Casida Linear Response
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.
Here are the input files for a linear-response calculation of the nitrogen dimer, found at share/testsuite/linear_response/01-casida.* .
Ground state calculation:
The first step is to converge the ground-state:
Unoccupied states
Once the ground state calculation is converged, it is necessary to add and converge more unoccupied states.
Casida calculation for excited states
See also the tutorial on Casida calculations.