The higher excited determinants (also called configuration state functions (CSFs) or shortly configurations) are then chosen either by the program according to some perturbation theoretical ansatz according to a threshold provided by the user or simply by truncating excitations from these references to singly, doubly, ... excitations resulting in MRCIS, MRCISD, etc.
For the ground state using more than one reference configuration means a better correlation and so a lower energy.
The problem of size inconsistency of truncated CI-methods is not solved by taking more references.
As a result of a MRCI calculation one gets a more balanced correlation of the ground and excited states.
The generally too-high excitation energies of CIS or CISD are lowered.