Manual:Casida
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Mark Casida's formulation of Linear-Response TDDFT allows calculations of the excitation energies of a finite system. For small molecules, this is normally the fastest way to calculate them.
To perform a Casida calculation you first need a ground-state calculation and a calculation of unoccupied states; then you run the code with CalculationMode=casida.
This is an input file for a linear-response calculation of the nitrogen dimer, found at SHARE/testsuite/finite_systems_3d/10-casida.01-N2.inp.
%CalculationMode
gs | unocc | casida
"ground_state_" | "unocc_states_" | "lrtddft_"
1 | 2 | 3
%
FromScratch = yes
bond_length = 2.0744
%Coordinates
"N" | -bond_length/2 | 0.0 | 0.0 | no
"N" | bond_length/2 | 0.0 | 0.0 | no
%
%Species
"N" | 14.0067000 | tm2 | 7 | 2 | 0
%
BoxShape = sphere
Radius = 12.0
Spacing = 0.36
SpinComponents = unpolarized
XFunctional = lda_x
CFunctional = lda_c_vwn
MaximumIter = 200
ConvRelDens = 1e-5
LCAOStart = lcao_full
LCAODimension = 18
EigenSolver = cg_new
EigenSolverInitTolerance = 1e-2
EigenSolverFinalTolerance = 1e-5
EigenSolverFinalToleranceIteration = 6
EigenSolverMaxIter = 20
unocc_states_EigenSolverMaxIter = 1000
TypeOfMixing = broyden
NumberUnoccStates = 9
PoissonSolver = fft_corrected
