[QE-users] A few questions about cell dynamics

[Andrii Shyichuk]

Dear Users,

I study a certain defect in a crystal, where there are two states 
(geometries), say, A and B.

Structure B is a meta-stable state:
- with PW vc-relax I get an optimized B structure;
- structure A total energy is lower than B total energy, and B can in 
principle morph into A;
- NEB shows presence of energy barrier of the B>A process, but it also 
shows that B is a local minimum.

In order to check if structure B is stable under heating, I've ran PW 
vc-md at different temperatures, and for lower temperatures B is stable, 
while for higher temperatures B morphs into A, as expected.

The questions:

1. How can I make a correspondence between the NEB barrier and the 
MD-estimated transition temperature? Should I expect the transition to 
happen when Ekin in MD is larger than E_barrier? (Both energies are per 
cell.)

2. I use ion_dynamics = 'beeman', ion_velocities = 'default', 
ion_temperature = 'rescaling' and nraise = 1. I've tested different 
values of dt, and it seems that with dt = 206.623 (10 fs) my system does 
not misbehave (the things go south with the atoms flying around at dt = 
500 and higher).
Should I assume that I play it safe, or is it better to use a smaller 
dt?
The transition takes about 500 fs, hence it's either long steps, or a 
lot of steps.

3. How much does the temperature set by tempw ( tolp = 50 ) correspond 
to the actual temperature? E.g. if I find that the morph happens at, 
say, tempw = 1000+-50 and higher, how safe it is to say that the same 
will happen at 1000+-X K and more in real life?

Thank you in advance.
Best regards.
Andrii Shyichuk
Department of Chemistry, University of Wrocław, Poland