[molpro-user] Strange CCSD(T) behavior

[Steven E. Wheeler]

I am running into some strange (T) behavior for an open-shell (doublet) 
system.  For certain geometries (for example, displaced geometries in a 
finite difference frequency or optimization calculation) I get a (T) 
correction that is about three times the size of the (T) corrections at 
nearby geometries.  Specifically, I will get small changes in RHF and 
UCCSD energies for a slightly displaced geometry but a drastically 
different (approaching 0.1 hart) UCCSD(T) energy.

Additionally, for certain basis sets I get well-behaved CCSD energies 
but again the (T) correction is about three times the size it "should 
be", compared to other basis sets.  For example, the following are at 
identical geometries but with different basis sets (the AVTZ results are 
the issue).  At other geometries the (T) behavior follows a more 
reasonable pattern.

basis=avdz

  !RHF-UCCSD STATE 1.1 ENERGY         -548.347334097101
  Reference energy                                         -547.793788648623
  RHF-UCCSD(T) triples contribution            -0.020888228005
  !RHF-UCCSD(T) STATE 1.1 ENERGY     -548.368222325106

basis=avtz

  !RHF-UCCSD STATE 1.1 ENERGY         -548.537527219381
  Reference energy                                         -547.865294087829
  RHF-UCCSD(T) triples contribution            -0.117411336047
  !RHF-UCCSD(T) STATE 1.1 ENERGY    -548.654938555428

basis=avqz

  !RHF-UCCSD STATE 1.1 ENERGY         -548.595621028713
  Reference energy                                         -547.883634562077
  RHF-UCCSD(T) triples contribution            -0.034331282651
  !RHF-UCCSD(T) STATE 1.1 ENERGY     -548.629952311364

I am converging everything tightly:
gthresh,energy=1.0d-12,orbital=1.0d-12,oneint=1.0d-16,twoint=1.0d-16,compress=1.0d-16

I'm not convinced this is a Molpro issue (running tests now with ACES), 
but wanted to throw it out on the list to see if anyone had any ideas.  
I have checked for orbital instability issues but have found none.

Sincerely,
Steven

-- 
Steven E. Wheeler
Postdoc, Houk Group
Department of Chemistry
University of California, Los Angeles
http://www.ccc.uga.edu/~swheele2