From owner-chemistry@ccl.net Sat Dec 1 18:33:01 2007 From: "Hod Greeley hod-ccl(0)greeley.org" To: CCL Subject: CCL: basis set erroneous concept Message-Id: <-35746-071201155232-20965-fekAZwaXftnFw1X2fksoHQ,,server.ccl.net> X-Original-From: "Hod Greeley" Content-Transfer-Encoding: 8bit Content-Type: text/plain;charset=iso-8859-1 Date: Sat, 1 Dec 2007 12:42:55 -0800 (PST) MIME-Version: 1.0 Sent to CCL by: "Hod Greeley" [hod-ccl-,-greeley.org] The really short answer is that the 6-31G basis is a set of functions defined totally independently of the orbitals those functions are used to describe. The idea is that within a molecule, the distribution of the electron densities can be approximately described by sums of basis functions. This is just like a Fourier transform. In fact, you could use sines and cosines as the basis functions for a molecular system. However, since sines and cosines do not look very similar to orbitals, you would have to include many, many terms in a (truncated Fourier) sum before your error (in neglecting the rest of the infinite number of terms) became small. This is theoretically fine, but impractical for actual computation. So, instead, comp. chemists choose other functions which more closely resemble orbitals. This way you only need a few functions to achieve a good approximation. Usually, you pick one defined set of functions for any one calculation, i.e. a basis set. So, yes, in your example you would use the 6-31G basis set functions in sums to approximate both sp2 and sp3 orbitals. Or, you might choose 6-31G**, which includes more functions. This means you would most likely get a more accurate answer, but at the expense of the calculations taking longer. I hope that helps. Best, Hod Greeley > 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 sp2 and sp3 > hybridization, for example CH3CH=CH2, and the basis set is 6-31G. Now my > question is: 6-31G basis set for Csp3 is similar to 6-31G basis set for > Csp2. Is this right??!! Please correct my concept. > > Thanks a Lot > > > > --------------------------------- > Be a better sports nut! Let your teams follow you with Yahoo Mobile. Try > it now.