Download CASTEP 19.11 computational chemistry software

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 in castep material studio”. 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.

CASTEP can compute vibrational (phonon) modes for metals and insulators using either of density functional perturbation theory (DFPT) or finite displacements in conjunction with supercells. In addition to the traditional method of a user-specified supercell (a.k.a. the “direct method”) CASTEP implements a new method which automatically selects and generates a series of supercells commensurate with the desired phonon wavevector criteria. CASTEP itself contains a full-featured lattice dynamics code, which takes control of setting up the entire calculation and and no external software (such as Phonoy, PHONON, Phon) is required.

The available capabilities and features include

• DFPT
• Full BZ sampling using Fourier interpolation for DOS and dispersion.
• Finite-displacements (Supports full range of Hamiltonians including DFT+U, DFT+SO, hybrids)
• Full BZ sampling using traditional finite-displacement/spuercell method for DOS and dispersion.
• Full BZ sampling using more efficient and automated non-diagonal supercell method for DOS and dispersion.
• Acoustic sum-rule enforcement correction in either real- or reciprocal space.
• Full use of space-group symmetry to reduce redundant computation of perturbations.
• Electric field response DFPT to give IR and Raman intensities and NLO coefficients.
• Full symmetry analysis to compute irreducible representaions.
• External tools for postprocessing and plotting phonon dispersion and DOS.
• External tool for isotopic substitution calculations.

CASTEP uses density functional perturbation theory and the GIPAW method to compute magnetic shielding (chemical shift) tensors in solids and molecules. It is also possible to compute electric field gradient tensors (EFG) and the spin-spin (J) coupling tensors.

CASTEP can calculate spectra involving transitions between core and conduction states (ELNES, XANES). Core-hole effects can be taken into account by using supercells and pseudopotential generated with a reduced occupancy.

CASTEP can also compute the complex dielectric function with the random phase approximation (neglecting local field effects). This can be used to obtain the refractive index and optical conductivity. It can also be used to obtain the loss function, providing a connection to low-loss EELS.

As well as computing band-structures and densities of states CASTEP has several tools for analysis of the electronic structure including:

• Mulliken population analysis
• Hirshfeld population analysis
• Electron Localisation Functions (ELF)

• CASTEP uses pseudopotentials to represent the interaction between core and valence electrons.
• CASTEP supports both norm-conserving and Ultrasoft pseudopotentials. Pseudopotentials can be read from file in various formats. CASTEP also has its own built in generator and can compute potentials ‘on-the-fly’ during a calculation. There is a built in database of well tested potentials. In particular it is possible to generate highly accurate semi-core pseudopotentials which have been used in high pressure studies.

CASTEP employs several electronic solvers. The default solver uses a density mixing (DM) algorithm in which the Kohn-Sham equations are solved for a fixed input density, and then a separate density mixing algorithm is used to evolve the density towards the groundstate.

For difficult to converge systems Ensemble Density Functional Theory (EDFT) can be used; this method is extremely robust, but much more computationally demanding than density mixing methods.

The present CASTEP code is written in Fortran 2003 using a carefully designed modular structure. It was written to be highly portable, and with parallel computing in mind. It can run on all levels of computing hardware from desktop pcs, through to HPC clusters and National-level supercomputers.

CASTEP employs three levels of parallelism: G-vectors (ie basis-set), k-points, and bands. For certain calculations an additional “Task farming” parallelism is available. All of these parallel strategies may be employed simultaneously to achieve good scaling to well over 1000 processing elements. A version of CASTEP which can take advantage of GPU-based clusters is also available.