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
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