Theoretical and computational studies focus on material properties under extreme conditions such as high pressure, high stress and/or high temperature. Staff in the theory group develop and implement a wide range of first-principles and many-body modeling and simulation approaches in theoretical exploration and also work closely with experimental groups.

## Computing Facilities

The HiPSEC theory group has access to many different supercomputers: XSEDE supercomputer network (https://www.xsede.org), Cherry Creek supercomputer in University of Nevada Las Vegas (https://www.nscee.edu/cherry-creek-resources.html), Nano cluster in Brookhaven national Laboratory (https://wiki.bnl.gov/CFN-Computation/index.php/Cluster) as well as our own in-house cluster.

Materials formed under ultrahigh pressure usually exhibit dramatic changes in physical, mechanical and functional properties and may offer significant improvements to the original forms. The theory group in HiPSEC is focusing on developing a heuristic strategy and a variety advanced methods to study the new phenomena and new materials under extreme conditions.

## Computational Methods

**Force Fields Modeling**

In atomistic simulation, the force field refers to the functional form and parameter sets used to express the interaction between atoms, molecules, or coarse-grained particles. Knowing the interactions of a given system, one could perform either static or dynamic simulation. In physical and material sciences, the *molecular dynamics* is a very popular method to study a wide range of behaviors in materials, such as geometry optimization, diffusion, melts, phase transition, mechanical deformation, and so on. Starting from an empirical or quantum mechanical model to describe the interactions between atoms or molecules, it simulates the dynamical evolution of the system as a function of time by numerically solving the Newton’s equations of motion (F=ma).

The available codes are:

LAMMPS is a classical molecular dynamics code, and an acronym for Large-scale Atomic/Molecular Massively Parallel Simulator).

** **GULP is a program for performing a variety of types of simulation on materials using different boundary conditions based on force field methods. The focus of the code is on analytical solutions, through the use of lattice dynamics, where possible, rather than on molecular dynamics.

**Electronic Structure Calculation: Density Functional Theory**

Density functional theory (DFT) is a computational quantum mechanical modelling method to investigate the electronic structure of many-body systems. Using this theory, the properties of a many-electron system can be determined by using functionals of electron density. DFT is among the most popular and versatile methods available in condensed-matter physics and solid state materials.

The available codes are:

VASP The Vienna Ab initio Simulation Package (VASP) is a computer program for atomic scale materials modelling, e.g. electronic structure calculations and quantum-mechanical molecular dynamics, from first principles. (https://www.vasp.at/)

Quantum Espresso is an integrated suite of Open-Source codes for electronic-structure calculations and materials modeling based on DFT, plane waves, and pseudopotentials. (http://www.quantum-espresso.org/)

**Crystal Structure Prediction**

The central activities in materials research lie in the structure-property relations. From the fundamental point of view, atomic structure is the most important piece of information about crystalline solids: just from the knowledge of topology of the structure, a precise structural model and many physical properties of crystals can be derived from first principle calculations. Under extreme conditions, the structure characterization becomes rather limited. With that, the computational techniques are essential to understand materials under high pressure. Crystal structure prediction is the calculation of crystal structures from first principles, with the aid of global optimization techniques like simulated annealing, genetic/evolutionary algorithm, basin hopping, minima hopping, etc.

The USPEX code (Universal Structure Prediction: Evolutionary Xtallography) can be used to predict stable crystal structures at given P-T conditions, knowing only the chemical composition (or to predict both the stable compositions and structures, given the element types). The USPEX code is based on an efficient evolutionary algorithm.