[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,
Thanks for your help.
Kirk and Gerald's suggestion of adding the option of start,atden or
start,h0 to the rhf directive also worked in my version (2006.1).
Still not sure why in a PES scan the rohf seems to converge to a
higher-energy state in the second step - this also holds true if one does
only hf (e.g. if one does a scan at the rhf level only), while e.g. a
geometry optimisation goes fine.
Apparently the orbital guess from a the previous scf is different for a
PES surface scan than it is for a geometry optimisation (at least at the
triplet ROHF level)?
With kind regards,
Fedor

> 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
> ---
>
>