The guided and radiation modes together form a complete basis set.
It comes about when trying to relax the volume of a crystal using a constant basis set.
Semiempirical calculations are usually carried out in a minimal basis set.
The finite basis set is assumed to be approximately complete.
The design of the laser determines which basis set of solutions is most useful.
Another, more precise notation indicates exactly which and how many functions are added to the basis set, such as (d, p).
These basis sets are popular in calculations involving periodic boundary conditions.
Secondly the correlation energy is highly dependent on the basis set used.
The largest basis set is 6-311+G(3df,2p) used at the MP2 level of theory.
The 6-311G basis set is replaced by the smaller 6-31G basis.