[molpro-user] MRCI symmetry breaking

[Jacky LIEVIN]

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
_____________________________________