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

[Gerald Knizia]

On Friday 12 September 2008 14:32, Fedor Goumans wrote:
> 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.

You could try specifying {rhf-uccsd(t); diis,1,1,6,1}. This often helps if 
convergence goes into the wrong direction before diis would usually start. 
(Starting with the first iteration has been changed to be the default in 
2008.1).

The input in the attachment did work in 2008.2. 

> 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. 

For reasons beyond my comprehension, RHF just reads the orbitals without doing 
the proper basis set projection between the old and the new basis. This 
apparently leads to poor starting orbitals.

> 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.

You can explicitly tell RHF to start with an atomic density guess:
	{rhf; wf,22,1,2; start,atden}
This should always work, regardless of restarts.
-- 
Gerald Knizia
-------------- next part --------------
memory,150,m;
DR=0.05
basis=cc-pVDZ
do i=1,40
   r2=2.0+i*dr
   rx(i)=r2
   r1=1.13599229
   a1=120.0
   geometry={Y
      angstrom;
      O1;
      C,O1,r1;
      O2,C,r2,O1,a1;
   }
   !{rhf,maxdis=25,shifta=-0.5,shiftb=-0.1; maxit,80; wf,22,1,2; start,atden; save,2100.2}
   {rhf; wf,22,1,2; start,atden; save,2100.2}
   rhf-uccsd(t),avg_denom=1;
   E(i) = ENERGY
enddo

table,rx,E