Messages

16

General Questions and Answers / Different Unit Cells for MoS2

« on: March 28, 2016, 09:08 »

I've created two MoS2 poscars; one from VNL and another from Python-ASE.  Attached are the files.  Why are there different variations for the hexagonal MoS2 unit cell?  Are there any advantages?

17

General Questions and Answers / kpoints and hexagons

« on: March 18, 2016, 09:13 »

I'm straining a transition metal dichalcognide (MoS2 for example), and my band diagrams are as expected.  However, I want to measure the change in geometry of the BZ unit cell, as shown in the attached image.  How do I measure geometrical distances with VASP?

The x,y,z positions for G, M and K are angular values, and don't tell you anything about the actual dimension. When the hexagon is strained, the geometry is changed.

How can I show there has been a physical dimension change in the hexagon from unstrained to strained?  The more I try this, the more confused I get.  Hope this makes sense.

Thanks
Mark

18

General Questions and Answers / Re: BZ Routes: K and K-prime

« on: March 16, 2016, 21:52 »

Thanks.  That's what I'm doing.  When you're stretching a molecule, how much actual change can be expected from a symmetric hexagonal lattice to an asymmetric one? When I run vasp calculations, the difference is size between the two BZ routes is so small, it's almost imperceptible.   I have to subtract the differences between eigenval data before I see any change in size.  Even then it's quite small.  Is that to be expected?

Here's the way I run my bandstructure calculations.  I allow the poscar to relax for all 3 runs.  Should I fix them after optimization?  Or is this OK and my data is as expected?  I don't have any data to compare to as a reference, so I don't know if my data is right or wrong.  There is a clear trend of percent strain vs the ratio change of the two K-K' routes at least.

1) Optimization.  Unfixed poscar
2) Self-consistency.  Unfixed poscar
3) Bandstructure.  Unfixed poscar.

Mark

19

General Questions and Answers / BZ Routes: K and K-prime

« on: March 15, 2016, 21:25 »

When using the VASP Scripter, how can you specify K-prime (as opposed to K).  I need a K-Gamma-K' because my k-space is strained, and won't be symmetric; so I get K'.  Do I do something like the following in the KPOINTS file?

0.333333 0.333333 0.000000 ! K
0.000000 0.000000 0.000000 ! G

0.000000 0.000000 0.000000 ! G
-0.333333 -0.333333 0.000000 ! K

Thanks,
Mark

20

General Questions and Answers / Documentation for VASPPlugins?

« on: March 2, 2016, 18:39 »

I attempted to find documentation for the VASPPlugin python commands, but was unable to locate them on the QuantumWise website.  Can someone point me in the right direction?

Thanks,
Mark

21

General Questions and Answers / Re: Band Gap calculation without ATK

« on: March 2, 2016, 08:17 »

Yes, you read my mind.  I needed to get bandgap data from VASP files.  Neglected to mention that.  Thank you!  Didn't realize I could do that so easily.  I've been looking for a simple way of getting VASP bandgap for a long time.  Thanks for coming to the rescue.   

Mark

22

General Questions and Answers / Re: Band Gap calculation without ATK

« on: March 1, 2016, 17:18 »

I'm a licensed user of VNL 2015 (not the free version).   I'm familiar with writing code using python-ASE.  I know there's ATK commands like bandstructure._directBandGap().inUnitsOf(eV),  but I'm unfamiliar with how to write scripts using these commands in VNL.  There is an "Editor" window, but not sure how to get started.  If I can use these commands, that would great! 

Mark

23

General Questions and Answers / Band Gap calculation without ATK

« on: March 1, 2016, 08:04 »

I can successfully create and view a bandstructure using VNL.  But how can I determine the band gap (direct and indirect) and Fermi level?   I get the impression their are scripts for this, but they require ATK.  Is there an alternative way of finding the band gap?

Mark

24

General Questions and Answers / Pov-ray for Dummies?

« on: February 2, 2016, 20:57 »

I'm looking to create professional quality images of charge transfer density plots.  What is the easiest method of generating publication-level images from chgcar data?  A commercial application is acceptable.   I can generate .pov files with VNL correctly.  I'd prefer something user-friendly.  Installing and using pov-ray has been unpleasant.   

Thanks,
Mark

25

General Questions and Answers / EIGENVAL files and BandStructure Analyzer

« on: January 6, 2016, 03:32 »

I'm attempting to use the Bandstructure Analyzer on a set of VASP files.  Although the eigenval file shows up in the Project File list, the Labfloor does not include eigenval, so the Analyzer option is disabled.   My eigenval file appears valid.   Any suggestions?   Doscar correctly shows the DOS, and chgcar provides a 2-D View.

Thanks,
Mark

26

General Questions and Answers / POSCAR: Import-Export Issues

« on: December 31, 2015, 21:23 »

I'm experiencing a really odd problem with importing/exporting poscar files with vasp.  Attached are two files: a good, working poscar before importing into VNL, and another one that is exported from VNL.  I made no changes during the export.

The exported version is unusable.  First of all, the atoms are set to fixed positions (which I didn't specify), and if delete the "T" option so they can relax, the vasp run crashes with errors that don't make any sense.  The "before" version works perfectly fine.  But the "after" version does not run on vasp.  I'm using the same INCAR and KPOINTS for both vasp runs.

I don't understand how to fix this behavior.  Any suggestions would be appreciated.

Thanks,
Mark

27

General Questions and Answers / Charge Density Plots

« on: December 7, 2015, 00:45 »

How can I create charge density plots using VNL?  I can do using VESTA (as shown), but not sure what the steps are in VNL.  I looked through the tutorials and videos, and didn't see any examples.

Mark

28

General Questions and Answers / 1T'-MoS2 lattice structure

« on: December 6, 2015, 06:16 »

Is there an "open-source" cif or poscar model for 1T'-MoS2?  2H and 1T are easy to come by, but since 1T' is a distorted structure, I'm not clear how to create this from scratch.

Mark

29

General Questions and Answers / WS2: hexagonal vs triclinic

« on: November 20, 2015, 05:57 »

For WS2, the VNL database uses a Hexagonal lattice.  I imported a CIF, and it's Triclinic.  I suppose they are equivalent, depending on how you define your unit cell, but which is the "real" stucture?  I wasn't aware of two different representations, so I will need to be careful and use one or the other.  But is one more common?

Thanks,
Mark

30

General Questions and Answers / Changes to POSCAR

« on: November 14, 2015, 22:15 »

I import a known, good WS2 POSCAR file (see the attached POSCAR-GOOD).   When I export this file I get the other attached file (POSCAR-GOOD-VNL).  However, the lattice parameters have changed.  A VASP run works OK on the POSCAR-GOOD file, but crashes on the POSCAR-GOOD-VNL file. 

How do I prevent VNL from modifying the poscar?   Or how do I make VASP work with the exported POSCAR-GOOD-VNL version?  Still learning how to use VNL!

Thanks,
Mark