CMSP

Most condensed matter ceases to exist in the form we know it at ambient conditions and transforms into something very different once brought to the highest pressures achievable today in the laboratory or found in the interiors of planets. Liquids solidify, crystalline solids repeatedly change their lattice structure, compounds often decompose, insulators generally turn to metals, magnetism fades away, etc. Understanding, explaining, predicting this evolution under pressure constitutes an important physics challenge as well as a potential opportunity to create new metastable but useful materials. Recent themes include the formation of non-molecular CO2 phases, the mixing behavior of methane/water fluids at planetary conditions, the role of anharmonicity in LiH, the computational discovery of a new spin liquid phase in O2, and new phenomena in high pressure frictional shear.

 Atomistic methods based on the ab-initio description of the electrons guarantee a chemically accurate description of the forces at play but their computational load grows rapidly with the size of the system.
Accurate force-fields parameterized on ab-initio calculations are a promising way to extend the size and time constraints of ab-initio simulations. Extensive work on polarizable models for oxides has shown that accurate potentials can be constructed that reproduce faithfully the ab-initio results on homogeneous bulk phases. Compounds studied so far include SiO2, MgO, MgSiO3, TiO2, and water. The force-field developed for water, for example, is all-atom, dissociable, and capable of describing with reasonable accuracy also the dissociated state.

 One of the most fundamental processes in acid-base chemistry is the ionization of water, a rare event where a water molecule will spontaneously ionize and form a hydronium ion and a hydroxide ion. The microscopic fluctuations that drive this process remains poorly understood. Research in this area is aimed at understanding the molecular mechanisms by which the proton and hydroxide ion diffuse through bulk water systems. How these processes change at hydrophobic interfaces such as the surface of water and protein surfaces and how these properties are related to spectroscopic features of experiments is an active area of research. A particular area of ongoing development is probing the properties of water as a 3D-network which will hopefully help its collective structural and dynamical signatures.

 When protein folding goes bad, aggregated structures of biological matter known as Amyloid plaques can form. These proteins have been implicated in neurodegenerative diseases such as Alzheimers and have rather unusual optical properties such as being able to fluoresce. An active area of ongoing research is to use a combination of both ground and excited state electronic structure calculations to understand the electronic and optical properties of these systems. Classical molecular dynamics combined with enhanced sampling methods like umbrella sampling and metadynamics are used to explore the conformational landscape of these proteins. These systems also provide good model systems to understand classical problems with bio-physical chemistry such as hydrophobic effects.

 Water is the most ubiquitous solvent for biological matter such as proteins and DNA – it acts a lubricant and maintains the structural integrity of biomolecules. Research in this area is aimed at understanding the intimate coupling between the fundamental interactions of water and the biological systems. The physical chemistry of how organic molecules such amino acids and DNA bases interact with water is at the heart of hydrophobic solvation which drives many processes such as protein folding, drug-ligand binding and protein aggregation. A combination of both computational simulations and statistical mechanical theory is used to examine how these solutes perturb the structural, dynamical, electronic and spectroscopic properties of water.