[Pw_forum] Units in dynamical matrix files

[Cristian Degli Esposti Boschi]

Dear all,

I would like to have some clarifications on the units used in dynamical 
matrix files produced by the ph.x program in the QE suite.

At the beginning of dynamical matrix file the masses are not expressed 
in the same units as in the input files used for pw.x, that is [amu].

It seems to me that in the dynamical matrix file the masses could be in
atomic units, which I would say correspond to electron masses.

So there should be a conversion factor

1 amu = (1.660538782 × 10^-27 kg / 9.1093826 × 10^-31 kg) me
       = 1822.8884 me

However there is still a factor 1/2 to include to pass from
the amu values to those reported at the beginning of the dynamical
matrix file.

Am I right? Why the 1/2 factor?

And what are the typical units used for force constants?

Thanks a lot. Cristian
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Cristian Degli Esposti Boschi
CNR, CNISM, Unita' di Ricerca di Bologna, c/o
Dipartimento di Fisica, Universita' di Bologna
viale Berti-Pichat, 6/2, 40127, Bologna, Italia
tel. ++39 051 2095114   fax ++39 051 2095113
e-mail: cristian.degliesposti -AT- unibo.it
web:    http://www.df.unibo.it/fismat/theory
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