[CPMD-list] calculation of deprotonation energies

[Christian Tuma]

Dear CPMD experts,

I have a question which concerns plane wave pseudopotential calculations
in general. The problem I have is the following: For the calculation of
the electronic deprotonation energy of a system "X-H" one usually calculates
the total energy of the neutral system "X-H", E(X-H), and the total energy
of the negatively charged system "X-", E(X-). The energy difference between
E(X-H) and E(X-) is the deprotonation energy since the energy of the removed
proton itself is zero (no electrons present). Now, the chosen type of pseudo-
potential for hydrogen in "X-H" affects the total energy of "X-H", whereas for
"X-" this doesn't matter since this pseudopotential is absent there. That means,
the deprotonation energy depends on the chosen pseudopotential for hydrogen in
"X-H". The thing that worries me now is that the error introduced by the
pseudopotential approach for hydrogen is present in E(X-H) but not in E(H+).
So there is no kind of cancelation of this error in the deprotonation energy,
in other words: the deprotonation energy contains an inherent error caused 
by the "disappearing" hydrogen pseudopotential (not by the special nature of
the type of chosen pseudopotential itself).
Is there a way to correct this error in the deprotonation energy? I think that
this error is not big enough to be considered important, so my question has
a rather theoretical and general background. Comments or help are welcome.

Thanks,

Christian Tuma.

-- 
Christian Tuma             Humboldt-Universitaet Berlin
ct at chemie.hu-berlin.de     Arbeitsgruppe Quantenchemie (Prof. Sauer)
phone: +49-30-20937140     Brook-Taylor-Str. 2, 12489 Berlin, GERMANY
fax:   +49-30-20937136     http://www.chemie.hu-berlin.de/ag_sauer