Vacuum_level of Huckel parameter sets

[ziand]

How are the vacuum_level parameters of the Cerda Huckel basis sets determined?
I know about their meaning, but how did your find the exact numbers?

[ziand]

I have another question that bothers me for quite a while:

Some time ago I created my own Huckel basis set for carbon, based on the Cerda_carbon_graphite set. It was a cumbersome fitting process, but I finally got what I wanted. I also took the E^vac into account, which was unnecessary, I guess???
At the end, I got new parameters including a new E^vac. I think I can use them for a pure-carbon system as they are. But suppose I want to mix them with other Elements. Do I have to change the E^vac, such that the Fermi energy goes back to where it was before the fitting procedure?

[ziand]

As there are no answers yet, let me add another short question:

How is the E^vac included in the Huckel formalism?
(This is described, somehow, in the manual. However, it could be added in the formulae of the semiempirical formalism, too.)

[Nordland]

How are the vacuum_level parameters of the Cerda Huckel basis sets determined?
I know about their meaning, but how did your find the exact numbers?

We calculated the work function for each of basis set in the Cerda parameters, and set the vacuum level such that it gives the correct work function.

[Nordland]

I have another question that bothers me for quite a while:

Some time ago I created my own Huckel basis set for carbon, based on the Cerda_carbon_graphite set. It was a cumbersome fitting process, but I finally got what I wanted. I also took the E^vac into account, which was unnecessary, I guess???

If you are going to do calculations that only involves Carbon and you are not interested in the work function, then it was unnecessary, since it only shifts the level of the orbitials.

At the end, I got new parameters including a new E^vac. I think I can use them for a pure-carbon system as they are. But suppose I want to mix them with other Elements. Do I have to change the E^vac, such that the Fermi energy goes back to where it was before the fitting procedure?

If you want to combine the new basis set with another basis, you will only get a good calculation of the vacuum level is defined using the same method in both basis set.

[ziand]

Sorry for the very late answer.

We calculated the work function for each of basis set in the Cerda parameters, and set the vacuum level such that it gives the correct work function.

Now I am curious. How did you calculate the work function for all those elements? What reference structures did you use? And what do you mean with "the correct work functions"? (The same as with Hoffmann or Müller basis?)

If you want to combine the new basis set with another basis, you will only get a good calculation of the vacuum level is defined using the same method in both basis set.

I combined the new basis set, which I derived form the Cerde_Graphite one, with other Cerda parameters of various metals. I determined the vacuum level of my new carbon set so that the Fermi level of the new set equals the Fermi level of the initial Cerda carbon set. This appeard reasonable to me. Now, how bad is this approach? And how should it really be done?

By the way: I combared the final results (transmission spectra) with DFT and it looks quite nice...

[kstokbro]

We calculated the workfunction in  the experimentally observed structure of the element, i.e. the geometry you get from the VNL crystal database.
The reference value for the Workfunction is the experimentally measured value.

You approach seems reasonable. You can check that the structure you want to combine with a metal, forinstance graphene, has the correct work function.