The relativistic configuration interaction (KRCI) for polyatomic molecules having heavy atoms has been implemented. The present KRCI is based upon determinants generated from two-component molecular spinors using effective core potentials with spin-orbit interactions and diagonalizes the energy matrix using the EVCHF of the IMSL diagonalization routine. Energy levels of Ge, Sn, Pb, Se, Te, and Po atoms are calculated and dissociation curves of HBr and $Po_2$ are obtained. Various calculations on the $Au_2$ are performed and potential energy curves of the $CH_3I$ are investigated for the excited states as well as the ground state. The Davidson``s diagonalization method is modified for the large sparse complex matrix and tested for Po, $Po_2$, and $Po_2H_2$.