CCL:G: Range of M.O.s used for correlation in CCSD calculations

 Sent to CCL by: Cory Pye [cpye ~~]
 Hello Andrea,
 MP2 defaults to frozen core, for calcium this is [1s2 2s2 2p6 3s2 3p6].
 For Ca+ you only have one electron, and you need at least two electrons to
 correlate. If you are examining, say Ca(H2O)8 + later on, and just want an MP2
 value to subtract, then the MP2 energy of Ca+ can be taken as the same as the
 HF energy. If however, you want to include all core orbitals in the correlation,
 then you would have to use MP2=full for all of your calculations (used to be
 the default MP2 method in Gaussian until, I think G92 or G98. If cost is
 prohibitive, then you could also use MP2=Window(m,n) which allows you to select
 which orbitals to use for correlation. This should be used with care, however,
 as in molecular systems, sometimes bonding orbitals can be lower in energy than
 the core orbitals in the s and early p (ord or f) blocks. See papers by Petrie
 on this topic.
 ---excerpt from Gaussian 03 manual----
 The "read window" option means that specific information about which
 orbitals are retained in the post-SCF calculation will be given in the input
 file. The additional input section consists of a line specifying the starting
 and ending orbitals to be retained, followed by a blank line. A value of zero
 indicates the first or last orbital, depending on where it is used. If the value
 for the first orbital is negative (-m), then the highest m orbitals are
 retained; if the value for the last orbital is negative (-n), then the highest n
 orbitals are frozen. If m is positive and n is omitted, n defaults to 0. If m is
 negative and n is omitted, then the highest |m| occupied and lowest |m| virtual
 orbitals are retained.
 Here are some examples for a calculation on C4H4:
 0,0 is equivalent to Full.
 5,0 freezes the 4 core orbitals and keeps all virtual orbitals (equivalent to FC
 if the basis has a single zeta core).
 5,-4 freezes the four core orbitals and the highest four virtual orbitals. This
 is the appropriate frozen-core for a basis with a double-zeta core.
 6,22 retains orbitals 6 through 22 in the post-SCF. For example, since C4H4 has
 28 electrons, if this is a closed shell calculation, there will be 14 occupied
 orbitals, 5 of which will be frozen, so the post-SCF calculation will involve 9
 occupied orbitals (orbitals 6-14) and 8 virtual orbitals (orbitals 15-22).
 -6 retains orbitals 9 through 20.
 ReadWindow is a synonym for RW.
 Performs the same function as the ReadWindow option, but takes its input as
 parameters in the route section rather than from the input stream.
 On Fri, 30 Sep 2011, Andrea Ciccioli wrote:
 > Sent to CCL by: "Andrea  Ciccioli"
 > Hello everybody.
 > I am doing CCSD(T) calculation with G03 on the Ca+ ion.
 > In the output, the message "There is no correlation energy for this
 system" is given, and the HF energy is not corrected by the CC procedure.
 > This is probably due to the criterium chosen to select the range of M.O.s
 used for correlation. Indeed, in the output the following message is reported:
 "Range of M.O.s used for correlation: 10   24". With this choice, I
 have seen that only 1 occupied MO is taken into account.
 > My question is: how can I extend the range of MO used for correlation ?
 > Thank you in advance
 > Andrea Ciccioli
 > Department of Chemistry
 > Sapienza - Rome>
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