[Pw_forum] alkali metal pseudopotentials

[Axel Kohlmeyer]

On Fri, 30 Dec 2005, W. YU wrote:

> Dear PW users,

dear w. yu,

> Happy and brighter new year!
>
> I have some problmes on using the alkali
> pseudopotentials;
>
> 1. Has anybody know any real system usage of the
> pseudopotenals of Li, Na, and K (both LDA and GGA)
> provided by the PW web?

some papers are actually mentioned in the details
section of the webpages, and the vanderbilt uspps
are also posted on the vanderbilt website and have
been used in different codes as well.

> 2. I did some calculations on alkali hydrides such as
> LiH and NaH. For LiH, the LDA using Von Barth and Car
> scheme give a much smaller lattice constant while GGA
> using uspp and a ld1-generated TM type Norm-conserving

there are several points to consider here:

LDA functionals in general are known for overbinding, as
are GGA for underbinding. the difference from the experimental
lattice constant can be easily 5% or even more.
some people claim, that using LDA at the experimental
lattice constant gives the most consistent results
for (pure) alkali metals. binary compounds (and beyond)
are - of course - more tricky.

using pseudopotentials with only one valence electron
for (partially) ionic compounds, is tricky as well.
i would suggest comparing to a 'semi-core' (or in the
case of Li all-electron) pseudopotential.

> LDA pseudopotential or the LDA pseudopotential
> converted from FHI format give much larger lattice
> constants than the experimental result. Bearing in

last time i checked, the FHI converter did not support
NLCC, so that potential may be totally bogus.
please recheck.

> mind the small mass of this system, the vibrating
> energy should play an important role and the
> theoretical LDA lattice constant would be much
> smaller. For NaH system, the VBC LDA gives a lattice
> constant very close to the experimental one and the
> GGA gives larger lattice constant. When the vibrating
> effect (Zero point energy and temperature effect) is
> taken into account, the resulting lattice constant is
> much larger than the experiment. While a
> full-potential calculation  with both LDA and GGA gives

can you please elabotrate, what you mean be 'full-potential'
calculation.

> much smaller lattice constants for both LiH and NaH
> than experimental ones. Now it seems that the
> calculations using pseudopotentials cannot give
> systematic results for LiH and NaH even with VBC
> norm-conserving pseudopotentials---that is, a much
> smaller zero-temperature lattice constant with a major
> improvement when the zero-point energy is included,
> but still less than or just close to the experimental
> one.
>
> (Note, the none-linear-core corrections are used on
> all alkali atom pseudopotentials).
>
> In this case, can I still trust the pseudopotential of
> Li and/or Na?

i have tested zv=3 pseudopotentials for Li (LDA and several GGAs)
vanderbilt uspps and troullier-martins nc and less well tested
Na semi-core potentials (also uspps and nc) and matching hydrogen 
PPs. you probably will have to test them for your system as well,
but give me a holler if you are interested.

axel.

>
> thanks a lot!
>
> W. YU
>
>


=======================================================================
Axel Kohlmeyer    e-mail: akohlmey at cmm.upenn.edu, tel: ++1-215-898-1582
     Center for Molecular Modeling    --    University of Pennsylvania
Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
=======================================================================
If you make something idiot-proof, the universe creates a better idiot.