[Wien] Comparison between ab inito calculation and "measurement" result

[Tomas Kana]

Dear Seongjae, 




 In addition to what Martin has written, I would like to point out that the 
difference between 

the description of a crystal at 0 Kelvin and the same crystal at a room 
temperature 

of 300 K corresponds to the difference between two 

approximations in solid state physics (see e.g. the textbook 

Neil Ashcroft and N. David Mermin, Solid State Physics): 

For 0 K you use the static lattice approximation, 

for 300 K you use the dynamic lattice approximation - it means include 
phonons 

(that can be regarded as quantum units of lattice vibrations, summing them 
up you 

get the whole vibration of your crystal). 

Treating temperature dependent parameters (such as heat capacity) one 
usually 

starts with the static lattice approximation and computing the ground state 
of 

a sufficiently large supercell (using for example the code Wien2k) and 

from there you get the second derivatives (Hessian) of the interatomic 
potentials 

needed to construct the equations valid for the phonons (the dynamic 
matrix). 

In real life this means that after finishing the calculations in Wien2k 

(using the static lattice approximation) you continue with a code like is 
for example 

PHONON by the group of prof. Krystof Parlinski and calculate the quantities 
you 

like to know. 

(see http://www.computingformaterials.com/index.html and others, see 

http://www.wien2k.at/reg_user/unsupported/ after the item Phonon). 

Best regards 

Tomas 






"

Dear group, 




As an engineering researcher with great lack in understanding the ab initio 
calculations, 

I have basically believed that the first-principle calculation results 
demonatrate rather 

"ideal" values presumably obtained at "0 K" and they need to be adjusted by 
proper mathematical 

models formulated as a function of temperature for reachiing the more 
practical values at non-0 K values. 




However, in many pieces of literature, they are trying to compare the ab 
initio calculation 

results and the measurement results at non-0 K, particularly at room 
temperature. 




I'm wondering what sort of foundation is required for believing that the 
simulation results 

can be treated as those obtained at 300 K. In other words, what models or 
equations can be 

adopted for taking the exact band structures and related parameters (Eg, 
effective mass, etc.) 

in hand in performing the first-principle simulations? 




It will be appreciated if you fix my fault and share some wisdom. Many 
thanks. 




- Sincerely, Seongjae. 




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