MD Overview

Instead of using the forces to optimise the structure of the system, CASTEP can instead use the forces to accelerate the atoms (and cell-shape) in order to simulate dynamical properties - a method known as "Molecular Dynamics" or simply "MD". To perform this kind of calculation, set

task : MD

in your param file.

CASTEP has a wide range of Molecular Dynamics (MD) capabilities, and can do equilibrium MD using a variety of ensembles:

  • NVE - the microcanonical ensemble - with fixed number of atoms, volume of cell, and total energy conserved
  • NVT - the canonical ensemble - with constant temperature not constant energy - due to the application of a thermostat
  • NPH - constant external pressure and enthalpy - due to the application of a barostat
  • NPT - constant external pressure and temperature - due to the application of a barostat and a thermostat
  • HUG - the Hugoniostat - a special thermo-barostat algorithm to find states on the Hugoniot curve

Of these, NPT is the closest to most real-life experiments.

Of the different thermostats, CASTEP supports Nose-Hoover, Nose-Hoover chains, Langevin and Hoover-Langevin.

Of the different barostats, CASTEP supports the isotropic Andersen-Hoover barostat, and the anisotropic Parrinello-Rahman barostat.

CASTEP also supports the Berendsen thermostat and barostat, as a route to faster equilibration before switching to one of the above thermostats/barostats for production data.

As well as generating different classical ensembles, CASTEP can also generate a state on the Hugoniot using the Hugoniostat algorithm. In fact, CASTEP can generate multiple sets of points on the Hugoniot curve in a single calculation.

CASTEP can also go beyond the Born-Oppenheimer approximation to do quantum dynamics, using Path Integral Molecular Dynamics (PIMD), in either NVT or NPT ensembles, using the Langevin thermostat.

In all MD schemes, CASTEP can support both linear and non-linear constraints on the ionic positions and/or on the cell vectors. The detailed trajectory information is written to a <seed>.md file, which is a structured text file (similar to the <seed>.geom file), for ease of manipulation and post-calculation analysis.