[molpro-user] dissociation to higher channel

[Kirk Peterson]

Dear Prof. Grein,

I ran through the CASSCF part of your input with orbital printing turned on.  At 2.7 Ang. one of the O 2p orbitals becomes exactly doubly occupied and for some reason gets swapped with a core orbital (3.1 and 4.1).  This persists out to your longest bond length. So the simple fix is to do a orbital rotation in the rs2 or mrci to swap these orbitals back into the correct order from 2.7 onwards:

if (ir2.lt.8) then
  {rs2;wf,24,1,0;maxit,100;option,maxiti=400}
else
  {rs2;wf,24,1,0;maxit,100;option,maxiti=400;rotate,3.1,4.1}
endif

This is a little strange to me since when orbitals are exactly doubly occupied in multi, Molpro used to canonicalize the closed-shell space.

best regards,

-Kirk


> On Aug 10, 2015, at 5:43 AM, Fritz Grein <fritz at unb.ca> wrote:
> 
> Dear MOLPRO users,
> I am encountering a problem with dissociation to a higher dissociation channel.
> When dissociating O3 to O2+O, in 1-A’ symmetry, then the MOLPRO calculated  CI energy (for fixed O-O distance and angle), is fine up to an O2-O separation of 2.6 A. Until here, the energy is close to the dissociated O2(3-Sigg) + O(3-P) energy. At 2.7 A, the calculated energy jumps higher, and stays higher thereafter. It appears that the O2(1-Delta) + O(1-D) level is approached.
> This happens with all CI methods, RS2, MRCI, AQCC and ACPF. Interestingly, the CASSCF energy dissociates to the lowest channel. Dissociation in symmetries, like 1-A”, also moves to a higher channel, although at a larger distance.
> Reading the literature, there are several MOLPRO calculations described for the 1-A’ dissociation of ozone, covering a large potential energy surface. Apparently, no dissociation problems as described above have been encountered. This means that there must be a way to avoid this problem.
> Any help would be appreciated.
> Fritz Grein
> Department of Chemistry
> University of New Brunswick, Canada
> --------------------------------------------------------------------
>      R2          ECAS                  ERS2
>     2.0   -224.5573513   -225.1020581
>     2.1   -224.5596135   -225.1015510
>     2.2   -224.5614908   -225.1012356
>     2.3   -224.5629154   -225.1009875
>     2.4   -224.5639408   -225.1007287
>     2.5   -224.5646529   -225.1003953
>     2.6   -224.5651329   -225.0997375
>     2.7   -224.5654847   -225.0134963
>     2.8   -224.5656828   -225.0134413
>     2.9   -224.5658012   -225.0133640
>     3.0   -224.5658675   -225.0132738
>  
> ***,o3-scan1
> memory,100,m
> symmetry,x
> angstrom
> geometry={o1
>           o2,o1,r1
>           o3,o2,r2(i),o1,a}
> r1=1.22,a=120.
> basis=avtz
> distance2=[2.0,2.1,2.2,2.3,2.4,2.5,2.6,2.7,2.8,2.9,3.0]
> i=0
> do ir2=1,#distance2
> i=i+1
> r2(i)=distance2(ir2)
> {hf;maxit,100}
> {casscf;wf,24,1,0;maxit,40}
> ecas(i)=energy
> {rs2;wf,24,1,0;maxit,100;option,maxiti=400}
> ers2(i)=energy
> enddo
> {table,r2,ecas,ers2
> head,r2,ecas,ers2}
> ----------------------------------------------------------------------
>    
>  
>  
>  
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