Assistance with Optimization of Potential Curve Calculation in CASSCF for CO Molecule

[白光如]

  Dear Molpro Users,  

  I am currently running CASSCF calculations to compute the potential curve of the CO molecule. However, I have encountered a small discontinuity in the potential curve, as shown in the blue curve in the figure, near R=1.5a.u..  I am wondering if anyone can suggest how to optimize the calculation of the potential curve in CASSCF to address this issue.  

  Below is the code I am using to calculate the potential curve for a single R:

  ***,CO,potential curve,trans-dipole

memory,300 ,m
symmetry,x,y

gthresh,gradient=1.d-10,energy=1.d-14,civec=1.d-12,step=1.d-8,orbital=1.d-12;
geometry={c;o,c,r}
basis=AVQZ
r=1.2
{hf
wf,14,1,0}
{hf
wf,13,1,1}
{casscf;
start,2100.2
orbital,ignore_error;
OCC,6,2,2,0;
FROZEN,2,0,0,0;
pspace,1;
WF,13,1,1;state,4;}
sE1=energy(1);
sE2=energy(2);
sE3=energy(3);

{table ,r ,sE1,sE2,sE3 }

I would appreciate any suggestions or advice on how to improve the stability of the potential curve in these calculations.

Thank you in advance for your help.

[Karl Irikura]

Hi Bai,

Molpro will choose the lowest states at each geometry.  This means that the states can change when the geometry changes.  Have you plotted the dipole moments, in addition to the energies?  If there is an abrupt change, you might be getting different states.  If that is so, you can try adding another state.  Or you could skip one or more states at certain geometries by setting their weights=0. 

Another thing to check is the angular momentum, using the TRAN2,LZZ command. If that shows a change in states, you can use the LQUANT directive (irrep #1 includes doublet Sigma+ states but also one component of Delta, Gamma, etc.). Maybe it's unnecessary, but I like to ensure C2v-symmetric orbitals by including both components of all states with Lambda > 0.  In this case, that would mean including some states in irrep #4. 

Good luck!

Karl 

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[白光如]

Dear Karl,

Thank you for your reply and suggestions. I appreciate your help!

Best regards,
Bai