The calculation of the excitation spectrum of a system in the Casida frequency-domain
formulation of linear-response time-dependent density functional theory (TDDFT)
implies the use of a basis set of occupied/unoccupied Kohn-Sham orbitals. This
basis set should, in principle, include all pairs formed by all occupied states,
and an infinite number of unoccupied states. In practice, one has to truncate this
basis set, selecting a number of occupied and unoccupied states that will form the
pairs. These states are specified with this variable. If there are, say, 15 occupied
states, and one sets this variable to the value "10-18", this means that occupied
states from 10 to 15, and unoccupied states from 16 to 18 will be considered.
This variable is a string in list form, i.e. expressions such as "1,2-5,8-15" are
valid. You should include a non-zero number of unoccupied states and a non-zero number
of occupied states.