From owner-chemistry@ccl.net Tue Aug 5 16:23:00 2008 From: "Wayne Steinmetz WES04747_._pomona.edu" To: CCL Subject: CCL: Ab initio on bridged rings Message-Id: <-37492-080805143414-30256-HMXrxJ2fZMORFvJNPL+Uyw:-:server.ccl.net> X-Original-From: "Wayne Steinmetz" Content-class: urn:content-classes:message Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="us-ascii" Date: Tue, 5 Aug 2008 11:33:56 -0700 MIME-Version: 1.0 Sent to CCL by: "Wayne Steinmetz" [WES04747:+:pomona.edu] You are having problems with convergence so your system shows sensitivity to structural details. I recommend that you first generate a structure using molecular mechanics and a good Class II force field, e.g. the Merck Molecular Force Field or Allinger's MM3. Sometimes, intuition in initially drawing the structure leads one astray. For example, when I drew the structure of calixarene, I started with a structure quite far in energy from the global minimum. Hence, after you have minimized your initial structure, you might also want to search conformational space. Spartan uses a Monte Carlo algorithm which usually works. In difficult cases such as calixarene, I have used distance geometry to get out of a trap. You can rely on the results of molecular mechanics if you use a Class II force field. Once you have located the global minimum, then proceed to Hartree Fock and then a more advanced method such as DFT with Becke's B3LYP functional or Truhlar's M06 functional. If the convergence falters, you might tweak a portion of this structure. This trick often helps getting out of a rut. Wayne E. Steinmetz Professor Emeritus of Chemistry USFS Volunteer Chemistry Department Pomona College 645 North College Avenue Claremont, California 91711-6338 USA phone: 1-909-621-8447 FAX: 1-909-607-7726 Email: wsteinmetz- -pomona.edu WWW: pages.pomona.edu/~wsteinmetz =20 -----Original Message----- > From: owner-chemistry- -ccl.net [mailto:owner-chemistry- -ccl.net]=20 Sent: Tuesday, August 05, 2008 12:30 AM To: Wayne Steinmetz Subject: CCL: Ab initio on bridged rings Sent to CCL by: "Simon Halstead" [shalstead:hit.edu.cn] Dear all, I am trying to optimize some bridged (1-4) cyclohexane rings. I initially optimized them using sempiempirical methods. These calculations converged very rapidly. However, I then tried to optimize the resulting structures at a higher level of theory. These high level calculations have failed to converge, despite being run for a large number of steps. I have tried HF, MP2 and B3LYP with a variety of basis sets but nothing appears to converge. Visualizing the outputs of the failed calculations shows no problems with the structure. Does anyone have any experience with structures like this or can anyone give me a suggestion of how best to optimize bridged rings? I am using GAMESS if this makes any differnce. Thanks for your help, Simon Halstead -=3D This is automatically added to each message by the mailing script = =3D-http://www.ccl.net/cgi-bin/ccl/send_ccl_messageSubscribe/Unsubscribe:=20Job: http://www.ccl.net/jobs=20http://www.ccl.net/spammers.txt------------------------------------------------------------- This message has been scanned by Postini anti-virus software. =0D