Bartomeu Monserrat

Report on the sixth blind test of organic crystal structure prediction methods

The results of the sixth blind test of organic crystal structure prediction methods are presented and discussed, highlighting progress for salts, hydrates and bulky flexible molecules, as well as on-going challenges.

Anharmonic vibrational properties in periodic systems: energy, electron-phonon coupling, and stress

A unified approach is used to study vibrational properties of periodic systems with first-principles methods and including anharmonic effects. Our approach provides a theoretical basis for the determination of phonon-dependent quantities at finite temperatures. The low-energy portion of the Born-Oppenheimer energy surface is mapped and used to calculate the total vibrational energy including anharmonic effects, electron-phonon coupling, and the vibrational contribution to the stress tensor. We report results for the temperature dependence of the electronic band gap and the linear coefficient of thermal expansion of diamond, lithium hydride, and lithium deuteride.

Quantum Monte Carlo study of the phase diagram of solid molecular hydrogen at extreme pressures

Establishing the phase diagram of hydrogen is a major challenge for experimental and theoretical physics. Experiment alone cannot establish the atomic structure of solid hydrogen at high pressure, because hydrogen scatters X-rays only weakly. Instead, our understanding of the atomic structure is largely based on density functional theory (DFT). By comparing Raman spectra for low-energy structures found in DFT searches with experimental spectra, candidate atomic structures have been identified for each experimentally observed phase. Unfortunately, DFT predicts a metallic structure to be energetically favoured at a broad range of pressures up to 400 GPa, where it is known experimentally that hydrogen is non-metallic. Here we show that more advanced theoretical methods (diffusion quantum Monte Carlo calculations) find the metallic structure to be uncompetitive, and predict a phase diagram in reasonable agreement with experiment. This greatly strengthens the claim that the candidate atomic structures accurately model the experimentally observed phases.

Comparing electron-phonon coupling strength in diamond, silicon, and silicon carbide: First-principles study

Renormalization of the electronic band gap due to electron-phonon coupling in the tetrahedral semiconductors diamond, silicon, and cubic silicon carbide is studied from first principles. There is a marked difference between the coupling of the vibrational state to the valence band maximum and to the conduction band minimum. The strength of phonon coupling to the valence band maximum is similar between the three systems and is dominated by vibrations that change the bond length. The coupling strength to the conduction band minimum differs significantly in diamond, silicon carbide, and silicon. In diamond, the coupling is dominated by six small pockets of vibrational states in the phonon Brillouin zone that are ultimately responsible for the stronger electron-phonon coupling in this material. Our results represent a first step towards the development of an a priori understanding of electron-phonon coupling in semiconductors and insulators that should aid the design of materials with tailored electron-phonon coupling properties.

Correlation effects on electron-phonon coupling in semiconductors: Many-body theory along thermal lines

A method is proposed for the inclusion of electron correlation in the calculation of the temperature dependence of band structures arising from electron-phonon coupling. It relies on an efficient exploration of the vibrational phase space along the recently introduced thermal lines. Using the

Temperature Dependence of the Energy Levels of Methylammonium Lead Iodide Perovskite from First-Principles

{G}_{0}{W}_{0}

Giant electron-phonon interactions in molecular crystals and the importance of non-quadratic coupling

approximation, the temperature dependence of the direct gaps of diamond, silicon, lithium fluoride, magnesium oxide, and titanium dioxide is calculated. Within the proposed formalism, a single calculation at each temperature of interest is sufficient to obtain results of the same accuracy as in alternative, more expensive methods. It is shown that many-body contributions beyond semilocal density functional theory modify the electron-phonon coupling strength by almost

Temperature effects in first-principles solid state calculations of the chemical shielding tensor made simple

50%

Vibrational renormalisation of the electronic band gap in hexagonal and cubic ice

in diamond, silicon, and titanium dioxide, but by less than

Quantum Monte Carlo study of the energetics of the rutile, anatase, brookite, and columbite TiO

5%