From owner-chemistry@ccl.net Sun Aug 31 18:11:01 2008 From: "David Hose anthrax_brothers*hotmail.com" To: CCL Subject: CCL:G: Easy convergence of Diffuse functions Message-Id: <-37689-080831180933-16680-7WkurxNLOk0pQmSFRb52Yg+/-server.ccl.net> X-Original-From: "David Hose" Date: Sun, 31 Aug 2008 18:09:30 -0400 Sent to CCL by: "David Hose" [anthrax_brothers:hotmail.com] Hi Ya, The addition of diffuse functions to the basis set is well known to cause problems with the convergence of the calculations. The most common fix is to use a better quality guess for the initial wavefunction in the second calculation. Assuming that you are using Gaussian, the simplest way is to optimise the geometry of the molecule at say HF/6-31G* level of theory. Duplicate the resulting checkpoint point file. Then set up the second (single point?) calculation, at say the HF/6-31+G* level of theory, with Guess=Checkpoint included in the route selection, ensuring that the %chk points to the previous calculation checkpoint file. On occasions, it is better if the previous calculation included a frequency calculation as the superior hessian allows the subsequent calculation converge more easily. There are some other approaches that can be used for difficult cases, but the above method typically works. I suggest that you check out David Young's book Computational Chemistry: A Practical Guide for Applying Techniques to Real World Problems. A great source of handy hints and tips. There is a version of his book here on CCL. The section of converge can be found here. http://www.ccl.net/cca/documents/dyoung/topics-orig/converge.html Regards, Dave. _____________ Sent to CCL by: "Mr Shabbir" [shabbir]^[nenu.edu.cn] Dear all I want to share some thing about optimization. I optimized several systems with 6-31G* basis set but when I use diffuse function (+) the same previously optimized system at (6-31G*) fails to converge by using diffuse function. How to treat the diffuse function especially in optimization? Is there any special keyword or way that can make the convergence easy with diffuse functions? Regards Mr.Shabbir shabbir::nenu.edu.cn