#!/bin/sh

# Calculation of the spin-flip excitation energies of the open shell molecule SiH2

$ADFBIN/adf <<eor
Title spin-flip excitation energies of SiH2
Atoms Zmatrix
 Si 0  0  0
 H  1  0  0  1.5145
 H  1  2  0  1.5145  92.68
End

excitations
  lowest 20
end

unrestricted
charge 0 2

SFTDDFT
FORCEALDA
TDA

Basis
 Type TZ2P
End

eor
mv TAPE21 SiH2_spinflip.t21
rm logfile

# 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.


$ADFBIN/adf <<eor
Title excitation energies of SiH2
Atoms Zmatrix
 Si 0  0  0
 H  1  0  0  1.5145
 H  1  2  0  1.5145  92.68
End

excitations
  lowest 20
end

Basis
 Type TZ2P
End

eor
mv TAPE21 SiH2_excit.t21
rm logfile

# 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.