled-cluster calculations, thereby establishing the usefulness of our approach. Since our density-functional based method is considerably faster than other standard methods, it provides a computational inexpensive alternative, which is of particular interest for larger systems where standard calculations are too expensive or infeasible.

cond-mat.mtrl-sci↗

arXiv2004

Transient currents and universal timescales for a fully time-dependent quantum dot in the Kondo regime

Using the time-dependent non-crossing approximation, we calculate the transient response of the current through a quantum dot subject to a finite bias when the dot level is moved suddenly into a regime where the Kondo effect is present. After an initial small but rapid response, the time-dependent conductance is a universal function of the temperature, bias, and inverse time, all expressed in units of the Kondo temperature. Two timescales emerge: the first is the time to reach a quasi-metastable point where the Kondo resonance is formed as a broad structure of half-width of the order of the bias; the second is the longer time required for the narrower split peak structure to emerge from the previous structure and to become fully formed. The first time can be measured by the gross rise time of the conductance, which does not substantially change later while the split peaks are forming. The second time characterizes the decay rate of the small split Kondo peak (SKP) oscillations in the conductance, which may provide a method of experimental access to it. This latter timescale is accessible via linear response from the steady stateand appears to be related to the scale identified in that manner [A. Rosch, J. Kroha, and P. Wolfle, Phys. Rev. Lett. 87, 156802 (2001)].

cond-mat.str-el↗

arXiv2002

Kondo effect in coupled quantum dots: a Non-crossing approximation study

The out-of-equilibrium transport properties of a double quantum dot system in the Kondo regime are studied theoretically by means of a two-impurity Anderson Hamiltonian with inter-impurity hopping. The Hamiltonian, formulated in slave-boson language, is solved by means of a generalization of the non-crossing approximation (NCA) to the present problem. We provide benchmark calculations of the predictions of the NCA for the linear and nonlinear transport properties of coupled quantum dots in the Kondo regime. We give a series of predictions that can be observed experimentally in linear and nonlinear transport measurements through coupled quantum dots. Importantly, it is demonstrated that measurements of the differential conductance ${\cal G}=dI/dV$, for the appropriate values of voltages and inter-dot tunneling couplings, can give a direct observation of the coherent superposition between the many-body Kondo states of each dot. This coherence can be also detected in the linear transport through the system: the curve linear conductance vs temperature is non-monotonic, with a maximum at a temperature $T^*$ characterizing quantum coherence between both Kondo states.

cond-mat.mes-hall↗

arXiv2001

Fundamental Time Constant for a Biased Quantum Dot in the Kondo Regime

It is shown that the rate limiting time constant for the formation of the Kondo state in a quantum dot can be extracted analytically from known perturbation theoretic results. The prediction obtained is verified via numerical simulations in the noncrossing approximation.

cond-mat.str-el↗

arXiv2000

Out-of-equilibrium Kondo effect in Double Quantum Dots

The out-of-equilibrium transport properties of a double quantum dot system in the Kondo regime are studied theoretically by means of a two-impurity Anderson hamiltonian with interimpurity hopping. The hamiltonian is solved by means of a non-equilibrium generalization of the slave-boson mean-field theory. It is demonstrated that measurements of the differential conductance dI/dV, for the appropriate values of voltages and tunneling couplings, can give a direct observation of the coherent superposition between the many-body Kondo states of each dot. For large voltages and arbitrarily large interdot tunneling, there is a critical voltage above which the physical behaviour of the system is again that of two decoupled quantum dots, i.e two Kondo singularities pinned at each chemical potential.

cond-mat.mes-hall↗

arXiv2000

Kondo time scales for quantum dots - response to pulsed bias potentials

The response of a quantum dot in the Kondo regime to rectangular pulsed bias potentials of various strengths and durations is studied theoretically. It is found that the rise time is faster than the fall time, and also faster than time scales normally associated with the Kondo problem. For larger values of the pulsed bias, one can induce dramatic oscillations in the induced current with a frequency approximating the splitting between the Kondo peaks that would be present in steady state. The effect persists in the total charge transported per pulse, which should facilitate the experimental observation of the phenomenon.

cond-mat.str-el↗