[molpro-user] MRCI symmetry breaking

[Toru Shiozaki]

Dear Jacky,
the jump in 90.06 is related to the pair elimination when orthogonal pair functions are generated.
Could you try the tighter thresholds for thrdlp and thrnrm as follows (or tighter value if you want):
{mrci; thresh,THRDLP=1.0d-10,THRNRM=1.0d-10}

In addition, it is not a good practice in general to use MRCI calculations with different symmetry (C2v and Cs in your case)
to construct the PES, since MRCI may not be invariant (but only slightly) with respect to the choice of the symmetry.
This is due to how we generate the CI basis functions.

Best wishes,
Toru

On Mar 24, 2011, at 9:30 PM, Jacky LIEVIN wrote:

> dear users,
> 
> The input below calculates few points of the potential curve for the rigid umbrella motion of CH3+ in its ground electronic state (2 A2" (D3h) --> 2 B2 (C2v) --> 2 A' (Cs)).
> The umbrella angle is changed from 90.0 to 90.1 degrees by steps of 0.02 degrees and the results are:
> 
>   ANGLE         ECASSCF          EMRCI             EMRCIDAV
>   90.00      -39.61901934   -39.71401806   -39.71754590
>   90.02      -39.61901934   -39.71401805   -39.71754589
>   90.04      -39.61901933   -39.71401804   -39.71754588
>   90.06      -39.61901932   -39.71401903   -39.71754705
>   90.08      -39.61901930   -39.71401900   -39.71754702
>   90.10      -39.61901928   -39.71401897   -39.71754699
> 
> The first point (D3h geometry) is calculated in C2v symmetry while the others (C3v) are calculated in Cs.
> A sudden stabilization (below the planar equilibrium energy) occurs in the MRCI and MRCI+Davidson curves at 90.06 degrees . Note that the CASSCF curve doesn't exhibit such a feature.
> Comparison of MRCI calculations below and above 90.06 shows that the number of N-2 electron functions increase from 48 to 49 and that the number of contracted configurations moves accordingly from 17183 to 17310.
> Would it be possible to control these numbers in order to obtain smooth curves correlating C3v point to planar D3h?
> 
> Note that this problem occurs in the frequency calculation with as a consequence wrong frequencies (too low) for the out of plane mode. 
> 
> Any ideas about the origin of this symmetry breaking and how to solve it are welcomed?
> 
> Many thanks in advance
> 
> Jacky
> 
> 
> 
> *** CH3 fondamental
> gthresh,energy=1.d-8,step=1.d-5
> gprint,orbitals,civector
> basis=vdz
> rx=1.0 ang
> r=1.09503    ang
> hx=90.0
> dhh=0.02
> hh=hx
> d=120
> geometry={C;H2 C r;X1 C rx H2 hh;H3 C r X1 hh H2 d;H4 C r X1 hh H2 -d}
> angle(1)=hh
> {hf
> wf,9,3,1}
> {multi
> wf,9,3,1}
> ecasscf(1)=energy(1)
> {ci
> maxiter,50
> wf,9,3,1}
> emrci(1)=energy(1)
> emrcidav(1)=energd(1)
> do i=2,6
> hh=hh+dhh
> symmetry,nosym
> geometry={C;H2 C r;X1 C rx H2 hh;H3 C r X1 hh H2 d;H4 C r X1 hh H2 -d}
> hf
> {multi
> closed,1
> start,2140.2
> wf,9,1,1}
> angle(i)=hh
> ecasscf(i)=energy(1)
> {ci
> wf,9,1,1}
> emrcidav(i)=energd(1)
> emrci(i)=energy(1)
> enddo
> table,angle,ecasscf,emrci,emrcidav
> ---
> 
> 
> _____________________________________
> Jacky Liévin
> Service de Chimie Quantique et Photophysique
> Université Libre de Bruxelles, CPi 160/09
> 50 av F.D. Roosevelt
> B-1050 Bruxelles
> Belgium
> Tel: +32-2-650 4089
> Fax: +32-2-650 4232
> _____________________________________
> 
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