The H, C, N, O, and F pseudopotentials are from the Rappe-Bennett
pseudopotential library (GGA-optimized):
  http://www.sas.upenn.edu/rappegroup/research/psp.html

ONETEP-compatible (*.recpot) versions were obtained from the CASTEP project
website:
  https://ccpforge.cse.rl.ac.uk/gf/project/castep/

The S pseudopotential was generated by K. Refson as part of a set of
pseudopotentials intended to supplement the Rappe-Bennett library. 
This was also obtained from the CASTEP project website:
  https://ccpforge.cse.rl.ac.uk/gf/project/castep/

