[QE-users] matdyn.modes phonon eigenvectors unit

[Andrea Urru]

Dear Wei Zhang, 

first of all, I would like to recall that it would be appreciated 
if you specify your name and affiliation when sending e-mails to 
the forum mailing list (I didn't spot it in your message). 

Coming to your point:

1. In the file matdyn.modes you find the eigenmodes, obtained from 
the eigenvectors of the dynamical matrix in the following way: 

d (s,alpha) = u (s,alpha) / sqrt(amass(s)), 

where: s labels the atoms in the unit cell, alpha is the cartesian 
index, u is the eigenvector of the dynamical matrix, d is the eigenmode,

amass(s) is the mass of the s-th atom. 
The eigenmodes are then normalized and written to file, so what you  
read in the file matdyn.modes is: 

d' = d / sqrt(|d|^2) 

2. The routine you refer to (WRITE_EIGENVECTORS.F90), in particular the 

lines you reported, is intended to write the eigenvectors of the 
dynamical matrix, not the eigenmodes. 
The routine which writes the eigenmodes is writemodes (you find it 
inside WRITE_EIGENVECTORS.F90, below the lines you refer to). 
So, before using the information contained in the output files, please 
be sure to know if you are reading (or if you are interested to) the 
eigenmodes rather than the eigenvectors of the dynamical matrix. 

3. About the units: in all the cases the vectors printed are normalized,

so they are dimensionless (since they are obtained by dividing the 
vector by its magnitude, both quantities having the same dimensions). 
To get a dimensioned quantity you should multiply the vector by a 
constant (that I will call amplitude from now on). The amplitude has 
the dimension of a length (Angstrom, bohr radii, m, cm, depending on 
which units you want to use) and defines the "total length" (or
magnitude) 
of the vector (which, otherwise, without being multiplied by the 
amplitude would have magnitude 1). 

Note: in order to do "frozen phonon calculations" the eigenmodes should 
be used, not the eigenvectors. 

I hope this helps, Best regards, 

Andrea Urru 

PhD student in Condensed Matter 
SISSA Via Bonomea 265 
34136 Trieste (TS)

On 2020-01-30 07:08, Wei Zhang wrote: 

> BLOCKQUOTE{margin-Top: 0px; margin-Bottom: 0px; margin-Left: 2em}
> body {border-width:0;margin:0} img {border:0;margin:0;padding:0} 
> Dear all, 
> I am not sure about the acuurate unit of phonon eigenvectors printed
> at the MATDYN.MODES. e.g. , 
> 
> In
> https://www.mail-archive.com/users@lists.quantum-espresso.org/msg04000.html
> 
> The authors mentioned : the units  is masses^(-1/2) (masses are in
> atomic rydberg units, i.e. sqrt(-911.444*amass(1) or (2) ......) ). 
> 
> In  WRITE_EIGENVECTORS.F90 OF QE, it writes 
> z_((na-1)*3+ipol,i) = z((na-1)*3+ipol,i) * SQRT(AMU_RY*AMASS(NTA)) 
> which has already been MULTIPLIED by sqrt(amu_ry*amass(nta)) , i.e.,
> sqrt(-911.444*amass(1) or (2) ......) ?? (As I have not found the
> value of constant AMU_RY used in QE ) 
> 
> The sum of the norm of XYZ of every modes in matdyn.modes equals to 1,
> which has been normalized, So I want to check that: the real
> displacement of XYZ should be divided by sqrt(911.444*amass(1) or (2)
> ......) with Bohr length unit? 
> 
> Thanks! 
> 
> 2020-01-30 
> -------------------------
> body {font-size:10.5pt; font-family:微软雅黑,serif}
> _______________________________________________
> Quantum ESPRESSO is supported by MaX (www.max-centre.eu/quantum-espresso [1])
> users mailing list users at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users
 

Links:
------
[1] http://www.max-centre.eu/quantum-espresso
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20200203/cfb4df58/attachment.html>