[molpro-user] open-shell systems with MOLPRO

[Lubomir Rulisek]

Dear Molpro users.

I encountered following problem running MOLPRO for seemingly simple 
open-shell systems.

My idea was to run few benchmark calculations for transition metal 
containing complexes.
One of the simplest is the linear [(CH3S-)Cu(H2O)]1+, i.e. with copper 
(II) ion. As it is the doublet
state, I had intention to run UCCSD(T) on top of UHF reference. I found 
out it is
not possible with MOLPRO. As the next option, I tried UCCSD(T)/ROHF 
approach.
ROHF is (in my hands) much more difficult, if not impossible, to 
converge (while UHF is usually much
easier,  actually in 15 years I am involved in "quantum bioinorganic 
chemistry", I never ran ROHF or
ROKS calcs). Then I thought for a moment to run UHF first, use the 
orbitals as the starting
guess for ROHF and run UCCSD(T) afterwards. This seemed to work out 
pretty well until the
moment when I compared two calcs where I got both ROHF/UHF and ROHF 
("from scratch")
convergence. These two results differed significantly (30 kcal.mol-1 or 
so). I am a bit puzzled
what to think about it. I attach both (~5 minute) inputs to this e-mail. 
The first
is running  ROHF/UHF few times to and back and the second is running it 
in reverse order.
It is possible, since I am not that experienced with MOLPRO that I might 
do something wrong...

I would be most grateful for any comment.

Best regards,
Lubomir Rulisek.
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