Specifying positions

Dear JD Gale,

Is it possible in GULP to specify the same site, occupy for more that one atoms. Like is it possible to specify that 50% of atoms occupy by one atom and 50% by the other. If so how to do.

I read from the manual that it only means that mixture of atoms is occupying the site. But if I want to perform doping like studies how should I do it ?

Suggest me some ideas. It will be of great help.

Thank you.

To mix atoms on the same site is easy - just put them at the same coordinates & give them an occupancy of (in this case) 0.5 each. The atoms will then automatically be constrained to move as a single entity during any optimisation. This will give you a mean field model for a doped system, which is OK for things like mean lattice parameters etc. For accurate thermodynamics you need to do explicit atomistic calculations as a function of concentration & arrangement of dopants with either a supercell or Mott-Littleton.