[DFTB-Plus-User] Possible bug, stress tensor and vibrational modes

Hegoi Manzano hmanzano at MIT.EDU
Thu Apr 29 22:07:54 CEST 2010


Dear all,

I will repeat a previous email without answer, just in case, adding this time a new question. I have started using the new dftb+ 1.1 release, and I found some problems.

1.- I am trying to calculate the elastic tensor coefficients for quartz as a test. My Si-O parameters are from the work

Guimaraes, L., et al., Imogolite nanotubes: Stability, electronic, and mechanical properties. Acs Nano, 2007. 1: p. 362-368.

so in principle should be quite correct. I am performing strain stress calculations, applying only one non zero strain to the unit cell and plotting the generated stress to fit the values and obtain the corresponding elastic coefficient. I would like to know the convention of the stress tensor and its units, as the results are completely different from the expected. The stress-strain relationship is lineal as it should be, so that is Ok. If the units are a.u. it should be Hartree/bohr^3, but when I made the conversion to GPa and the results are really crazy. By the way, the same happens for calcium oxide stress tensor, even if energy-volume calculations give a quite accurate bulk modulus (102 GPa DFTB and 112 GPa experiments)

2.- I have some slako files with a polynomial representation of the repulsion instead a spline. With the previous release they work (the snapshot) they work, but now I get the error:

ERROR!
-> Incompatible repulsive polynomial coefficients  for the species pair  1- 2

I don't know where is the error, as the same file works for the previous version

3.- How can I calculate the vibrational modes for a given small cluster?

Thank you and best regards!

_____________________________________________
Dr. Hegoi Manzano

Concrete Sustainability Hub.
Massachusetts Institute of Technology, Room 1-370
77 Massachusetts Avenue, Cambridge, MA02139

e-mail: hmanzano at mit.edu // hegoi.manzano at gmail.com
           www.hegoi-manzano.blogspot.com
_________________________________________________
________________________________________
From: dftb-plus-user-bounces at dftb-plus.info [dftb-plus-user-bounces at dftb-plus.info] On Behalf Of Benjamin Hourahine [benjamin.hourahine at strath.ac.uk]
Sent: Thursday, April 22, 2010 2:39 AM
To: User list for DFTB+ related questions
Subject: Re: [DFTB-Plus-User] DFTB+ 1.1 release candidate

Hello,

sorry for the delay in replying, the stress tensor is calculated in atomic units,
and printed in atomic unit of force / atomic length^2.

Regards

Ben
________________________________________
From: dftb-plus-user-bounces at dftb-plus.info [dftb-plus-user-bounces at dftb-plus.info] On Behalf Of Hegoi Manzano [hmanzano at MIT.EDU]
Sent: 17 April 2010 01:42
To: User list for DFTB+ related questions
Subject: Re: [DFTB-Plus-User] DFTB+ 1.1 release candidate

Dear dftb+ community,

Anyone knows the unit of the stress tensor in dftb+ 1.1 release?

Thank you!


_____________________________________________
Dr. Hegoi Manzano

Concrete Sustainability Hub.
Massachusetts Institute of Technology, Room 1-370
77 Massachusetts Avenue, Cambridge, MA02139
e-mail: hmanzano at mit.edu // hegoi.manzano at gmail.com
           www.hegoi-manzano.blogspot.com
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