CCL: NBO vs Loewdin Charges



 Sent to CCL by: =?iso-8859-1?Q?V=EDctor_Lua=F1a?= Cabal
 [victor%x%fluor.quimica.uniovi.es]
 Tom,
 My message was not exactly directed to you. It just happens that CCL
 mixes two types of people: (a) students having an incomplete knowledge
 on quantum chemistry and trying to find someone that helps with his/her
 problems, (b) The persons with much clear knowledge on the subject
 trying to provide the help. In this respect CCL plays the role that
 tutors do on any good research university.
 Sometimes I find it difficult to pass remarks that might misdirect the
 audience. In this case, a non-nuclear attractor should be considered
 (in my opinion) another chemical actor of the molecule if (an only if)
 it is confirmed that the NNA is real and not an artifact of the calculation
 (low level basis set, inadequate xc, ...). There are cases where an H may
 lose completely its electron and be converted functionally into a proton
 and cases where a lonely electron can be stabilized by the rest of the
 molecule.
 On Thu, Sep 03, 2015 at 08:46:27AM -0600, Thomas Manz thomasamanz-,-gmail.com
 wrote:
 > Sorry, I did not mean to imply that non-nuclear attractors do not exist.
 > They do exist in some materials.  The challenge that occurs is that
 > when a non-nuclear attractor is present, it does not fulfill the goal
 > to compute net atomic charges, since some of the electrons are assigned
 > to the non-nuclear attractor instead of to one of the atoms.
 That's what it must be done imho. That electron does not correspond
 to any particular nucleus (i.e. atom) but to the whole sytem, as it
 is the electrostatic potential of the whole system what stabilizes it.
 The NNA *is* another chemical species, and an important one whem present:
 solved electrons are highly suggestive in Na[A]NH3(l).
 > I realize there are various good reasons to perform a topological
 > analysis of the electron density. I'm just saying that if the goal is to
 > create a default method for assigning atomic properties, it is not the
 > most straightforward approach since some of the properties may belong
 > to the non-nuclear attractors instead of the atoms.
 True. I agree completely on that.
 Any good software must consider both situations and deal with them
 correctly.  I love developing modellers (modelling codes) and analyzers
 (modellers by other name) on gas phase molecules and condensed matter
 problems by the way ;-)
 The key is, of course, ensuring that the independent electron is not
 an artifact for the case under study. Electrides (electrons stabilized
 by the electrostatic field of the environment) do exist, and sometimes is
 difficult to convince chemists of that exotic chemical species.
 Condensed matter physicasits are more open in this aspect, as F centers in
 MgO and alkaline halides are known since irradiated matter was studied
 as part of nuclear efforts after WWII.
 > Trying to assign the non-nuclear attractor to one of the atoms (i.e.,
 > closest atom) may not be possible without breaking the crystal symmetry
 > (i.e., what happens if the non-nuclear attractor is located the same
 > distance from multiple atoms?
 High symmetry is not a requirement for NNA's to occur, even thought
 not having a nucleus makes the electron easy to move when the molecule
 experiences vibrations. Examining how vibrations influence NNA's is
 (in my opinion) an excellent subject to study. My next paper might
 verse on that :-)
 It has been a pleasure exchanging opinions on this issue,
                                                          Víctor
 --
      .  .    "In science a person can be convinced by a good argument.
     / `' \   That is almost impossible in politics or religion"
    /(o)(o)\  (Adapted from Carl Sagan)
   /`. \/ .'\  "Lo mediocre es peor que lo bueno, pero también es peor
  /   '`'`   \ que lo malo, porque la mediocridad no es un grado, es una
  |  \'`'`/  | actitud" -- Jorge Wasenberg, 2015
  |  |'`'`|  | (Mediocre is worse than good, but it is also worse than
   \/`'`'`'\/  bad, because mediocrity is not a grade, it is an attitude)
 ===(((==)))==================================+=========================
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