From owner-chemistry@ccl.net Wed Oct 7 16:25:01 2020 From: "Radoslaw Kaminski rkaminski85:+:gmail.com" To: CCL Subject: CCL:G: Interaction energy between open-shell systems Message-Id: <-54179-201007162415-25241-Gf932Oxxv/lQaTgr9uK80A!A!server.ccl.net> X-Original-From: "Radoslaw Kaminski" Date: Wed, 7 Oct 2020 16:24:14 -0400 Sent to CCL by: "Radoslaw Kaminski" [rkaminski85^gmail.com] Dear All, We frequently do interaction energy computations using Gaussian (counterpoise keyword etc.). In our case we cut out molecules from the crystal and compute int. energies between molecules for various motifs. For simple organic molecules this is simple since they are usually singlets, thus the corresponding spin and multiplicity line looks like that (assuming all molecules are neutral): 0 1 0 1 0 1 The situation is much more complex if unpaired electrons are present. If molecule has a multiplicity of 2 (one unpaired electron) the total multiplicity of the system can be both 1 or 3. Thus, two are possible (again, for neutral molecules): 0 1 0 2 0 2 0 3 0 2 0 2 My question is then what would be the "correct" approach to compute interactions energies in such case? In this sample example we can compute both, and from our tentative results the energies are very mych different. Which are then the most "correct" ones? We plan to compute such int. energies between molecules with transition metals such as Fe or Co as well. In such cases the metal can be, for example, in high-spin state. Computation of int. energies between two HS molecules leads to variety of multiplicity states to consider... If someone has experience and would be kind enough to provide an comment on such problems I would be grateful. Some references would also be of benefit (I could not easily find any unfortunately). With many thanks in advance. Regards, Radek Kaminski