Structural Properties and Dynamics

CASTEP can calculate the optimal arrangement of atoms in a structure, a process called 'geometry optimisation', and can also simulate the dynamical properties of a system using different forms of 'molecular dynamics' (MD).

Geometry optimisation

CASTEP has a varity of geometry optimisation scheme including

LBFGS - the low-memory version of BFGS - the default option
BFGS - widely-used quasi-Newton minimization
TPSD - two-point steepest descent
DMD - optimally damped MD

A variety of constraints can be applied including: fixed cell, fixed volume, fixed ions. Non-linear constraints such as fixed atom seperation can also be applied.

Molecular dynamics

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

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
HUGINIOT - for shock waves

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.

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.

For more information on structural properties in CASTEP see the following presentations:

Structural Calculations and Phase Stability - Prof Matt Probert (York)
Introduction to CASTEP MD - Prof Matt Probert (York)
Introduction to CASTEP PIMD - Prof Matt Probert (York)

Key References

Improved algorithm for geometry optimisation using damped molecular dynamics - M.I.J. Probert, Journal of Computational Physics, 191, p 130 (2003)

Hamiltonian Vibrational Spectroscopy