Hi Partha,
Thank you for your question.
Atom-in-material properties should not explicitly depend on the basis
set used to perform the quantum chemistry calculation. The reason for
this is that nature corresponds to the limit of an arbitrarily large
basis set (aka 'complete basis set limit'). For reasons of
computational efficiency, in practical calculations a finite basis set
is usually used to get an answer quickly. So, we want to use
population analysis methods that are not too sensitive to the choice
of basis set, so that reasonably sized basis sets will produce answers
that are not much different than what would be computed if an
arbitrarily large basis set (aka 'complete basis set limit') were used.
The Mulliken method explicitly depends on the basis set choice and has
no complete basis set limit. In other words, the Mulliken populations
are not defined for arbitrarily large basis set. This is very bad,
because it means that as the accuracy/precision of the quantum
chemistry calculation is improved by using larger and larger basis
set, the Mulliken charges get worse and worse with no mathematical
limits or meaning. So, you should not use Mulliken populations. It
also means that if you run a similar calculation twice, but using
different basis sets for each calculation, the Mulliken charges that
you get from the two similar calculations may not be similar. For
example, Mulliken population analysis may predict the cations have
turned into anions, and vice versa, even though it is exactly the same
material. In other words, Mulliken population analysis often gets
cations and anions confused; it does not know the distinction between
them.
Between the two methods you mentioned, natural population analysis
(NPA) is the better choice. An even better choice would be to use a
method like DDEC6 for which the atom-in-material properties are a
functional of the electron and spin density distributions with no
explicit basis set dependence, and for which the atom-in-material
properties are chemically meaningful across a broad range of material
types.
Sincerely,
Tom
On Sun, Jun 28, 2020 at 8:43 AM Partha Sengupta anapspsmo%x%gmail.com
<http://gmail.com>; <owner-chemistry*_*ccl.net
<mailto:owner-chemistry*_*ccl.net>> wrote:
Sir, For a metal complexes involving Cu[N,O donor], [Chlorine and
fluorine atoms are in the benzene ring]. I found that cu has
0.96091 charges( Natural Population analysis) while Mulliken
atomic charge is 0.457333. On the same process N has -0.53717 and
-0.29941 respectively. What is the reason behind this? What will
be the better choice to represent?
Partha Sengupta
--
*/Dr. Partha Sarathi Sengupta
Associate Professor
Vivekananda Mahavidyalaya, Burdwan/*