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

Lorenzo Lodi l.lodi at ucl.ac.uk
Fri Feb 29 04:46:57 GMT 2008

```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.