  We present a comprehensive relativistic coupled cluster study of the
electronic structures of the ThO and ThS molecules in the spinor basis.
Specifically, we use the single-reference coupled cluster and the
multi-reference Fock Space Coupled Cluster (FSCC) methods to model their ground
and electronically-excited states. Two variants of the FSCC method have been
investigated: (a) one where the electronic spectrum is obtained from sector
(1,1) of the Fock space, and (b) another where the excited states come from the
doubly attached electronic states to the doubly charged systems (ThO$^{2+}$ and
ThS$^{2+}$), that is, from sector (0,2) of the Fock space. Our study provides a
reliable set of spectroscopic parameters such as bond lengths, excitation
energies, and vibrational frequencies, as well as a detailed analysis of the
electron correlation effects in the ThO and ThS molecules. Finally, we examine
the first ionization potential and electron affinity of the above mentioned
molecules.
