Hello everyone,
I am doing NVT MD calculations using pw.x code. During the simulation, my
pressure was constantly oscillating at a particular value of around 50 GPa.
Many literature review papers* mention this line: "The supercell **volume
was relaxed in such a way to minimize the system's energy." In principle,
during the simulations, one could adjust the supercell volume to make the
external pressure of the inherent **structures equal to zero." I* have
tried a lot of different things to maintain my pressure at zero, but as
soon as I manage to achieve oscillatory zero pressure, some other
parameters seem to shift. For example, I ran "md" at 3000K using the
rescaling method, then restarted and ran "vc-relax" with fixed ion
positions to achieve lower pressure, as shown below.
" total stress (Ry/bohr**3) (kbar) P=
123.26
total stress (Ry/bohr**3) (kbar) P=
116.29
total stress (Ry/bohr**3) (kbar) P=
89.51
total stress (Ry/bohr**3) (kbar) P=
77.12
total stress (Ry/bohr**3) (kbar) P=
60.67
total stress (Ry/bohr**3) (kbar) P=
55.80 "
However, in the same vc-relax output*,** temperature* moved as follows:
" Ekin = 0.00000000 Ry T = 0.0 K Etot = -13138.95318083
Ekin = 0.30599294 Ry T = 253.6 K Etot = -13138.94891345
Ekin = 1.41097581 Ry T = 714.8 K Etot = -13138.96700552
Ekin = 1.65432345 Ry T = 878.9 K Etot = -13139.19284122
Ekin = 1.87157146 Ry T = 1101.3 K Etot = -13139.61462781
Ekin = 1.67673049 Ry T = 1142.5 K Etot = -13140.01208055 "
Is this temperature gain due to electron kinetic energy or why does T begin
at zero? Is there something I am missing? Additionally, I used "vc-md" to
control the temperature and pressure, but after that, I was unable to
restart the file
My exact methodology is:
1. restart_mode ='from_scratch',----> MD calculation (NVT) at 1000K
with simple rescaling method up to 10 picoseconds
2. restart_mode = "restart"-----> 'vc-relax' up to pressure convergence
threshold (0.5d0 Kbar) !!! { problem: temperature not maintained shown
above}
My main question is:
Can I restart the calculation from MD to vc-md/relax and again from
vc-md/relax to md ?
And if I run vc-relax or vc-md calculations, my supercell volume fluctuates
slightly, requiring me to switch from an old supercell celldm to a new one
in order to maintain pressure zero..
regards
Jayraj Anadani
Ph.D Scholar, SPU
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