CCL:G: basis set erroneous concept

 Sent to CCL by: errol lewars [elewars[a]]
 2007 Dec 3
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 _ 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!
 John McKelvey
On Dec 1, 2007 6:35 AM, mohamed aish mhmdaish!^! <owner-chemistry*_* <mailto:owner-chemistry*_*>> 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
Thanks a Lot
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