[molpro-user] Calculating molecular oxygen excited state

[Jacky LIEVIN]

Hi Lorenzo,


Le 28 sept. 09 à 20:52, Lorenzo Lodi a écrit :

> Jacky LIEVIN wrote:
>> Dear Lorenzo,
>> Please find below a modified version of your input. The results  
>> are  looking fine:
>
> [cut]
>
> Thanks a lot Jacky, it did the trick and it's now working fine!
> I spent some time studying your sample input (I'm new to state- 
> averaged calculations) and there a few things I'm not too sure of.
> For the excited-state state-averaged CASSCF your input reads:
> {MCSCF;
>  WF, NELEC=16, SYMMETRY=1, SPIN=0;state,2;lquant,2,0
>  WF, NELEC=16, SYMMETRY=1, SPIN=0;state,1;lquant,2}

>
>
> The second line specifies the two lowest-states with "1" (=Ag)  
> symmetry, namely the <sigma g+> and <delta g xx-yy>.
> If the third line is to specify the degenerate <delta g xy> state  
> with symmetry "4" (=B1g), shouldn't we specify SYMMETRY=4 ?

OUPS! You are right! sorry for this mistake: I did the test too quickly.
The correct input is actually:
{MCSCF;
WF, NELEC=16, SYMMETRY=1, SPIN=0;state,2;lquant,2,0
WF, NELEC=16, SYMMETRY=4, SPIN=0;state,1;lquant,2}

>
>
>
> Also, I have a slightly different question. Is it advisable/ 
> necessary to in the excited-state RHF step to have it to converge to  
> the desired excited state too? Couldn't we use the ground state as  
> RHF reference for the subsequent steps?

The RHF calculation only serves to provide an initial guess to the  
CASSCF calculation. So you can actually use the ground state RHF  
orbitals as starting point for the CASSCF step

best wishes

Jacky




>
>
> Thanks again,
>
> Lorenzo
>
>> Modifications are:
>> 1) the use of state averaged MCSCF
>> First for ensuring the degeneracy of the delta state (one component  
>> of  symmetry 1 and one of symmetry 4): group theory tells us that  
>> deltag  correlates to Ag + B1g in D2h symmetry
>> Second for describing the second state (sigma + g) of symmetry 1:   
>> sigma + g correlates to Ag
>> Both electronic states must thus be calculated in a single MCSCF  
>> state  averaged calculation defined by 2 WF directives.
>> Note the lquant directive given on each WF line and for each   
>> calculated state
>> Note the degeneracy CASSCF(1)=CASSCF(2)= energy of delta state
>


_____________________________________
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
_____________________________________