Strain after optimization

[MB]

Sir,
When we make an interface using the interface builder a strain is applied onto one of the surfaces. After that the structure is optimized. I would like to know if there is a change in the distribution of strain and its magnitude after geometry optimisation of the structure.

[zh]

The stress of a bulk configuration can be obtained by calling Stress(..).
Please see the manual:http://quantumwise.com/documents/manuals/latest/ReferenceManual/index.html/ref.stress.html

[MB]

I went through the tutorial and inserted the necessary command n the script file. The stress report printed in the log file is shown here

Stress Report                                                                |
+------------------------------------------------------------------------------+
| [[ -1.81118773e-02   3.92228089e-06  -4.73882461e-07]                        |
|  [  3.92228089e-06  -2.01589654e-02   5.15914664e-07]                        |
|  [ -4.73882461e-07   5.15914664e-07  -7.64831780e-04]] eV/Ang**3             |
+---------------------------------------

Can you please explain about the different elements in this matrix and what they denote.

[Umberto Martinez]

The matrix printed is the stress tensor, each element represents the stress acting in a particular face in a particular direction.

As zh suggested, check also the manual http://www.quantumwise.com/documents/manuals/latest/ReferenceManual/index.html/ref.stress.html

[MB]

Can you please explain which are the corresponding faces for the elements given in the stress report. I saw the link but I could not figure out from it. Please explain in details.

[zh]

It is quite simple. The subscript (alpha, beta) runs over 1, 2, and 3 (i.e., x, y, z).
See below for the matrix notation of stress and strain:
http://en.wikipedia.org/wiki/Voigt_notation

Stress Report                                                                |
+------------------------------------------------------------------------------+
| [[ -1.81118773e-02   3.92228089e-06  -4.73882461e-07]                        |
|  [  3.92228089e-06  -2.01589654e-02   5.15914664e-07]                        |
|  [ -4.73882461e-07   5.15914664e-07  -7.64831780e-04]] eV/Ang**3             |
+---------------------------------------

It is exactly the matrix notation of stress.

[MB]

While building the interface using the interface builder I have chosen to strain the second surface only. After this , I have put the entire structure for geometry optimization. What I am actually interested to know is,  how the interface strain on the second surface gets changed after geometry optimization, not the stress of the entire system. Kindly suggest a way.

[Anders Blom]

The Interface Builder does not report stress, it shows strain, which is defined as a/a' where a and a' are the different lattice constants of the two surfaces. The stress is something quite different, it's a computed property of the entire, composite new cell, and can't easily be subdivided between the two original systems. Of course you can still take the ratio of a'' (the new, optimized lattice constant) to a and a', but I'm not sure what use that number is... More relevant is probably to optimize the stress of the two subsystems before making the interface to see how large the strain really is, not just based on experimental lattice constants but those obtained from DFT.