From owner-chemistry@ccl.net Thu Nov 17 14:03:00 2011 From: "Johannes Hachmann jh .. chemistry.harvard.edu" To: CCL Subject: CCL: StateAveraged CASSCF calculations Message-Id: <-45895-111117120650-5040-DLlIbQYtBjb3CmQ29WzH5w---server.ccl.net> X-Original-From: "Johannes Hachmann" Content-Language: en-us Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii" Date: Thu, 17 Nov 2011 12:06:32 -0500 MIME-Version: 1.0 Sent to CCL by: "Johannes Hachmann" [jh(-)chemistry.harvard.edu] Hi Saman, If you try to obtain an excited state in a variational method you often get root-flipping, i.e., your optimization collapses down to the ground state. In certain cases you can utilize spin or spatial symmetry constraints to prevent this but otherwise this is a touch problem. If you take a ground state result and assume that the higher roots are the excited states it leads to bad excitation energies because the expansion is tailored towards one state and may not represent the other states well, which leads to an unbalanced description. Therefore, the weights should be chosen according to what you want to get out of your calc. There is a way around root-flipping which is described here: http://jcp.aip.org/resource/1/jcpsa6/v127/i8/p084109_s1 This paper also contains useful references. Helgaker's 'Molecular Electronic-Structure Theory' book discusses this in some technical detail as well, if I remember correctly. Best wishes Johannes ----------------------------------------------- Dr. Johannes Hachmann Postdoctoral Fellow Aspuru-Guzik Research Group Harvard University Department of Chemistry and Chemical Biology 12 Oxford St, Rm M104A Cambridge, MA 02138 USA eMail: jh _ chemistry.harvard.edu ----------------------------------------------- > -----Original Message----- > From: owner-chemistry+jh==chemistry.harvard.edu _ ccl.net > [mailto:owner-chemistry+jh==chemistry.harvard.edu _ ccl.net] On Behalf > Of Saman Mandegar mandegar_saman::yahoo.com > Sent: Thursday, November 17, 2011 10:08 > To: Hachmann, Johannes > Subject: CCL: StateAveraged CASSCF calculations > > > Sent to CCL by: "Saman Mandegar" [mandegar_saman[]yahoo.com] > > Dear All, > > I would be very happy to have the ideas about state average > calculations in CASSCF. I am not pretty sure why state average > calculations are done. What is the reason? Why it is necessary for > excited state computations? And how we determine the weighting > factor for each state for example in SA3-CAS(4,3) calculation? Any > help and pointer to literature is greatly appreciated. > > Regards, > Saman > > > > -= This is automatically added to each message by the mailing script > =- > To recover the email address of the author of the message, please > change> Conferences: > http://server.ccl.net/chemistry/announcements/conferences/