SeqQuest requires pseudopotentials and Gaussian basis sets for every atom type in a given calculation. These pseudopotentials are not generated within Quest itself: the Gaussian basis sets are atom- and potential-specific and must be provided. Quest accesses the pseudopotentials and associated basis sets it uses from atom files that have been previously generated, and collected into libraries. This page summarizes the availability of atom files for the various functionals available in the code.
One should not mix different functionals in the atom pseudopotential generation and SeqQuest calculations. If necessary, it is probably safe to use PBE atoms in PW91 calculations given the close correspondence of these two functionals, but we recommend simply keeping to PBE calculations.
The code uses non-local ("semi-local" in common physics jargon) norm-conserving pseudopotentials (not "ultra-soft" nor "separable" potentials). All the pseudopotentials currently in the libraries are either of the Hamann type or the Troullier/Martins type. The pseudopotentials are generally constructed to be very "hard" to ensure maximum transferability. Compromising the transferability of the PP for greater softness is unnecessary, as the cost of a local orbital calculation is relatively insensitive to the PP,
For each available atom of a given pseudopotential and given DFT functional, a Gaussian basis set is provided. Development of effective basis sets is something of an art. The basis sets for Quest are carefully optimized pseudo-variationally, refined and tested in chemical environments representative of typical expected Quest use. Users should check internal documentation of atom files for basis author. These basis sets are intended for periodic calculations. The basis design avoids the very diffuse gaussians that can lead to linear dependence (but can become linearly dependent under significant compressions). Ghost atoms (or "floating orbitals"), basis sets without atomic potentials, are also possible, to augment the atom-centered basis (e.g. to get more accurate work functions for slab surfaces).