Ivan Scivetti ivansc... at gmail.com
Thu Feb 21 12:26:22 UTC 2019

Dear CP2K community,

For cell optimization one has the option of EXTERNAL_PRESSURE. In the 
manual it is explained that this input parameter should be set as a list of 
values, which I assume they represent the components of a tensor. I find 
this a bit confusing since pressure is an scalar and equal to the trace of 
the stress tensor. Still, in the special case of isotropic systems, each 
diagonal component is equal to the external pressure. Indeed, this seems to 
be the default case  for EXTERNAL_PRESSURE, where each diagonal component 
is set to 100 bars. My question is the following: how should 
EXTERNAL_PRESSURE be defined for an hexagonal cell with a=b neq c, with  
alpha=beta=90 and gamma=120 degrees? 

Thanks a lot in advance   

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