Tohoku University · Materials Science
Professor Chris J. Pickard's research lab specializes in first-principles computational materials science, focusing on the prediction and understanding of novel electronic, magnetic, and structural properties of materials under extreme conditions. The lab develops advanced density functional theory (DFT) methods for accurate simulation of NMR chemical shifts, EPR g-tensors, and electronic responses in insulators and paramagnetic systems. A central theme is the discovery of new stable phases—especially in hydrogen-rich and light-element materials—using ab initio random structure searching (AIRSS), particularly under high pressure. The lab also investigates high-pressure metallization, superconductivity, and exotic bonding in systems such as silane, nitrogen, and hydrogen.
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A theory for the ab initio calculation of all-electron NMR chemical shifts in insulators using pseudopotentials is presented. It is formulated for both finite and infinitely periodic systems and is based on an extension to the projector augmented-wave approach of Bl\"ochl [P. E. Bl\"ochl, Phys. Rev. B 50, 17 953 (1994)] and the method of Mauri et al. [F. Mauri, B. G. Pfrommer, and S. G. Louie, Phys. Rev. Lett. 77, 5300 (1996)]. The theory is successfully validated for molecules by comparison wit
It is essential to know the arrangement of the atoms in a material in order to compute and understand its properties. Searching for stable structures of materials using first-principles electronic structure methods, such as density-functional-theory (DFT), is a rapidly growing field. Here we describe our simple, elegant and powerful approach to searching for structures with DFT, which we call ab initio random structure searching (AIRSS). Applications to discovering the structures of solids, poin
High-pressure phases of silane SiH4 are predicted using first-principles electronic structure methods. We search for low-enthalpy structures by relaxing from randomly chosen initial configurations, a strategy which is demonstrated to work well for unit cells containing up to at least ten atoms. We predict that silane will metallize at higher pressures than previously anticipated but might show high-temperature superconductivity at experimentally accessible pressures.
Density-functional-theory calculations and a structure-searching method are used to identify candidate high-pressure phases of nitrogen. We find six structures which are calculated to be more stable than previously studied structures at some pressures. Our four new molecular structures give insight into the most efficient packings of nitrogen molecules at high pressures, and we predict two new nonmolecular structures to be stable at very high pressures.
We have studied solid hydrogen up to pressures of 300 GPa and temperatures of 350 K using density functional theory methods and have found ``mixed structures'' that are more stable than those predicted earlier. Mixed structures consist of alternate layers of strongly bonded molecules and weakly bonded graphene-like sheets. Quasiharmonic vibrational calculations show that mixed structures are the most stable at room temperature over the pressure range 250--295 GPa. These structures are stabilized
A theory for the reliable prediction of the EPR g tensor for paramagnetic defects in solids is presented. It is based on density functional theory and on the gauge including projector augmented wave approach to the calculation of all-electron magnetic response. The method is validated by comparison with existing quantum chemical and experimental data for a selection of diatomic radicals. We then perform the first prediction of EPR g tensors in the solid state and find the results to be in excell
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