From owner-chemistry@ccl.net Fri Dec 21 23:20:00 2007 From: "Brian Salter-Duke brian.james.duke(~)gmail.com" To: CCL Subject: CCL:G: "Low frequencies" in Gaussian Freq Jobs Message-Id: <-35909-071221220336-27759-N2QJP4rB7JaarXNxntqBug[a]server.ccl.net> X-Original-From: "Brian Salter-Duke" Content-Disposition: inline Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset=UTF-8 Date: Sat, 22 Dec 2007 13:00:36 +1100 MIME-Version: 1.0 Sent to CCL by: "Brian Salter-Duke" [brian.james.duke::gmail.com] The frequencies for translational motion are generally small. Those for rotational motion can be large for two reasons:- 1. For all methods they will not be close to zero if the geometry is not at a stationary point. If in doubt about their values repeat optimization with opt=tight in Gaussian. 2. In DFT in Gaussian they can be non-zero, particularly for heavy atoms (1st transition row and heavier) because the way the grid is set up means the calculation is not rotationally invariant. You can try a bigger grid but this lowers the largest rotational frequency only slowly and the disk and memory use increases fast. You probably should use a grid larger than default for transition metals but the largest rotational frequency will still not be really satisfactory It is general considered that the "zero" frequencies should be in the range -10 to +10, but some people insist on even closer to zero such as in the -5 to +5 range. The sign is not important. Of course the big problem is that a large negative rotational frequency confuses seeing whether there is an imaginary frequency and hence not at local minimum. Hope this helps, Brian. On Dec 19, 2007 10:40 PM, Andrea Ciccioli andrea.ciccioli+*+uniroma1.it wrote: > > Sent to CCL by: "Andrea Ciccioli" [andrea.ciccioli_-_uniroma1.it] > Dear friends, > > it is commonly asserted in textbooks and software manuals that the obvious test to recognize minima in PES among stationary points is that the vibrational frequencies have to be real. > However, I wonder if besides this criterium one should also check carefully the values of the frequencies reported as "Low frequencies" in Gaussian outputs (just before the list of Harmonic frequencies). These are the "frequencies" actually corresponding to translations and rotations, and they should be ideally equal to zero, and indeed in many cases they are very low. However, it happens not seldom to me, e.g. for triatomic species containing heavy elements such as transition metals, to obtain outputs where, although the harmonic vibrational frequencies are all real (positive numbers in the Gaussian output), ie the structure should be a minimum, nevertheless one or two "Low Frequencies" are not that low. Furthermore, they are in general both positive and negative. For example, low frequencies as high as +/- 10 to 40 cm-1 are obtained. Moreover, these values are apparently larger for analytic second-derivative frequency calculations than for numerical calculations (I use DF! > T methods). > As far as you know, these relatively high values of the "Low Frequencies" can indicate that the calculated structure is not a true minimum, in spite of having real harmonic frequencies ? What could be a reasonable criterium to consider the "Low frequencies" small enough to be sure that the stationary point is a true minimum ? > Has anyone some suggestions/indications to give me ? > Thanks to all, and season's greetings. > Andrea Ciccioli > University of Rome (ITALY) > Sapienza> > > -- Brian Salter-Duke (aka Brian Duke) Brian.James.Duke^gmail.com