# Example: Excitation energies open shell molecule: CN¶

```
#!/bin/sh
# Calculation of the excitation energies of the open shell molecule CN
# In this example, the lowest 20 excitation energies of CN are calculated in a
# spin-unrestricted TDDFT calculation. 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. The transitions
# are always from alpha spin-orbital to alpha spin-orbital or from beta spin-
# orbital to beta spin-orbital.
AMS_JOBNAME=exci $AMSBIN/ams <<eor
System
atoms
C 0.0000 0.0000 0.0000
N 0.0000 0.0000 1.1718
end
end
Task SinglePoint
Engine ADF
title excitation energies of CN
excitations
lowest 20
nto
sfoanalysis
end
basis
type AUG/ADZP
end
spinpolarization 1
unrestricted
EndEngine
eor
# Next the same example for CN is given with the Tamm-Dancoff approximation
# (TDA) approximation (including TDA in the input). Due to this approximation
# the calculated excitation energies will not be exactly the same as in the
# first example.
AMS_JOBNAME=exci_tda $AMSBIN/ams <<eor
System
atoms
C 0.0000 0.0000 0.0000
N 0.0000 0.0000 1.1718
end
end
Task SinglePoint
Engine ADF
title excitation energies (TDA) of CN
excitations
lowest 20
nto
sfoanalysis
end
basis
type AUG/ADZP
end
spinpolarization 1
tda
unrestricted
EndEngine
eor
# Note that the basis set is still far from complete, the ATZ2P is better. For
# accuracy reasons one may need to increase the numerical quality. If one uses
# the augmented basis sets for accuracy reasons one could use NumericalQuality
# Good.
# The third calculation is the calculation of spin-flip excitation energies for
# CN. Again these energies will not be exactly the same as in the first example.
# For open-shell molecules, spin-flip transition can result in transition to the
# ground state with a different S_z value, while the symmetry of the transition
# density is A1 (Sigma+ for linear molecules). 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 CN shows one value which is close to zero and has a transition density of
# Sigma+ symmetry.
AMS_JOBNAME=exci_tda_spinflip $AMSBIN/ams <<eor
System
atoms
C .0000 .0000 .0000
N .0000 .0000 1.1718
end
end
Task SinglePoint
Engine ADF
title spin-flip excitation energies (TDA) of CN
excitations
lowest 20
nto
sfoanalysis
end
basis
type AUG/ADZP
end
sftddft
spinpolarization 1
tda
unrestricted
EndEngine
eor
```