[Pw_forum] cell optimization

[Nicola Marzari]

> On Sun, 15 Jun 2008, hania djani-ait aissa wrote:
>
> 
> HDA> I am trying new calculation with ecutoff=40Ry, and will see if the 
> HDA> discontinuous energy-volume persist. however for better 


A quick comment - the discontinuous energy-volume issue is one
where the energy *drops down* discontinuously when the lattice parameter
is increased, due to a discontinuous increase in the size of the basis
set when new plane waves enter your calculation. Here:

a=7.28    E=-1282.635
a=7.29    E=-1282.639
a=7.30    E=-1282.658
a=7.31    E=-1282.684
a=7.32    E=-1282.654
a=7.33    E=-1282.624
a=7.34    E=-1282.679
a=7.35    E=-1282.669
a=7.36    E=-1282.652

in going from 7.31 to 7.32 the energy *increases*; same from 7.32 to
7.33 .

While increasing the cutoff might make your calculations physically
accurate, it will not explain why the energy jumps up in going
from 7.31 to 7.32.

My suggestion (did you try it ?) was to make sure that the
calculations were converged to the correct ground state - in exotic
cases (seen only twice by me - in graphene and in the cubic barium
titanate perovskite), and depending on the algorithm, the top band at
some k-point might get stuck in a higher eigenstate.

My limited experience is that startingwfc="atomic+random"
cured this problem in those two cases.


			nicola


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