ANADDB: exclude acustic modes from thermodynamic properties [SOLVED]

[ChrisKue]

Hey,

During the calculation of the phonon free entalphy with ANADDB I discovered differences for own values and ANADDB values.

The code of ANADDB (src/77_ddb/thm9.F90, Var: gdos ) reveals that all phonon modes (also the acustic modes) are included in the calculation of thermodynamical properties. Since the partition function is a function of (h_quer*omega), acustic modes should be excluded from the function. Or not ?

As the contribution of low frequencies ( in cm^1) to the values of the thermodynamic properties is big, the error of the values is large.

Chris

Version: abinit 7.6.4

[jzwanzig]

I don't see why you would exclude acoustic modes--all modes contribute to thermodynamic properties, acoustic modes more than most exactly because of their low frequencies.

[ChrisKue]

Usually the acoustic modes in solids at gamma should be zero. Due to numerical inaccuracies the modes are near to zero.
If omega is zero the partition function will have the value zero. sinh(0) = 0.
But log(sinh(0)) is not defined.

I do not quite understand how to deal with the acoustic frequencies which are actual zero at gamma.

1) Anaddb sets the acoustic frequencies (If I use the acoustic sum rule correction) to 1.5cm**-1

2) Exclude the modes because they have no energie (h_quer*omega = 0) and therefore no contribution to the thermodynamical properties.

[jzwanzig]

The acoustic contributions to thermodynamic properties are detailed nicely in Ashcroft and Mermin chapter 23.

[ChrisKue]

Thank you for the hint. Understood something wrong.