# Example: Spin-flip excitation energies: SiH2¶

```
#!/bin/sh
# Calculation of the spin-flip excitation energies of the open shell molecule SiH2
AMS_JOBNAME=SpinFlip $AMSBIN/ams <<eor
System
atoms
Si 0.0 0.0 0.0
H 0.0 -1.095664024353027 -1.045576810836792
H 0.0 1.095664024353027 -1.045576810836792
end
end
Task SinglePoint
Engine ADF
title spin-flip excitation energies of SiH2
excitations
lowest 20
end
forcealda
basis
type TZ2P
end
sftddft
spinpolarization 2
tda
unrestricted
EndEngine
eor
# In this example, the lowest 20 spin-flip excitation energies of SiH2 are
# calculated in a spin-unrestricted TDDFT calculation.
# In this case an excited state is used as reference, which means that there can
# also be a negative excitation energy, which is indeed the case. The electron
# configuration used in the SCF is (a1)^1 (b1)^1, with Sz=1, thus a ^3B_1
# state, which is an excited state. The ^1A_1 state with electron configuration
# (a1)^2 is lower in energy, and is the ground state.
# There is also an excited 1A1 state with electron configuration (b1)^2. The
# transition from the ground 1A1 state to the excited 1A1 state is an excitation
# from the electron configuration (a1)^2 to (b1)^2. This transition is actually
# a double excitation, which means that some double excitations can be reached
# using spin-flip TDDFT with carefully selected reference states.
# In the MO -> MO transitions part for the excitations of the output file, the
# spin of each molecular orbitals are also specified to help assign the spin
# state of the excited states. Note that in these spin-flip calculations the
# transitions are always from alpha spin-orbital to beta or from beta spin-
# orbital to alpha spin-orbital.
# For open-shell molecules, spin-flip transition can result in transition to the
# ground state with a different Sz value, while the symmetry of the transition
# density is A1. The excitation energy of this transition should be zero and
# this can be used to test the reliability of spin-flip TDDFT. Indeed the
# calculation of the spin-flip excitation energies of SiH2 shows one value which
# is close to zero and has a transition density of A1 symmetry.
# The 1A1 state with electron configuration (a1)^2 can also be used in the
# calculation of the excitation energies. This is a closed shell configuration,
# in which case we do not need the spin-flip method.
AMS_JOBNAME=Excitation $AMSBIN/ams <<eor
System
atoms
Si 0.0 0.0 0.0
H 0.0 -1.095664024353027 -1.045576810836792
H 0.0 1.095664024353027 -1.045576810836792
end
end
Task SinglePoint
Engine ADF
title excitation energies of SiH2
excitations
lowest 20
end
basis
type TZ2P
end
EndEngine
eor
# The transition from the ground ^1A_1 state to the excited ^1A_1 state, which
# is an excitation from the electron configuration (a1)^2 to (b1)^2, can not be
# reached in this calculation, since it has mainly double excitation character.
# Of course, other excited ^1A_1 states can be reached.
```