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
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:
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.