CCL: origin of ab initio

 Sent to CCL by: Herbert Fruchtl [herbert.fruchtl[]]
This discussion is as exciting and useless as C/Fortran or vi/emacs, but since it has started, here are my two pence worth of why using a basis set doesn't break the "ab initio" definition, by way of an analogy.
We know how to get to a more complete basis set. We just can't afford to go all the way. So we choose one, and possibly a second (better) one to check how close we are.
We know how to do Full CI. Can't afford that either. So we stick with some limited number of excitations, and possibly do a few test calculations with a better method to check.
So, if we call HF, MP2 and CCSD ab initio, we can as well call minimal basis, vdz and vtz ab initio. There is no systematic way to make B3LYP (in the complete basis set limit) more accurate, and this is why DFT isn't ab initio.
 John McKelvey wrote:
Your comment is correct with respect to construction of basis sets; however in any of the ZDO schemes, or other more elegant schemes, the choice of basis set, the choice of post-SCF treatment, or the choice of a DFT functional [or its construction] to get results that best reproduce experiment is routinely done and is either semi-empirical or purely empirical. Which term suits the best, I do not know. IMHO, "Everybody adjusts something to get the best answer."
 John McKelvey
On Dec 12, 2007 6:59 PM, Kirk Peterson kipeters-* <>; <owner-chemistry],[ <mailto:owner-chemistry],[>> wrote:
     Sent to CCL by: Kirk Peterson [kipeters-$ <>;]
     As a sidebar to this discussion, I have to strongly disagree that
     basis set parameters, exponents or contraction coefficients,
     use empirical data in their construction.  All commonly used basis
     sets, even the Pople-style sets, are generated by optimizing
     exponents and contraction coefficients to minimize (ab initio)
     energies of atoms and sometimes molecules.  Some of the Pople-style
     basis sets utilize scale factors to apply to atom-optimized exponents,
     but these were based on (ab initio) molecular calculations
     and not experimental data.
     On Dec 12, 2007, at 8:21 AM, Rene Fournier renef+* wrote:
      > Sent to CCL by: Rene Fournier [renef\a/]
      > David Craig and Robert Parr first used "ab initio" in
      > chemistry,
      > see
      > Near the middle of that page, Parr recounts:
      > " Craig and I published this paper on "configuration
 interaction in
      > benzene", where we took the pi-system and did essentially a
      > configuration interaction calculation on it.
      > That has some trivial historical interest in that it was there that
      > the
      > word, the term ab initio was introduced. Craig and Ross had computed
      > everything from the start in London and I had personally computed
      > everything from start in Pittsburgh. Then we compared our answers
      > when we
      > were finished- This involved computing of all the integrals as
     best as
      > they could be done and selecting the configurations to mix for the
      > ground
      > and exited states because there were electronic states that were of
      > experimental interest and we checked our answers one against each
      > other
      > when we were finished. And what the paper says is, that these
      > calculations
      > were done ab initio by Craig and Ross and by me, independently. And
      > Mulliken later said that this was the introduction of the term ab
      > initio
      > into quantum chemistry. In the short review that you have, I talk
      > about
      > this and reproduce a picture of a letter from Craig to me where he
      > uses
      > the term ab initio in a different context. So ab initio was
      > introduced in
      > the quantum chemistry by Craig in a letter to me and I put it
     into the
      > manuscript. That's where ab initio came from.  "
      > Funny thing is Parr later became a champion of Density Functional
      > Theory
      > and for many years (70's, 80's) DFT practitioners were often
      > criticized
      > for doing calculations that were not "ab initio".  I think
 views have
      > changed now;  "first-principles" was introduced probably to
      > "mostly
      > not empirical" but without the implications "ab initio"
 had acquired
      > over
      > the years.  The term "ab initio calculation", as it's
 commonly used,
      > very rarely refers to a calculation "devoid of empiricism",
      > example
      > the choice of basis set parameters is almost always empirical,
      > see discussion on
      >  Rene Fournier                   Office:  303 Petrie
      >  Chemistry Dpt, York University  Phone: (416) 736 2100 Ext. 30687
      >  4700 Keele Street,  Toronto     FAX:   (416)-736-5936
      >  Ontario, CANADA   M3J 1P3       e-mail: renef*_*
      > On Wed, 12 Dec 2007, Christoph Etzlstorfer christoph.etzlstorfer .
      > <>; wrote:
      >> There is a story about that in Michael J.S. Dewars biography
      >> semiempirical life", American Chemical Society, 1992, p. 129.
      >> Best regards
      >> Christoph
      >> Am 11.12.2007 um 03:13 schrieb Tommy Ohyun Kwon ohyun.kwon _
      >> <>;:
      >>> Sent to CCL by: Tommy Ohyun Kwon [ohyun.kwon . <>;]
      >>> Dear CCLers;
      >>> I would appreciate it if anyone could tell me who used the
 term of
      >>> "ab initio
      >>> calculations" first.
      >>> Thank you very much for your kind attention.
      >>> Best wishes,
      >>> Tommy
      >>> --
      >>> Tommy Ohyun Kwon, Ph.D
      >>> School of Chemistry and Biochemistry
      >>> Georgia Institute of Technology
      >>> Atlanta Georgia, 30332
      >>> Email: ohyun.kwon]*[
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      >> Dr. Christoph Etzlstorfer       Phone:  *43-732-2468-8750
      >> Universitaet Linz              Fax:    *43-732-2468-8747
      >> A-4040 Linz               E-mail: christoph.etzlstorfer,+,
      >> Austria         
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 Herbert Fruchtl
 EaStCHEM Fellow
 School of Chemistry
 University of St Andrews