Subject: A MOPAC Error Message; Geometry Optimize at Excited Singlet State

From: YONG HUANG <[email-masked]>
Date: Thu, 13 Jun 1996 13:55:12 -0500 (CDT)

I'm using CAChe MOPAC 94 on a Mac. From time to time I get an error message 
  in the output file upon finishing a geometry optimization
  
  "Trust radius now less than 0.00100 Optimization terminating
    Geometry may not be completely optimized"
  
  What does it mean? Other than this message, everything is fine except 
  sometimes the gradient accompanying the final heat of formation is too big 
  (>5 e.g.). (Only simetimes though). I use EF, ANALYT, PRECISE, PM3, etc. 
  keywords. Thanks for help.
  
  Another thing I'd like to say. I used to have problems geometry optimizing a
  molecule (a typical size like C10H10O10) in its excited singlet state. Got 
  help from CCL and used keyword NOANCI. Worked for only some of my molecules.
  Besides, NOANCI tended to give me large final gradient (>1 if I remember
  right). Now I find that if I run a CI at excited singlet state and then
  geometry optimize it, it works much better. Run CI at as high levels as you
  and/or computer can tolerate. [So far I have tested C10H10O10 only since each
  test takes many hours. But I feel this "CI->GeomOpt" method is generally
  applicable.] Any comments are welcome.
  
  Yong
  [email-masked]