Trial structures generated by USPEX are relaxed and then evaluated by an external code interfaced with USPEX. Based on the obtained ranking of relaxed structures, USPEX generates new structures—which are again relaxed and ranked. Our philosophy is to use existing well-established ab initio (or classical forcefield) codes for structure relaxation and energy calculations. Currently, USPEX is interfaced with:
VASP — https://www.vasp.at/
LAMMPS — http://lammps.sandia.gov/
DMACRYS — http://www.chem.ucl.ac.uk/basictechorg/dmacrys/index.html
CP2K — http://www.cp2k.org/
Quantum Espresso — http://www.quantum-espresso.org/
FHI-aims — https://aimsclub.fhi-berlin.mpg.de/
ATK — http://quantumwise.com/
CASTEP — http://www.castep.org/
Tinker — http://dasher.wustl.edu/tinker/
MOPAC — http://openmopac.net/
The choice of these codes was based on 1) their efficiency for structure relaxation; 2) robustness; and 3) popularity. Of course, there are other codes that can satisfy these criteria, and in the future we can interface USPEX to them.