[Pw_forum] why SCF calculation for isolated atom is tricky

[Lorenzo Paulatto]

In data 24 settembre 2009 alle ore 06:53:06, udayagiri sai babu  
<udayagiri3 at gmail.com> ha scritto:
> In the forum I have seen the statement like doing scf calculation for an
> isolated atom is tricky. Can somebody explain me why is this so?

Because finding the ground-state of an isolated atom often involves  
breaking the symmetry, which requires reading the manual (hint:  
nosym=.true. and specify a starting magnetization).

The most serious problem is that the ground state is usually degenerate,  
e.g. Boron can put it's higher-energy electron in p_x, p_y or p_z or any  
linear combination of three and still get exactly the same energy.
As a consequence Self-consistency cycle can start to oscillate between  
degenerate ground-states and never converge. That's why an abundant number  
of bands and a small smearing is usually a good idea. Other people prefer  
to break  symmetry by hand, e.g. specifying a non-cubic unit cell, but I  
personally find it to be not very effective.

> and then
> while testing a pseudo potential is it a good idea to calculate cohesive
> energies (which involves isolated atom) and compare as we do for lattice
> parameter and bulk modulus.

Yes, it is good idea.

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Lorenzo Paulatto
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