[Wien] Gmax and geo optimization

[Peter Blaha]

> 1. Is it possible to optimize the geometry (with the lattice parameters and atom positions simultaneously, not just E-V or fixed c/a ratios, etc.) with wien2k? 

No and yes.
WIEN2k does not have a "stress tensor", thus optimization of lattice parameters can
only be done by total energy.
In volume optimization (x optimize; optimize.job) one can easily replace   run_lapw   by  min_lapw
and thus simultaneously optimize vol and internal position.

Similar, 2D optimizations (a,c) can be done fairly easily at moderate costs (scripts available),
maybe also a 3D case is still feasable.

However, for a "triclinic" case a 6D search by E-tot only is hardly possible.

> 2. I wish to know the unit or dimension of Gmax (usually taken as 14)? I thought it is the unit of displacement (bohr radius). Any comments?

GMAX is the magnitude of the largest K-vector in the Fourier expansion of rho and V. The k-vectors are
given in units of 2pi/a and a is specified in atomic units (bohr). Thus Gmax has bohr^(-1).
Gmax^2 is the "energy cutoff" in Ry, and such a definition is often used in PP calculations.

-- 
-----------------------------------------
Peter Blaha
Inst. Materials Chemistry, TU Vienna
Getreidemarkt 9, A-1060 Vienna, Austria
Tel: +43-1-5880115671
Fax: +43-1-5880115698
email: pblaha at theochem.tuwien.ac.at
-----------------------------------------