[Wien] [Wien2k User Group] Lattice stability in bcc, fcc and hcp and other crystal structures

[Stefaan Cottenier]

> We consider that the stable ground state of a metal is bcc. The fcc 
> structure is the high temperature phase. To calculate the enthalpy of 
> transformation of the fcc to bcc, we consider the following
> 
> Enthalpy of transformation =ENE(fcc)-ENE(bcc)
>
> Now for this above equation, do we have to do a geometry optimization 
> and then calculate the ENE?

Yes (geometry optimization for bcc or fcc = find the optimal lattice 
constant, no troubles here with internal coordinates).

> Now, for example, We want to calculate the ENE of Fe in a monoclinic 
> lattice, then doing a geometry optimization and then finding the ENE has 
> a purpose.
> 
> What about the stable phases? And what about the high temperature phases?

For more complex structures, the unit cell volume and shape (a, b, c, 
alpha, beta, gamma) as well as the internal coordinates have to be 
optimized for all phases.

Mind that in this way you find a zero kelvin tranformation enthalpy, 
which is for a temperature-induced transformation somewhat a flawed concept.

Stefaan