Setting up CASTEP calculations

The topics in this section describe how to set up CASTEP calculations of various types, as well as the ways in which the job control facility can be used.

There are a number of options that are understood by CASTEP which are not currently accessible through the Materials Studio interface. Such options can be utilized by manually editing the CASTEP input files. Information about how to do this is also presented in this section.

An important concept that affects a large number of settings exposed in CASTEP is that of a calculation quality. The Quality setting on the Setup tab of the CASTEP Calculation dialog provides a shortcut for setting up a balance between the accuracy and speed of calculations. The range of values from Coarse to Ultra-fine allows you to increase the accuracy gradually by imposing more strict tolerances, improving accuracy of reciprocal space sampling, and increasing the number of basis functions. Nevertheless there are cases where there is no need to systematically change the quality settings and a fast qualitative description is sufficient. The Express quality setting is provided to allow many simple cases to get reasonable results quickly. Please note that these settings cannot be translated into the Coarse - Ultra-fine accuracy scale. This setting is especially useful for semiconductors and insulators and can be fairly accurate for nonmagnetic metals. The study of magnetic metallic systems requires more accurate calculation settings than those defined by the Express quality setting.

CASTEP does not support all possible permutations of electronic settings for all possible properties requests. In cases where the input settings are contradictory the Materials Studio client will display messages explaining the problem. However, sometimes the combinations selected during the setup of a CASTEP calculation are impossible to satisfy. The main source of such contradictions is in the type of pseudopotentials used. For example, NMR calculations and core level spectroscopy calculations require the use of on-the-fly generated (OTFG) pseudopotentials for at least some elements. On the other hand, nonlocal exchange-correlation functionals (for example B3LYP) or linear response phonon calculations are limited to norm-conserving pseudopotentials. It is therefore impossible to use the same settings for requesting, for example, phonon dispersion using linear response and core level spectra, or to calculate band structures with B3LYP and request NMR chemical shifts.

Accurate calculations for periodic systems may require a large number of k-points if the lattice has a strongly skewed unit cell (for example, some cell angles are very small). In such cases it might be beneficial for the speed of calculations to convert the structure to reduced (Niggli) cell representation using the Redefine Lattice dialog.

The following topics contain information on how to set up a CASTEP calculation:

Setting up CASTEP calculations

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