From owner-chemistry@ccl.net Sun Oct 16 13:00:01 2011 From: "George B Fitzgerald gfitzgerald[-]accelrys.com" To: CCL Subject: CCL:G: opt=Z-matrix convergence problems in Gaussian Message-Id: <-45650-111016125858-23564-995VNH2IwKtpkFCtZ9G9Zw-.-server.ccl.net> X-Original-From: "George B Fitzgerald" Date: Sun, 16 Oct 2011 12:58:55 -0400 Sent to CCL by: "George B Fitzgerald" [gfitzgerald : accelrys.com] I dont know specifically about Gaussian, but I've seen energy bouncing between 2 values in other applications. In the cases I've seen, it was oscillating between 2 nearly degenerate states. It sounds like you were ok when you started the optimization, and then the geometry found a spot on the potential energy surface where these 2 states are near degenerate. I solved my problem by going to an open-shell calculation, which resulted in a lower energy and no more oscillation. If you're currently running closed shell, you might try that. -george -----Original Message----- Sent to CCL by: Tanja van Mourik [tanja.vanmourik]=[st-andrews.ac.uk] Hi All, We are trying to optimise a cluster consisting of ~50 molecules with M06-L with Gaussian 09. We are using Opt=Z-matrix, as optimisation in redundant internal coordinates does not work ("Error in internal coordinate system"). With Opt=Z-matrix, the energy appears to converge nicely up to 100 cycles, but after that the energy starts to oscillate wildly. This happens always after 100 cycles. Does anyone know why this is, and how to solve this problem? Currently we restrict MaxCycle to 100, and restart the calculation repeatedly (it needs many hundreds of optimisation cycles to converge, particularly with Opt=Z-matrix), but I am hoping for a better solution. Any ideas? Best wishes, Tanja