From owner-chemistry@ccl.net Tue Apr 18 10:35:00 2006 From: "Pablo Echenique Robba pnique^unizar.es" To: CCL Subject: CCL:G: SCF and Opt convergence criteria Message-Id: <-31541-060418103427-13397-WHMdOwdiAMMuaDJQp+4Mzg|,|server.ccl.net> X-Original-From: Pablo Echenique Robba Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset=ISO-8859-1; format=flowed Date: Tue, 18 Apr 2006 16:34:13 +0200 MIME-Version: 1.0 Sent to CCL by: Pablo Echenique Robba [pnique]^[unizar.es] Dear CCLers, I am performing constrained geometry optimizations at the Hartree-Fock level with Gaussian03 in the HCO-Ala-NH2 molecule (8 heavy atoms, 8 hydrogens) fixing the phi and psi Ramachandran angles and relaxing the rest of the internal coordinates. With some basis sets and at certain points of the phi,psi space, I have experienced some problems to achieve geometry convergence if Opt=Tight is specified. If I specify SCF=(Conver=10) (which, by the way is more restrictive than Gaussian's default), the geometry does not converge and I invariably obtain the same scheme: Maximum Force and RMS Force converged, but Maximum Displacement and RMS Displacement not converged, as in the following example: Item Value Threshold Converged? Maximum Force 0.000000 0.000015 YES RMS Force 0.000000 0.000010 YES Maximum Displacement 0.000256 0.000060 NO RMS Displacement 0.000044 0.000040 NO However, if I change to SCF=(Conver=12), the problem seems to fix and I obtain: Item Value Threshold Converged? Maximum Force 0.000006 0.000015 YES RMS Force 0.000002 0.000010 YES Maximum Displacement 0.000057 0.000060 YES RMS Displacement 0.000011 0.000040 YES In a number of optimization steps much lower than the maximum allowed. Has anyone experienced this? What may be the reason? It seems as if the Maximum and RMS Displacement were badly computed with Conver=10 and correctly computed with Conver=12. If this is the case, why is Gaussian default (equivalent to Conver=8) so low? Thanks in advance, Pablo. -- ------------------------------------ Pablo Echenique Robba Departamento de Fisica Teorica & Instituto de Biocomputacion y Fisica de los Sistemas Complejos BIFI Universidad de Zaragoza 50009 Zaragoza Spain Tel.: 34 976761260 E-mail: pnique^-^unizar.es ------------------------------------