# 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:

Ground State input file

#### Unoccupied states

Once the ground state calculation is converged, it is necessary to add and converge more unoccupied states.

Unoccupied States input file

#### Casida calculation for excited states

Casida input file