This FAQ explains what does the expression “sff” in “Phase details” window stand for.

• In “Phase details” window, there is an expression “sff” with some value next to it. What is the meaning of it?

This expression is an artifact from the previous considerations and it is not very important for the MatCalc users. Nevertheless, some explanation is given below:

Some phases in MatCalc contain vacancies as components. In this regard, the sites that are normally considered as “unoccupied” are treated as “occupied by vacancy”. An important example is FCC_A1 phase, where the interstitial sites are represented by the sublattice with vacancies set as major constitutent resulting in the considerable value of the vacancy site fraction on this sublattice as expressed by Y$FCC_A1$VA$1 variable.

“sff” is an abbreviation of “site fraction factor”. It was an auxiliary variable for the considerations of the amount of the sites occupied by the atoms. It is taken as:

\[\Large sff=(\large\sum\limits_j \normalsize (n_{j}\sum\limits_i y_{ij})\Large)^{-1}\]

with

$n_j$ - number of sites on the sublattice $j$
$y_{ij}$ - site fraction of the element $i$ on the sublattice $j$

Note, that the site fractions of the vacancies at a given site are not included in the above sum.