Messages

1186

General Questions and Answers / Re: Calcite and Water

« on: December 5, 2016, 10:23 »

Could you share some information on what the difference between  LCAO in ATK and plane wave method is ?

The main difference between the two methods is that the LCAO and plane wave methods adopt two different basis sets: localized atomic orbitals (LCAO) and plane waves (PWs), respectively, to solve the same Kohn-Sham equations of density functional theory (DFT), which is implemented in the ATK-DFT for the LCAO basis set. However, we are currently working on the development of the ATK-DFT plane-wave based calculator as well. You may consult textbooks on Solid State Physics for more details on these two basic methods used in atomistic modeling of materials and devices.

1187

General Questions and Answers / Re: define constraints for Left direction in graphene sheet

« on: December 5, 2016, 10:08 »

What do you mean by plot "strain"?

1188

General Questions and Answers / Re: How can I analysis the bonding way of Ni-C, and choose the interface distance?

« on: November 29, 2016, 09:29 »

Have you followed any of the references in the paper and/or did google or Web of Science search for this topic? There are tons of papers about a graphene sheet on metal surfaces, in particular, on the Ni surface.  You may have a look at this one http://journals.aps.org/prb/abstract/10.1103/PhysRevB.79.195425, and follow the papers that have cited it.

1189

General Questions and Answers / Re: Transition Energy of Defect Center

« on: November 29, 2016, 09:19 »

If you are really interested in accounting for two-particle excitations (excitonic effect), there is no way you can do it with ground state density functional theory. You then need to adopt the BSE approach, which is not implemented in ATK. For very accurate calculations of single-particle excitations, the GW approach is an option, but it is not yet in ATK.

However, you can still calculate the single-particle spectrum, using the LDA or GGA functional, as descried in this tutorial http://docs.quantumwise.com/tutorials/optical/optical.html about absorption spectrum. I think something like this was calculated in one of the papers you have mentioned. I note again that the excitonic shift, which is calculated with the BSE, will be missing in this spectrum. If your material of interest is h-BN, you might get away with the band gap problem of DFT since h-BN has a pretty large gap.

Regarding optical transitions related to the defects, I think you still need to do it in the way Ulrik suggested, see Eq. 3 in that work. It might be useful to have a look at the following paper by Chris G. Van de Walle1 and Jörg Neugebauer  [J. Appl. Phys. 95, 3851 (2004)]; http://dx.doi.org/10.1063/1.1682673, especially, Sec. 7D, where they do discuss thermodynamic transition levels versus optical levels, in particular, related to photoluminescence. That review paper appears to provide useful information how experimental measurements for defects can be interpreted from first-principles calculations.

1190

General Questions and Answers / Re: How can I analysis the bonding way of Ni-C, and choose the interface distance?

« on: November 28, 2016, 14:57 »

There is a vast literature on this topic, e.g., see http://journals.aps.org/prb/pdf/10.1103/PhysRevB.85.184426 and references therein.

1191

General Questions and Answers / Re: Problem with Bandstructure

« on: November 28, 2016, 14:54 »

Try using a different text editor, and make the dos input file from scratch.

1192

General Questions and Answers / Re: VNL - effective mass

« on: November 28, 2016, 00:42 »

I think so. Actually, you can also calculate effective masses in the band structure analyzer, see http://docs.quantumwise.com/tutorials/effective_mass/effective_mass.html. That allows you to identify the coordinates of the k-points directly in the band structure plot.

1193

General Questions and Answers / Re: Curie temperature

« on: November 26, 2016, 19:03 »

You may find a detailed answer to your question,  for example, at https://www.researchgate.net/post/Curie_Temperature_Calculation_by_DFT_MD_calculations_vasp.

1194

General Questions and Answers / Re: VNL - effective mass

« on: November 26, 2016, 18:48 »

You may have a look at Appendix in W. Setyawan, S. Curtarolo / Computational Materials Science 49 (2010) 299–312, where the Brillouin zones for all 14 Bravais lattices are given together with the coordinates of the symmetry k-points. Using this information, you must be able to define the direction along the symmetry k-line of your interest to calculate the corresponding effective mass in this specific direction of the Brilloiun zone.

For example, if your crystal structure is orthorhombic, the G (0,0,0) -> X (0.5, 0, 0) direction can be defined as (0.5,0,0) or any vector (x,0,0) given in fractions of the reciprocal lattice vectors  b1, b2, b2, see Figure 7 and Table 8, where 0<x<0.5. Note that you have to verify that the direct/reciprocal lattice vectors as defined in VNL are consistent with their definition adopted in the paper mentioned in the previous paragraph.

1195

General Questions and Answers / Re: crystal structure

« on: November 26, 2016, 17:13 »

You can import the structure to the Builder from an external crystallography database: Add -> From Plugin -> Crystallography Open Database, and do search for compounds with K, Nb, O chemical elements. You have to select the KNbO3 crystal structure of your interest from the search list.

1196

General Questions and Answers / Re: VNL - effective mass

« on: November 26, 2016, 11:19 »

This will be exactly the G->X direction.  I am not sure I understand what is your actual question.

1197

General Questions and Answers / Re: how to obtain the phonon transmission of a desired layer in heterostructure

« on: November 26, 2016, 11:07 »

To do that, you may have a look at the following two tutorials http://docs.quantumwise.com/tutorials/phonon_calcs/phonon_calcs.html and http://docs.quantumwise.com/tutorials/thermoelectrics_cnt_isotope/thermoelectrics_cnt_isotope.html.

1198

General Questions and Answers / Re: Spin Orientation

« on: November 24, 2016, 10:57 »

I would suggest having a look at https://en.wikipedia.org/wiki/Ferrimagnetism and references therein to better understand the physics of ferro- and ferrimagnetism.  This should make it easier for you to figure out what is the actual magnetic state of your material considered.

1199

General Questions and Answers / Re: spin-dependent Seeback coefficients

« on: November 24, 2016, 10:47 »

UPDATE: I have added a Device_New_Configuration.py to the zip-file in case someone wants to redo the calculation for the carbon chain, and then run the seebeck_coefficient.py with the corresponding Device_New_Configuration.nc file generated.  I note that the Device_New_Configuration.nc in the updated zip-file has been generated using the ATK2016.3 version. 

1200

General Questions and Answers / Re: MGGA formulation

« on: November 24, 2016, 10:24 »

This is a typo in the tutorial. Thanks for spotting it. But it is implemented correctly in the ATK code, where the factor of sqrt(5/12) was used in the formula for the MGGA-TB09 exchange potential, in consistency with the original work by Tran and Blaha.