Hello,
The values for a, b, and c depend on your system. If, for example, your
system is finite in the x and y direction, but infinite in the
z-direction, then you will choose 1 k-point for the x- and y-
directions, but (perhaps - it depends on the length) more in the
z-direction. To figure out how many k-points you need, you have to do
multiple runs and find the lowest number of k-points necessary to get an
accurate energy value. It would also be instructive to read other
published results that discuss the chosen number of k-points for
calculations on systems similar to the one you're considering.
TranSIESTA does not take c into account, because the boundary conditions
for transport are different than they are for DFT. Specifically, you
change from an infinitely periodic system in all directions, to
semi-infinite electrodes in the transport direction, while the other two
directions retain the same boundary conditions. The reason that changing
c may change your results is because it /does/ affect the SIESTA run -
and therefore your DM, H, and S - which will then change your TranSIESTA
results.
Here
<http://193.146.160.29/gtb/sod/usu/$UBUG/repositorio/10300141_Monkhorst.pdf>
is Monkhors and Pack's paper.
<http://iopscience.iop.org/0953-8984/14/11/302>Here
<http://prb.aps.org/abstract/PRB/v65/i16/e165401> is the TranSIESTA
paper, which discusses their chosen boundary conditions.
Hope this was helpful,
-Jonathan
On 06/13/2012 07:48 AM, 隆小江 wrote:
Dear Siesta Users,
I was confused about the K-point in transiesta calculation. for example:
%block kgrid_Monkhorst_Pack
a 0 0 0.0
0 b 0 0.0
0 0 c 0.0
%endblock kgrid_Monkhorst_Pack
some articles set a=b=1, c about 20 to 30.
while some others set a=b=3~6, c=1.
I hear from the http://emuch.net/bbs/ that transiesta would not take c
in to account, namely there would be no difference between c=1 and c=30.
but I got different results when set a=b=2, c=1 and a=b=2, c=9. so I
think that it might be incorrect.
some other people says that the value of a, b, c should be different
for the periodic systems and nonperiodic systems.
I am wondering how to set them on earth.
Thank you very much for your help. I look forward to your responses!
