CCL: StateAveraged CASSCF calculations

 Sent to CCL by: "Johannes Hachmann" [jh(-)]
 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:
 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
 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
 eMail: jh _
 > -----Original Message-----
 > From: _
 > [ _]
 On Behalf
 > Of Saman Mandegar
 > Sent: Thursday, November 17, 2011 10:08
 > To: Hachmann, Johannes
 > Subject: CCL: StateAveraged CASSCF calculations
 > Sent to CCL by: "Saman  Mandegar" [mandegar_saman[]]
 > 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
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