Vacuum level for charged systems

[Nuri Yazdani]

Hi all,
First to summarize the question: how/can one extract the vacuum level from 
a calculation done on a charged (non periodic!) system, such that the 
energies taken from two calculations with different amounts of charge can 
be compared on an absolute scale to one another? Also, if someone could 
provide a reference for where this or a similar situation has been looked 
at/addressed? 
Why I want to know this is explained in more detail below.

I am doing calculations on charged molecules, including HF. I am trying to 
look into the reorganization energy associated with hopping of carrier from 
one charge molecule to another. To estimate this, I calculate the energies 
for a charged (+1e-) and neutral molecule:

E0{A,n} = E{A,n}+E{F,ref}, where E{A,n} are the energy spectra I get from 
CP2k for the neutral molecule, and E{F,ref} is the reference level to vacuum

E0{C,n} = E{C,n}+E{C,ref}, where E{C,n} are the energy spectra I get from 
CP2k for the charged molecule, and E{F,ref} is the reference level to 
vacuum for that calculation

If there are N occupied molecular orbitals in the neutral molecule, than 
the reorganization energy can be approximated as:

lambda = Sum{1:N}(E{C,n}+E{C,ref}) + Sum{1:N+1}(E{A,n}+E{A,ref}) - 
Sum{1:N+1}(E{C,n}+E{C,ref}) - Sum{1:N}(E{A,n}+E{A,ref})
lambda = Sum{1:N}(E{C,n}-E{A,n}) + Sum{1:N+1}(E{A,n}-E{C,n})  - E{C,ref} + 
E{A,ref}

So the end product is directly proportional to the difference in reference 
energies.

Cheers,
Nuri
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