[Pw_forum] pressure from pw calculation

[Paolo Giannozzi]

On Friday 14 January 2005 11:29, W. YU wrote:

> I calculated the energy as a function of lattice parameter,
> then fitted them to Murhaghan equation of state and got 
> the minimum energy lattice parameter. The fitted zero
> pressure is also corresponds to the this lattice
> parameter. But when I put "tstress=.true." keyword in
> the input, the output P is a very large negative value
> at the fitted zero pressure lattice parameter. What's
> wrong here, or just I have some misunderstanding?

there is nothing (possibly) wrong. The pressure calculated
as the trace of the stress is the derivative of the energy
with respect to the volume, at fixed basis set, i.e. fixed
number of plane waves. At convergence this has to be
the same as the pressure one gets from the equation
of state. As a matter of facts, the latter converge much
better than the former with respect to the cutoff. This 
behavior is documented and explained ("Pulay stress")
in the literature.

Paolo