CCL: Is CASSCF appropriate for study of PES?

 Sent to CCL by: makowskm]![chemia.uj.edu.pl
 > Hi CCLers,
 >  I am studying the PES of a small molecule and wondering
 > whether CASSCF or CASPT2//CASSCF is an appropriate
 > method for the PES study since these methods are not
 > size consistent. Another downside of using CAS is that there
 > is no good way of choosing virtual orbitals into the active space.
 >  Any comments?
 >  Thanks a lot!
 > Yingbin
 The statement that they are not size consistent is not true. It depends
 how you choose your active space.
 If you study A + B = AB reaction size consistency would be kept if active
 space for AB is a direct sum of active spaces for A and B.
 Yes, this is true that finding active space appropriate for specific
 problem  might be a hard work. Point is, if your system is
 multiconfigurational in its nature or electronic state changes on scan
 surface, CASSCF/CASPT2 or some kind of MRCI might be the only valid
 choice.
 Yours,
 Marcin Makowski
 --
 Dr Marcin Makowski
 Jagiellonian University
 Department of Theoretical Chemistry
 email:makowskm[a]chemia.uj.edu.pl
 or
 Kyushu University
 email:marcin[a]cube.kyushu-u.ac.jp