From owner-chemistry@ccl.net Fri Sep 11 00:35:00 2015
From: "N. Sukumar nagams^rpi.edu"
To: CCL
Subject: CCL:G: Case Studies of QM Computational Chemistry in Reactivity
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Date: Fri, 11 Sep 2015 09:37:54 +0530
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Sent to CCL by: "N. Sukumar" [nagams]![rpi.edu]
Since this list includes a large number of non-specialists, one should
be careful to avoid making sweeping statements like "While we may not be
able to measure atomic charges as precisely as energies in experiments,
it is not true to say atomic charges are not experimentally observable.
They can be observed and measured through spectroscopy experiments,
albeit with much less precision than we are able to measure energies."
"Atomic charges" are about as measurable as the divinity of an orbital!
Both are entirely theoretical properties of theoretical objects. I joint
Stefan in asking how to "measure atomic charges" - and before that
please also clarify what you mean by "atomic."
--
N. SUKUMAR
Professor & Head, Department of Chemistry
Director, Center for Informatics
Shiv Nadar University, India
On 2015-09-11 05:32, Thomas Manz thomasamanz-*-gmail.com wrote:
> Hi Stefan,
>
> In regards to your questions about MP2, one has to be extremely
> careful with such an approach, because the denominator is of the form
> (energy_1 - energy_2) which causes the denominator to become zero when
> energy_1 = energy_2. This can cause perturbation methods to blow up.
> For this reason, I generally prefer non-perturbative methods such as
> CCSD, when a higher-level calculation result is needed. I would
> recommend CCSD as opposed to MP2, simply because CCSD has a more
> well-defined mathematical limit on the results of the calculation.
> Personally, I don't use MP2 calculations for this reason, but this
> doesn't necessarily mean others can't. However, I wouldn't go so far
> as to say that MP2 calculations don't have a well-defined basis set
> limit. I believe that for most systems the complete basis set limit
> would be well-defined for MP2 calculations. In this sense, the MP2
> calculations are much more well-defined than Mulliken or Lowdin
> populations, which definitely do not have a basis set limit.
>
>> If the set is small (minimal) the derived atomic charges are
> chemically reasonable and correlate well with those from other methods
> for well understood reasons.
>
> The populations of the density matrix projected onto a smaller basis
> set is usually referred to by a different name. At least in Gaussian
> programs, it is called Pop=MBS. In Gaussian programs, this is a
> different algorithm than Pop=Regular which performs Mulliken analysis
> in the current basis set. In my experience, the Pop=MBS method is not
> very useful and tends to crash a large percentage of the time. It
> seems to crash especially often for heavier atoms and for those with
> pseudopotentials. Also, people have tested the idea to project
> plane-wave basis sets onto minimal localized atomic orbital basis
> sets, but this results in charge leakage where the density matrix in
> the smaller basis set does not accurately represent the true density
> matrix. In general, the small basis sets do not represent the density
> matrix with high accuracy. Therefore, in general, I cannot recommend
> the approach you mentioned. There are certainly much better approaches
> if the goal is to compute net atomic charges.
>
> Best,
>
> Tom
>
> On Thu, Sep 10, 2015 at 2:04 PM, Stefan Grimme
> grimme,,thch.uni-bonn.de [1] wrote:
>
>> Sent to CCL by: "Stefan Grimme" [grimme|*|thch.uni-bonn.de [1]]
>> Dear Tom,
>> I followed this discussion quietly for some time but now can't
>> resist to
>> comment on this too extreme viewpoint:
>>
>> 1. Methods can be useful and reasonable without a definite
>> mathematical limit. A Mulliken or Loewdin population analysis gives
>> a definite result for a given well-defined AO basis set. If the set
>> is small (minimal) the derived atomic charges are chemically
>> reasonable and correlate well with those from other methods for well
>> understood reasons. I don't want to defend orbital based
>> partitionings (I prefer observables) but making the mathematical
>> limit
>> to the encompassing requirement seems nonsense to me.
>> There are other useful and widely used QC methods like
>> Moeller-Plesset
>> perturbation theory which are often divergent (or at least
>> convergence is
>> unlcear) in large one-particle basis sets and hence also do not
>> have a
>> definite mathematical limit. Is this a good reason to abandon all
>> MP2
>> calculations?
>>
>> 2. The word "observe" in our context can only mean "observable" in
>> a QM
>> sense. Hence, because there is no operator for "atomic charge" an
>> observable atomic charge does not exist in a strict sense. You
>> probably mean
>> correlations of spectroscopic signatures with atomic charges when
>> writing
>> "They can be observed and measured through spectroscopy
>> experiments".
>> If you have another opinion on that I would like to know more
>> details on
>> how to measure atomic charges.
>>
>> Best wishes
>> Stefan
>>
>>> Hi Peeter,
>>
>>> There is a fundamental distinction between the current
>> conversation focused on exchange-correlation theories and basis sets
>> and the earlier discussion focused on atomic properties. If one
>> increases the basis set size, exchange-correlation functionals such
>> as B3LYP, M06, or whatever one you care to use will approach a
>> well-defined mathematical limit. We can then discuss what the
>> relative accuracy of that mathematical limit is in comparison to
>> experimental properties and also discuss how close we are to that
>> mathematical limit with a particular basis set. Thus, it is
>> meaningful to discuss how adequate an exchange-correlation theory or
>> basis set are for a particular research problem. Of course, the goal
>> is to choose an adequate level that is not too computationally
>> expensive for the particular research question being studied.
>>
>>> In contrast, Mulliken and Lowdin population analysis schemes do
>> not have any defined mathematical limits. As the basis set is
>> increased and the energy and electron density approach the complete
>> basis set limit, the Mulliken and Lowdin populations behave
>> erratically and blow up. This is how we know for sure that Mulliken
>> and Lowdin population analysis schemes are utter nonsense and should
>> never be used for publication results. As pointed out by one person,
>> their only purpose is for debugging calculations to see if the
>> symmetry or other basic features of the input geometry are
>> malformed.
>>
>>> It is not the earlier discussion on atomic charges that is
>> "nonsense" but rather the Mulliken and Lowdin populations that are
>> nonsense, because they have no defined mathematical limits. This has
>> nothing to do with atomic charges, per se. The Mulliken and Lowdin
>> populations do not measure anything physical. They do not measure
>> atomic charges. Probably the confusion has been propagated by
>> calling Mulliken and Lowdin populations as types of "atomic
>> charges", but really the Mulliken and Lowdin populations cannot be
>> atomic charges, because they have no defined mathematical limits. In
>> the future, I shall try to avoid referring to Mulliken and Lowdin
>> populations as types of atomic charges, because I think this error
>> is responsible for the confusion surrounding the definition of
>> atomic charges. While we may not be able to measure atomic charges
>> as precisely as energies in experiments, it is not true to say
>> atomic charges are not experimentally observable. They can be
>> observed and m!
>> easured through spectroscopy experiments, albeit with much less
>> precision than we are able to measure energies. I could go into more
>> extensive details and examples if you are interested.
>>
>> -= This is automatically added to each message by the mailing
>> script =-
>
>
>
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--
N. SUKUMAR
Professor & Head, Department of Chemistry
Director, Center for Informatics
Shiv Nadar University, India