[Pw_forum] energy origin in espresso

[Nicola Marzari]

Paolo Giannozzi wrote:

>> then one have to use a common reference to compare
>> the levels of different calculations. For this purpose one
>> can use e.g. take a hydrogen molecule 10 Å away from
>> the slab or molecule, whose bonding orbital can be taken
>> as reference.
> 
> yes, this is a simple way to find a common alignment


Dear Ambavale,


to elaborate on Paolo's comment: this is one of the two common
ways to do it. The other would be to do a plot of the electrostatic
potential - far away form the slab it will become flat (unless your
slab, e.g., has a surface dipole - then you'll learn you have
a small problem, and need to be careful about electrostatic
interactions between different charges).

But suppose it will become flat - that will point to the reference
zero energy of that calculation. You redo this for all calculations,
and you have reference points of each of them.

Use this approach, and the one where you add the hydrogen molecule.
Do they give you the same results ? In principle they should - if they
do not, you need to figure out what is the problem, and in the process 
you learn a lot about accuracy.

Warmly recommended,

			nicola

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Prof Nicola Marzari   Department of Materials Science and Engineering
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