3 Overview of input and output files

Input/output files depend on the external code used for structure relaxation.

An important technical element of our philosophy is the multi-stage strategy for structure relaxation. Final structures and energies must be high-quality, in order to correctly drive evolution. Most of the newly generated structures are far from local minimum and their high-quality relaxation is extremely expensive. This cost can be offset if the first stages of relaxation are done with cruder computational conditions — only at the last stages is there a need for high-quality calculations. The first stages of structure relaxation can be performed with cheaper approaches or lower computational conditions (basis set, -points sampling, pseudopotentials) or level of approximation (forcefields vs. LDA vs. GGA) and even different structure relaxation code (see Subsection 2.5 for a list of supported codes) during structure relaxation of each candidate structure. We strongly suggest you initially optimize the cell shape and atomic positions at constant unit cell volume, and only then perform full optimization of all structural variables. While optimizing at constant volume, you do not need to worry about Pulay stresses in plane-wave calculations — it is OK to use a small basis set; however, for variable-cell relaxation you will need a high-quality basis set. For structure relaxation, you can often get away with a small set of -points — but don’t forget to sufficiently increase this at the last stage(s) of structure relaxation, to get accurate energies. Use your (and our) wisdom, be a strategist, and remember that poor relaxation can ruin your results.