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2.4 Checking for transferability
</H2><EM>
A simple way to check for correctness and to get a feeling for
the transferability of a PP, with little effort, is to test the
results of PP and AE atomic calculations on atomic configurations
differing from the starting one. The error on total energy
differences between PP and AE results gives a feeling on how
good the PP is. Just to give an idea: an error <!-- MATH
$\sim 0.001$
-->
∼0.001 Ry
is very good, ∼0.01 Ry may still be acceptable.
The code <TT>ld1.x</TT> has a ``testing'' mode in which it does
exactly the above operation. You provide the input PP file and
a number of test configurations.
</EM>
<P>
<EM>You are advised to perform also the test with a basis set of spherical
Bessel functions <I>j</I><SUB>l</SUB>(<I>qr</I>). In addition to revealing the presence of
``ghosts'', this test also gives an idea of the smoothness of the
potential: the dependence of energy levels upon the cutoff in the kinetic
energy is basically the same for the pseudo-atom in the basis of <I>j</I><SUB>l</SUB>(<I>qr</I>)'s
and for the same pseudo-atom in a solid-state calculation using PW's.
</EM>
<P>
<EM>Another way to check for transferability is to compare AE and pseudo (PS)
logarithmic derivatives, also calculated by <TT>ld1.x</TT>. Typically
this comparison is done on the reference configuration,
but not necessarily so. You should supply on input:
</EM><DL class="COMPACT">
<DT>–</DT>
<DD>the radius <I>r</I><SUB>d</SUB> at which logarithmic derivatives are
calculated (<I>r</I><SUB>d</SUB> should be of the order of the
ionic or covalent radius, and larger than any of the <I>r</I><SUB>c</SUB>'s)
</DD>
<DT>–</DT>
<DD>the energy range <!-- MATH
$E_{min}, E_{max}$
-->
<I>E</I><SUB>min</SUB>, <I>E</I><SUB>max</SUB> and the number
of points for the plot. The energy range
should cover the typical valence one-electron energy
range expected in the targeted application of the PP.
</DD>
</DL><EM>
The files containing logarithmic derivatives can be easily read and
plotted using for instance the plotting program <TT>gnuplot</TT>
or <TT>xmgrace</TT>.
Sizable discrepancies between AE and PS logarithmic derivatives
are a sign of trouble (unless your energy range is too large or
not centered around the range of pseudization energies, of course).
</EM>
<P>
<EM>Note that the above checks, based on atomic calculations only,
do not replace the usual checks (convergence tests, bond lengths,
etc) one has to perform in at least some simple solid-state or
molecular systems before starting a serious calculation.
</EM>
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