Number of orbitals in full configuration interaction I am using one quantum chemistry package where I am supposed to assign a number of active orbitals I want to be included in the FCI (full configuration interaction) calculation. For example if I have  ${H_2}$ molecule (singlet) and I use $cc-pV6Z$ basis set of Dunning, what would be the maximum number of active orbitals in the FCI? Can somebody provide me insight into this problem?
In simple terms, Full CI uses all of the possible orbitals (a "full" active space). Thus the number of active orbitals should equal the size of your basis set. If you have 10 basis functions, and then perform a Hartree-Fock calculation, you get 10 orbitals out (some occupied, some virtual).

Complete Active Space (CAS) methods, on the other hand, only use some of the orbitals and then perform a Full CI on that subset of orbitals, leaving the other orbitals (inactive space) alone.

Full CI is CAS in the total orbital / total active space limit.