Computational Crystallography with quantum ESPRESSO (a few years - - PowerPoint PPT Presentation

Computational Crystallography with quantum ESPRESSO (a few years later) Paolo Giannozzi, Universit` a di Udine, Italy CECAM Discussion Meeting on Quantum Crystallography, Nancy, 2017/06/18 A very short introduction to “traditional” applications of quantum ESPRESSO (QE) to Quantum Crystallography (QC), and to recent developments that may be useful in the fields of QC First known (to me) occurrence of QC and QE in the same title:

– Typeset by FoilT EX –

What is QE? What is it useful for?

QE is an integrated suite of codes for atomistic calculations based on electronic structure, using density-functional theory, a plane-wave basis set, pseudopotentials. The applications described in the above-shown paper: Phonons in CsI and Ice under pressure MgO and MgSiO4 under geophysical conditions Models of amorphous silica Molecular crystals formed by CO2 and H2 under pressure are a good though incomplete sample of what is typically achieved using QE: Structural and vibrational data of materials having a complex structure or under unusual conditions, also with limited or unavailable experimental data Calculations typically performed with GGA-type functionals. The accuracy of data so computed is often hard to assess, though.

Verification and Validation of electronic-structure codes

Systematic comparisons of different pseudopotential and all-electron DFT codes: Reproducibility in density-functional theory calculations of solids, K. Lejaeghere et multis aliis, Science 351 (6280), aad3000 (2016), DOI 10.1126/science.aad3000 Tests precision of the computational methods, not physical accuracy of results. Main outcome: everybody is converging towards the same set of results.

Comparing QE with Gaussian-based code CRYSTAL

Effect of the basis set Comparison of charge density in Si Comparison of charge density in Al

Non-local functionals for molecular crystals

New non-local (vdW-DF) functionals allow to deal with molecular crystals without semi-empirical schemes, with a computational effort comparable to plain DFT:

Summary: new trends and developments, useful for QC

• Extensive efforts of verification and of validation of the results
• QE as “quantum engine” for advanced minimization algorithms (e.g., basin

hopping, genetic algorithms) and for “high-throughput” calculations (AiiDA)

• Improved pseudopotential tables, all-electron Kohn-Sham orbitals and charge

density using projector-augmented waves (PAW)

• Nonlocal functionals describing van der Waals (dispersive) interactions in

molecular crystals

• Hybrid functionals: fast algorithms for the calculation of computationally heady

exact-exchange term