Hi,
Quoting: "..there is no intrinsic reason why one cannot optimize
alpha and beta spin orbitals of the active space separately..."
As Nuno pointed out CASSCF WF i.e. the multi determinantal (or
configurational) expansion as a whole is spin adapted. So zero spin
contamination. This makes the WF a proper eigenfunction of both Sz
and S2 operators.
If you select different spatial orbitals for alpha and beta you will
have a non-zero spin contamination, hence contrary to the spirit of
the method.
Of course you can start form any orbital set, in that case natural
orbitals from an unrestricted calculation can be used, however they
are still a single set of spatial orbitals for alpha and beta pairs.
In short (U) in CASSCF does not exist, unless it is someone's
misnomer for some mathematical trick in a certain code, however I've
never heard of something like that.
Suggested reading should involve Mark Gordon's, Ruedenberg's and
late Roos'.
Best,
Yavuz
On 21.12.2012 11:41, J D Whitfield
jdwhitfield-$-gmail.com wrote:
First thanks for the feedback so far. More
specific information about my study: I am considering a small
CASSCF(4,5) calculation for the triplet state of Be in the TZ
Dunning basis. This amounts to FCI for this toy model. I
am
interested in the natural orbital occupation numbers, but, in
this case, the restricted orbitals are not sufficient as the
alpha and beta spin orbitals should each have different
energy.
To Sam, My input script was the following:
%Chk=CAS1
#P UCASSCF(4,5)/Gen POP=NOAB Density=All
Z=4, TZ, FCI
0
3
4
0.0 0.0 0.0
!
TZ (Dunning) EMSL Basis Set Exchange
Library
...
The output was:
QPErr
--- A syntax error was detected in the input line.
#P UCASSCF(4,5)/Gen POP=NOA Density=All
'
Last state="GCL"
TCursr= 1046 LCursr= 3
To Ljiljana, would using the
UHF orbitals be sufficient to obtain a solution where
the alpha and beta orbitals are optimized
separately.
To Nuno, thank for the suggestion but please
refer to a specific reference that would discuss the
implementation details mundane to CASSCF. I can believe
that CASSCF is not implemented in Gaussian using an
unrestricted formalism but there is no intrinsic reason
why
one cannot optimize alpha and beta spin orbitals of the
active space separately.
Thanks,
JDW
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