From owner-chemistry@ccl.net Fri Mar 24 05:29:01 2017 From: "Stefan Grimme grimme-*-thch.uni-bonn.de" To: CCL Subject: CCL: Becke Johnson correction on vibrational mode intensities, e.g. Si6 Message-Id: <-52717-170324043740-2361-uDWg4muj8ZX74LsUs+8iwQ a server.ccl.net> X-Original-From: "Stefan Grimme" Date: Fri, 24 Mar 2017 04:37:34 -0400 Sent to CCL by: "Stefan Grimme" [grimme=-=thch.uni-bonn.de] Hi, I can't say much about your actual calculations but want to clarify this issue. The dispersion energy with BJ-damping becomes constant at short interatomic distances meaning that the forces vanish. This is more physical than the previously used zero-damping which introduces artificial repulsive dispersion forces a short distances. When the forces vanish there can not be a contribution to the force constant (i.e. vibrational frequencies). Because in Si6 dispersion corrections are anyway small (its a small molecule), your observation seems to me the result of a bug in the code rather than a physical effect. Best Stefan