# DEBYE

Useful

DEBYE temperature Debye-temperature [idwopt]


The effect of temperature on FMS is approximated by multiplying each free propagator by $\exp(-\bar\sigma^2k^2)$, which gives correct DW factors for single scattering. The DW factors for multiple scattering are not exact, but their contribution is reduced both by thermal factors and by the mean free path. Also if you are running the FMS module, then you can only obtain XANES, where this approximate treatment of thermal effects is probably adequate.

The DEBYE card is used to calculate Debye–Waller factors for each path using the correlated Debye Model. The model is best suited for homogeneous systems, where it is quite accurate. CAUTION: in heterogeneous systems the model only gives approximate values which can easily be off by factors of two or more. If this card is present, the correlated Debye model Debye–Waller factors will be summed with the DW factors from the SIG2 card and from the ‘list.dat’ file, if present. Note that the DEBYE card is currently incompatible with the CFAVERAGE card for options other than the correlated Debye model (idwopt > 0). Temperatures are specified in kelvin.

*Debye-Waller factors for Cu at 190K with correlated Debye Model
DEBYE 190 315


By default, idwopt = 0 specifies that the correlated Debye model is used to calculate EXAFS Debye–Waller factors. Two additional models for calculating DW factors are available in FEFF8 based on the information about the harmonic force constants in the material. idwopt = 1 means the equation of motion (EM) method is used to get Debye–Waller factors and idwopt = 2 means the recursion method (RM) which is an improved correlated Einstein model. Both methods are faster than molecular dynamics simulations, and the recursion method is much faster than the equation of motion method. However, the equation of motion method leads to somewhat more accurate results than the recursion method. These additional methods seem to be superior to the correlated Debye model in cases with tetrahedral coordination, such as solid Ge and many biological materials. Both EM and RM methods need additional input (the force constants) and a complete description of both is given in Anna Poiarkova’s thesis (see the FEFF Project web site) and in the associated documentation.

* Calculate Debye-Waller factors for Cu at 190K with equation of motion
DEBYE 190 0 1