*From*: Herbert Homeier t4720 <Herbert.Homeier-: at :-chemie.uni-regensburg.de>*Subject*: Re: CCL:G:[Q]: How to keep a particular state in G94?*Date*: Thu, 16 Nov 95 10:44:51 +0100

yu (yu-: at :-infiniti.wavefun.com) wrote (Wed, 15 Nov 1995 07:57:54 -0800): > On Nov 15, 11:29am, <bcnlma-: at :-hkpucc.polyu.edu.hk> wrote: > > Subject: CCL:G:[Q]: How to keep a particular state in G94? > > Dear all, > > > > I had a question that I hope someone on the list can help. I am > > doing some calculations on small transition metal compound using Gaussian > > 94. I carried out an SCF calculation which gives me a particular state but > > I would like to calculate a DIFFERENT state. After using > > guess=(read,alter) to get to the state I like, the SCF brings it back to > > the original state that I do not want. > > > > My question is: is there anyway I can get Gaussian to keep the same > > state from my guess=(read,alter) job? > > > > yours sincerely, > > Ida N. L. Ma > > ... > > Dear Dr. Ma, > > The reason causing your problem isn't program you used, but a basic theoretical > one. If the excited state you want to get has same symmetry with ground state, ======================================================================== > according to the Variational Principle (SCF bases on the principle), if you ====== > only do One-Determinant calculation, the energy you get is always the ground ============================================================================ > state's. To study your problem, you need to use MCSCF or sophisticated CI ======= > methods. > > Good luck! > > Jianguo Yu > Is the underlined sentence really true? As far as I remember the excited states of the same symmetry as the ground state are saddle points of the functional that is minimized in a variational calculation (as are the excited states of the other symmetries). That would imply that an SCF calculation can converge to a particular excited state of ground state symmetry if the initial guess is close enough. (in principle, not paying attention to rounding errors etc.) I faintly remember that some people did a similar thing in DFT to obtain excited states from one-determinant wave functions. If this saddlepoint approach is in principle ok, the main problem would be to get accurate initial guesses. In a situation where many excited states are close together in energy or closely coupled (like in many transition metal compounds), this is expected to be rather difficult. By the way: The above should not be interpreted in the sense that I recommend not to use MCSCF / CI. 8^) Best regards Herbert Homeier -------------------------------------------------------------- Dr. Herbert H. H. Homeier Institut fuer Physikalische und Theoretische Chemie Universitaet Regensburg D-93040 Regensburg, Germany Phone: +49-941-943 4720 FAX : +49-941-943 2305 email: na.hhomeier-: at :-na-net.ornl.gov <A HREF="http://rchs1.uni-regensburg.de/%7Ec5008/";>HOMEPAGE</A> ---------------------------------------------------------------