[Molpro-user] how to avoid SCF restart in a CCSD(T) scan and why does the ROHF SCF seem to converge to a wrong state?

[Fedor Goumans]

Dear all,
Just getting started with molpro I wanted to use to scan PESs.
It turns out that for this particular coordinate (see input below),
CCSD(T) always crashes in the 2nd step, with this message:
UNREASONABLE NORM. CALCULATION STOPPED
In the CCSD bit of the programme.

It looks like in the second step the RHF reference wavefunction
converges to a wrong state (e.g. for the input below -187.2009 H vs.
-187.5115 H for a 'fresh start' on the same geometry) giving rise to
this problem, because when I scan the coordinate by hand, the RHF is
not restarted and the CCSD converges just fine (also the RHF energy is
lower). I am surprised that this happens even at these small step
sizes of 0.05A or even 0.01A at long distances. I've tried to turn off
restart as in the example below, but the rhf is restarted every time in
the second step even though restart,0 is in the do-loop. Also, just
scanning at the RHF for small displacement (triplet, ROHF) gives a drop of
half a Hartree when going from the first structure to the second.

Any ideas about why the SCF converges to a wrong state? Could that have to
do with the ROHF state for a triplet?
Any ideas how to switch of RHF SCF restart in a PES scan?

Thanks in advance for your help.

Fedor Goumans

Example input for scan:

***,O+CO long range complex
memory,450,m
DR=0.05
do i=1,40
 restart,0
 r2=2.0+i*dr
 r1=1.13599229
 a1=120.0
 geometry={Y
 angstrom;
 O1;
 C,O1,r1;
 O2,C,r2,O1,a1;
 }
 basis=cc-pVDZ
 wf,22,1,2 ! A' surface
 rhf
 rhf-uccsd(t)
enddo
---