[QE-users] Inconsistency in pressure and energy in optimizing degauss

[Sudip Kumar Mondal]

Hi all ,
I haven't encountered this kind of a problem with QE before.I was
performing an optimization on degauss value. Upon a relaxation ,I obtained
the following data :


degauss = 0.02  Final energy = -4970.7298625102 Ry (kbar) P= 3.80
degauss = 0.04  Final energy = -4970.7331133155 Ry (kbar) P= 0.02
degauss = 0.05  Final energy = -4970.7357925053 Ry (kbar) P= -0.66
degauss = 0.06  Final energy = -4970.7395506085 Ry (kbar) P= -4.54
degauss = 0.07  Final energy = -4970.7451525972 Ry (kbar) P= -8.43.

I have already optimized the lattice parameters , ecut , mixing beta ,
k-pts etc. following a vc-relax. The threshold parameters e.g.,
etot_conv_thr , forc_conv_thr  & conv_thr in &electrons were set
accordingly to satisfy scf corrections to forces. As you can see that the
final energy and the pressure are changing in an inverse way as the degauss
decreases.
Is there any reason behind this kind of unstable behavior of P and total
energy?
Further which set should I choose to proceed ? Is it the one with P=0.02
with somewhat larger energy  or the one with the minimum energy but with a
larger negative pressure ?
Is there any reason behind this kind of unstable behavior of P and total
energy?

QE version - 5.4.0

Thank you.
-- 
Sudip Kumar Mondal
DST-INSPIRE Junior Research Fellow
High Pressure Temperature Laboratory
Dept. Of Physics/Geological Sciences
Jadavpur University
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