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

[Lorenzo Lodi]

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