Queries about oprical spectrum

[ramkrishna]

Dear Sir,
        I am very new in optical spectrum calculations and so I have some queries,

1.  http://www.quantumwise.com/documents/tutorials/latest/SiliconOptical/index.html/ . This tutorial shows the calculations of optical spectrum by using DFT+ meta-GGA for bulk Si. Is the meta-GGA only exchange correlation function which can be used for the calculation of optical spectrum? Can we use semiemperical extended huckel (slater-coster)  for this calculation (as DFT is more time consuming)??

2. Can we calculate optical spectrum by considering meta-GGA for nanowire/nanoribbon?

MGGA gives error: "Diagonalization did not converge" and "mathutils.cpp:1086"

in this topic, I have seen that Mr Anders has replied

MGGA isn't really designed to work with systems that contain vacuum.

, So which method can be suitable for the calculations in case of nanoscale system?

3. When I am calculating the static dielectric constant of bulk Si (as mentioned in the tutorial) for device application, then due to the symmetry it is easily understandable but when there is no symmetry (please find the attachment) then what will be the appropriate choice of dielectric constant of the material for the device application? Is there any concept of "effective dielectric constant" of material? If you please discuss about this then it will be too helpful for me. (if possible, please provide a ref)

Regards
Ramkrishna     

[Nordland]

Dear Sir,
        I am very new in optical spectrum calculations and so I have some queries,

1.  http://www.quantumwise.com/documents/tutorials/latest/SiliconOptical/index.html/ . This tutorial shows the calculations of optical spectrum by using DFT+ meta-GGA for bulk Si. Is the meta-GGA only exchange correlation function which can be used for the calculation of optical spectrum? Can we use semiemperical extended huckel (slater-coster)  for this calculation (as DFT is more time consuming)??

Any model will work provided it gives the correct energies and orbitals shapes.

3. When I am calculating the static dielectric constant of bulk Si (as mentioned in the tutorial) for device application, then due to the symmetry it is easily understandable but when there is no symmetry (please find the attachment) then what will be the appropriate choice of dielectric constant of the material for the device application? Is there any concept of "effective dielectric constant" of material? If you please discuss about this then it will be too helpful for me. (if possible, please provide a ref)

I would recommend to try the new version 12.8, since there has been fixed issues with symmetries of the optical spectrum. But in general, materials may have different optical axises with a different dielectric constant in different directions.

[ramkrishna]

Dear Sir,
         In case of bulk Si, the Extended Huckel method underestimates the static dielectric constant (almost half the value from DFT) than DGT-MGGA but the nature of spectrum remains almost same. Please find the attachments. Why is it then coming such a low value?? Can you please check the scripts and tell me where is my mistake?? It will be too helpful for me. 

In case nano-wire/ribbon... what should I do?

Regards
Ramkrishna