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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