The atomic simulation environment (ASE) is a software package written in the Python programming language with the aim of setting up, steering, and analyzing atomistic simulations. In ASE, tasks are fully scripted in Python. The powerful syntax of Python combined with the NumPy array library make it possible to perform very complex simulation tasks. For example, a sequence of calculations may be performed with the use of a simple 'for-loop' construction. Calculations of energy, forces, stresses and other quantities are performed through interfaces to many external electronic structure codes or force fields using a uniform interface. On top of this calculator interface, ASE provides modules for performing many standard simulation tasks such as structure optimization, molecular dynamics, handling of constraints and performing nudged elastic band calculations.

Using a first-principles approach we calculate the electron-phonon couplings in graphene for the transverse and longitudinal acoustic phonons. Analytic forms of the coupling matrix elements valid in the long-wavelength limit are found to give an almost quantitative description of the first-principles matrix elements even at shorter wavelengths. Using the analytic forms of the coupling matrix elements, we study the acoustic phonon-limited carrier mobility and quasiparticle lifetime observable in photoemission spectroscopy for temperatures 0-200 K and high carrier densities of 10 12 -10 13 cm −2 . We find that the intrinsic effective acoustic deformation potential of graphene is eff = 6.8 eV and that the temperature dependence of the mobility μ ∼ T −α in the Bloch-Grüneisen regime increases beyond an α = 4 dependence even in the absence of screening when the true coupling matrix elements are considered. The α > 4 temperature dependence of the mobility is found to originate in a similar temperature dependence of the relaxation time at the Fermi level. The large disagreement between our calculated deformation potential and those extracted from experimental measurements (18-29 eV) indicates that additional or modified acoustic phonon-scattering mechanisms are at play in experimental situations.

We study the phonon-limited mobility in intrinsic n-type single-layer MoS2 for temperatures T > 100 K. The materials properties including the electron-phonon interaction are calculated from first-principles and the deformation potentials and Fröhlich interaction in single-layer MoS2 are established. The calculated room-temperature mobility of ∼ 410 cm 2 V −1 s −1 is found to be dominated by optical phonon scattering via intra and intervalley deformation potential couplings and the Fröhlich interaction. The mobility is weakly dependent on the carrier density and follows a µ ∼ T −γ temperature dependence with γ = 1.69 at room temperature. It is shown that a quenching of the characteristic homopolar mode which is likely to occur in top-gated samples, increases the mobility with ∼ 70 cm 2 V −1 s −1 and can be observed as a decrease in the exponent to γ = 1.52. In comparison to recent experimental findings for the mobility in single-layer MoS2 (∼ 200 cm 2 V −1 s −1 ), our results indicate that mobilities close to the intrinsic phonon-limited mobility can be achieved in two-dimensional materials via dielectric engineering that effectively screens static Coulomb scattering on e.g. charged impurities.

We theoretically study the acoustic phonon limited mobility in n-doped two-dimensional MoS2 for temperatures T < 100 K and high carrier densities using the Boltzmann equation and firstprinciples calculations of the acoustic electron-phonon (el-ph) interaction. In combination with a continuum elastic model, analytic expressions and the coupling strengths for the deformation potential and piezoelectric interactions are established. We furthermore show that the deformation potential interaction has contributions from both normal and umklapp processes and that the latter contribution is only weakly affected by carrier screening. Consequently, the calculated mobilities show a transition from a high-temperature µ ∼ T −1 behavior to a stronger µ ∼ T −4 behavior in the low-temperature Bloch-Grüneisen regime characteristic of unscreened deformation potential scattering. Intrinsic mobilities in excess of 10 5 cm 2 V −1 s −1 are predicted at T < 10 K and high carrier densities (n 10 11 cm −2 ). At 100 K, the mobility does not exceed ∼ 7 × 10 3 cm 2 V −1 s −1 . Our findings provide new and important understanding of the acoustic el-ph interaction and its screening by free carriers, and is of high relevance for the understanding of acoustic phonon limited mobilities in general.

We present a practical scheme for performing error estimates for Density Functional Theory calculations. The approach which is based on ideas from Bayesian statistics involves creating an ensemble of exchange-correlation functionals by comparing with an experimental database of binding energies for molecules and solids. Fluctuations within the ensemble can then be used to estimate errors relative to experiment on calculated quantities like binding energies, bond lengths, and vibrational frequencies. It is demonstrated that the error bars on energy differences may vary by orders of magnitude for different systems in good agreement with existing experience.