[Pw_forum] pressure from pw calculation-further

[W. YU]

>> 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

Dear Paolo and pw users,

The following is the relationship between lattice
parameter a0 and pressure from the results of pw
calculation.The fitted zero pressure lattice parameter
is about 7.74au. If the pressure is the derivative of
the energy with respect to the volume (given by -dE/dV
?) at fixed basis set,the absolute value of pressures
might be different from the fitted ones, but the sign
of the pressure should be at least positive with a0
less than about 7.7au. But the output pressures from
pw are all negtive. Why?

By the way, where can I find the literature explain
this "Pulay stress". I didn't find it in the mannual 
or the input pw.

best regards!

W. YU
======================================================
  
a0(au)      P(ressure,kbar)

7.4     -2966.80
7.5     -3074.92
7.6     -3148.36
7.7     -3186.48
7.8     -3243.19
7.9     -3286.29
8.0     -3315.97
8.1     -3327.72
8.2     -3336.04
8.3     -3336.26
8.4     -3309.90
8.5     -3227.79


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