From owner-chemistry ^at^ ccl.net Mon Dec 3 13:35:00 2007
From: "errol lewars elewars||trentu.ca"
To: CCL
Subject: CCL:G: basis set erroneous concept
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X-Original-From: errol lewars
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Date: Mon, 03 Dec 2007 10:38:14 -0500
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Sent to CCL by: errol lewars [elewars[a]trentu.ca]
2007 Dec 3
Hello,
re "question is: 6-31G basis set for C_sp3 is similar to 6-31G basis set
for C_sp2 ."
Basis sets are sets of mathematical functions whose purpose is to
approximate, via molecular orbitals and a (or more than one) Slater
determinant, the molecular wavefunction. The standard procedure is to
put basis functions on atoms, so in an oversimplified way they are akin
to conventional atomic orbitals.
What you do with basis functions other than approximating a molecular
wavefunction is optional. Hybridization, for example, is only a kind of
mathematical trick, a convenient way of viewing the electron
distribution around an atom.
E. Lewars
===
John McKelvey jmmckel _ gmail.com wrote:
> This is a very good question!!
>
> The short answer is that the iterative SCF process, whether
> Hartree-Fock or DFT, allows the distribution of electrons to be
> optimal for any local chemical environment. This process has optimal
> chance of representing the "true chemical picture" the larger the
> number of mathematical electronic distribution functions, here basis
> orbitals. Hence, a simple Slater orbital, or its popular Gaussian
> approximation STO-nG, is at the most restrictive end of the spectrum,
> while something like 6-311++G** is in the direction of the much more
> flexible set of distribution functions, but not necessarily _the_ most
> flexible, but may be close enough for practical application. Now
> something like 6-31G is in between, among many other possibilities.
> Which of the intermediate set of functions one chooses is controlled
> by how well it allows the most suitable results for a given application.
>
> There are many papers by the Pople group published during the
> development of various basis sets, just to mention one of many places
> to read about this.
>
> Hope this helps!
>
> Cheers,
>
> John McKelvey
>
> On Dec 1, 2007 6:35 AM, mohamed aish mhmdaish!^!yahoo.com
> > wrote:
>
> Hi All;
> I am a new in computational chemistry field. Now, I am reading
> about the basis sets. There are something confused me and would be
> appreciative if anyone helps me to correct my erroneous concepts.
> Suppose that we have a compound containing carbons with sp^2 and
> sp^3 hybridization, for example CH_3 CH=CH_2 , and the basis set
> is 6-31G. Now my question is: 6-31G basis set for C_sp3 is similar
> to 6-31G basis set for C_sp2 . Is this right??!! Please correct my
> concept.
>
> Thanks a Lot
>
>
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