[molpro-user] Difference between CLOSED and CORE orbitals is MRCI

[Kirk Peterson]

Dear Lorenzo,

the electrons in a closed orbital are not excited in any reference  
CSF, but they are involved in excitations into the virtual orbitals  
and hence they are correlated.  If you think of a single reference  
closed-shell CISD calculation, the orbitals outside the core are  
obviously constrained to be doubly occupied in the reference but are  
correlated since single and double excitations are taken into the  
virtual orbitals.

hope that helps,

-Kirk

On Feb 28, 2008, at 8:46 PM, Lorenzo Lodi wrote:

> How do not understand very well the difference between CLOSED and CORE
> orbitals in MRCI calculations. Molpro 2006.2 manual says:
>
> "These orbitals do not form part of the active space, i.e., they are  
> doubly occupied in all reference CSFs; however, in contrast to the  
> core orbitals (see CORE), these orbitals are correlated through single
> and double excitations."
>
> I don't understand very well what this means, if they are not  
> excited in any CSF, how can they be correlated at all? Could anyone  
> explain this to me in a few words? I see that it does make a  
> difference. Consider this small input for water:
>
> r1=1.7, r2=1.9, theta=104.5
> geometry={O;  H1,O,r1;  H2,O,r2,H1,theta}
> basis=cc-pVDZ
> {RHF}
> {MCSCF; OCC, 7, 2 ; CLOSED, 1,0 ; FROZEN, 0,0}
> {MRCI ; OCC, 7, 2 ; CLOSED, 0,0 ; CORE, 0,0}
>
> If run with molpro 2006.2 I get
> Number of internal configurations:                 2744
> Number of singly external configurations:         66836
> Number of doubly external configurations:          4930
> Total number of contracted configurations:        74510
> Total number of uncontracted configurations:     787136
> !MRCI STATE 1.1 ENERGY               -76.235776417153
>
> On the other hand, if I use the same input but with the last line  
> changed to
> {MRCI ; OCC, 7, 2 ; CLOSED, 1,0 ; CORE, 0,0}
> I get
> Number of internal configurations:                 2744
> Number of singly external configurations:         66836
> Number of doubly external configurations:          4930
> Total number of contracted configurations:        74510
> Total number of uncontracted configurations:     698432
> !MRCI STATE 1.1 ENERGY               -76.235767414467
>
> which is indeed different, although by a small amount.
> It I want to run a calculation correlating ALL the electrons,
> would it make sense to use anyway
> {MCSCF; OCC, 7, 2 ; CLOSED, 1,0 ; FROZEN, 0,0}
> at the cas-scf stage and then
> {MRCI ; OCC, 7, 2 ; CLOSED, 1,0 ; CORE, 0,0}
> at the MRCI stage? Is the difference with respect to using
> {MRCI ; OCC, 7, 2 ; CLOSED, 0,0 ; CORE, 0,0}
> likely to be large?
>
>
>
> A part from this, I also would like to ask something about the  
> notation. Is there any difference between a line like
> {MRCI ; OCC, 7, 2 ; CLOSED, 0,0 ; CORE, 1,0}
> and
> {MRCI ; OCC, 7, 2 ; CLOSED, 1,0 ; CORE, 1,0}
> ?
> My own tests suggest that there isn't, and if this is the case it  
> seems to me that the second line uses a rather ambiguous notation  
> and should perhaps be avoided.
>
> Thank you in advance,
> Lorenzo Lodi