Dear All,

Sometime ago I asked about the difference between
delta-SCF and TD-DFT methods.

I got two messages, pasted bellow. I thank
Dr.Christopher J. Cramer and Dr. Fedor Goumans.

Fedor Goumans writes
Dear Dr Kiran,

The Delta SCF approach is based on single
determinantal energies.
Basically it means that for a given excited state that
corresponds to a
one-electron excitation, you relax the ground state,
plant the electron you
want excited from the orbital you want to the orbital
you want it
excited to (using the occupations key). The energy you
obtain by this is a
50/50 mixture of singlet +triplet excitation
excitation energy as it is
not a single determinant. You can retrieve the singlet
energy by
calculating the triplet energy of this state (which is
a sing slater
determinant): E (singulet) = 2 E (s+t) - E (triplet).
In terms of
SlaterDeterminants:
  E(1/2 SQRT (2) * |1(alpha)2 (beta)| - 1/2 SQRT (2) *
|1 (beta)2
(alpha) | ) = 2 E ( |1(alpha)2 (beta) | ) - E (
|1(alpha)2 (alpha) | ),
which follows from analysing the differences between
the the energies
of the determinants (in terms of J and K).
See: http://www.scm.com/Doc/Welcome.html and download
'Theory'; see
also: Ziegler, Rauk, Baerends, Theor. Chim. Acta, 43,
261-271.; Daul, Int.
J. Quantum Chem. 52, 867-877.
As this Delta-SCF approach is theoretically not well
founded, some
inconsistencies may occur.

TDDFT is theoretically justified, although besides the
xc-potential,
the xc-kernel has to be approximated. In ADF this is
done rather crudely,
by setting both frequency and spatial dependency to
zero (Adiabatic
LDA). This approximation appears not to be too rigid.
See van Gisbergen's thesis
(http://www.scm.com/Doc/publist.html) for
comparison of TDDFT, Delta SCF and CASPT2 results for
CrCO6. See also
Rosa et al J. Am. Chem. Soc. 121, 10356-10365.
With the TDDFT-module more excitations can be
calculated at once, in
one step. Furthermore, you can specify the number
excitations you want
for each irrep. Singlet and/or triplet energies may be
calculated.

I hope this helps,
kind regards,
Fedor Goumans
--------
Dr. Cramer writes
Kiran,

Delta-SCF simply implies that the two states you are
interested can
both
be found as single-configurational variational minima
within their
respective spin-state symmetries. For example, the
splitting between
the
lowest singlet and the lowest triplet of methylene,
CH2, can be found
by
delta-SCF because the two states differ by BOTH spin
and spatial
symmetry,
and each is lowest in energy for its respective set of
states.
Similarly,
the difference between the singlet A1 ground state and
the lowest
triplet
A1 state could be found by delta-SCF because the two
states differ in
spin
symmetry. However, if either of the two states in
which you are
interested
does NOT represent a lowest-energy spin-spatial state,
then delta-SCF
cannot be employed with any real hope of success. Even
in those rare
circumstances where you can get an orbital occupation
in the converged
SCF
to correspond to the dominant single configuration
associated with the
excited state (if there is one...) since there is no
enforcement of
orthogonality between that excited state and those of
lower energy in
the
spin-spatial set, the excited state is likely to be
considerably too
low
in energy.

To deal with excited states in the latter class,
multiconfigurational
methods, or CIS, or time-dependent methods like RPA,
TD-DFT, etc., are
more
appropriate.

Chris Cramer

kiran boggavarapu
>
> Dear All,
>
> There is some mention of delta-SCF method for
excited
> state energies by ADF. This is not TD-DFT.
> I failed to locate how to run those calculations in
> the user's guide.
>
> CAn some help me?
>
> Is it better or wrose compared to TD-DFT?
>

-- 
Christopher J. Cramer
University of Minnesota
Department of Chemistry
207 Pleasant St. SE
Minneapolis, MN 55455-0431
--------------------------
Phone:  (612) 624-0859 || FAX:  (612) 626-2006
cramer_at_pollux.chem.umn.edu
http://pollux.chem.umn.edu/~cramer
(website includes information about new textbook
"Essentials
    of Computational Chemistry:  Theories and Models")
 
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