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
``<owner-chemistry*_*ccl.net <mailto:owner-chemistry*_*ccl.net>> 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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