[molpro-user] Size consistency issue

[george]

Dear sir

                Thank you for the valuable suggestion. I will have a look
at the issues you mentione soon.

On Fri, Feb 12, 2016 at 4:49 PM, Lorenzo Lodi <l.lodi at ucl.ac.uk> wrote:

> Hello George,
> I had a look at this using small basis sets and I found results similar to
> yours, ie the CASSCF energies are not size-consistent in this case. So
> `trivial' errors such as incorrect convergence or getting the wrong
> electronic states should be excluded.
> Specifically:
>
> 1) Using the small STO-3G basis set CASSCF is FCI in the valence space,
> and in this case the energies *are* size consistent. I get using MCSCF
> E1=-73.804150 for O, E2=-73.443584 for O+, E1+E2=-147.247734 which is also
> the value I get for O2+ at r=100 a0.
>
> 2) Using 6-31G or cc-pVDZ the sum of the atomic energies is lower than the
> energy of stretched O2+ by about 0.0047 Eh, as you reported in the
> aug-cc-pV5Z basis set.
> On the other hand if we do FCI using the MCSCF orbitals we get
> size-consistent energies (attached outputs).
> As suggested by Kirk, perhaps different virtual orbitals enter into the
> active space (AS) in the molecular and atomic cases. If you want to
> investigate this further I'd have a look at the orbital expansion
> coefficients (use ORBPRINT) in the atomic and molecular cases (using the
> smallest basis set which reproduces the problem, eg 6-31G or even smaller)
> and see if you can understand what is going into the AS.
>
> I also point out that in this sistem there are lots of degeneracies. In
> the atomic calculation (attached) I average over the spacial components (3
> components for oxygen 3P and 1 component for oxygen+ 4S^o), while for the
> molecule I average over the two degenerate components of the 2Pi_g term. At
> r=infty a part from 2Pi_g there is also a 2Pi_u and two 2Sigma_g^+ terms,
> as well as {2Pi_g, 2Pi_u, 2x2Sigma_g^+} quadruplets and sextets, I wonder
> if the size-consistency violation you observed is related to which states
> are included and averaged over.
>
> Lorenzo
>
>
> On 10/02/16 18:36, george wrote:
>
> Dear molpro users
>
>
>        I am computing potential energy curve for ground state O2+ at multi
> level using av5z basis set. This dissociates to O(3P)+O+(4S) fragments. Now
> I add the energies of these two fragments and get -149.18369960 hartrees at
> the same level. When I look at the energies of curve at 100 angstrom, the
> multi value at this long distance is  -149.13557933 hartrees. I wonder why
> and how this numerical discrepancies arise? My input file is very simple
> without any complicated keyworkds. Am I missing something relevent to this
> type of problem? Any suggestion please.
>
>
> My input
>
>  ***,o2+
>  memory,125,m
>  print,orbitals,civector
>
> !file,1,o2+_ci_d2h_6_1_mar5.int
>
> !file,2,o2+_ci_d2h_6_1_mar5.wfu
>
> !symmetry,nosym
> !symmetry,x
>
> !rcm=[1.11780256,0.7,0.75,0.8,0.85,0.9,0.95,1,1.025,1.05,1.075,1.1,1.110,\
>
> !      1.125,1.15,1.175,1.2,1.25,1.3,1.35,1.4,1.5,1.6,\
>  !
> 1.7,1.8,1.9,2,2.125,2.25,2.5,2.75,3,3.25,3.375,3.5,3.625,3.75,4,4.25,4.5,4.75,5,5.25,5.5,5.75,6,6.5,7,7.5,8,9,10,11,12,13,14,15,16,18,20,21,22,23,24,25]
> ANG
>  rcm=[100] ANG
>
>  geometry
> !geometry specification, using z-matrix
>  o1
>  o2,o1,r
>  end
>
>  basis=aug-cc-pV5Z
>
>  do i=1,#rcm
>
>  r=rcm(i)
>
>  {multi;maxiter,40;config;pspace,0.1;wf,15,7,1;dm}
>  emulti_gs(i)=energy(1)
>
> !{ci;option,maxiti=10000;wf,15,6,1}
>
> !eci_gs(i)=energy(1)
>
> !eci_david(i)=energd(1)
>
>  {table,rcm,emulti_gs
>  Title,Results for O2+ dissociation 2Pi_g state
>  format,'(f15.8,f30.15)'}
>
>
> !{table,rcm,eci_gs
>
> !format,'(f15.8,f30.15)'}
>
>
> !{table,rcm,eci_david
>
> !format,'(f15.8,f30.15)'}
>
>  end do
>
>
>
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