= Open-shell Systems : Spin-flip TD-DFT for spin-flip excitations =

Although SF-TD-DFT is possible by using isf=3, which calculate a->b and b->a excitation at the same time, however, the TDA version is preferred. 
This allows separate calculations of a->b and b->a types of excitations.

Using a UKS/ROKS reference in SCF, the input for spin-flip TDA (flip-up) reads:
{{{
$TDDFT
IMETHOD 
 2
ISF
 1
...
ialda
 2
$END
}}}

The keyword ''ialda'' controls the spin-flip kernel when using GGA functionals in ground state calculations. 
The option "ialda=2" means a ALDA0 type approximation is used, which always gives numerical stable results.

Flip-down excitations can be calculated by choosing isf=-1. 
{{{
$TDDFT
IMETHOD 
 2
ISF
 -1
...
ialda
 2
$END
}}}

There are spin-adapted versions of flip-down excitations with imethod=3, but they are not thoroughly tested and to be explored in future.