The atomically oriented valence-universal coupled-clusters method in the intermediate Hamiltonian formulation with one-and two-body part(s) included in the cluster operator (VU-CCSD/R) is applied to describe the ground state and some excited states of the Mg atom. Two Slater-type orbitals (STO) basis sets are employed to see the effect of the basis set on the results. In addition to that, several complete model spaces are used in the calculation starting with (3s,3p) and ending with (3s,3p,3d,4s,4p) orbitals as active. It has been found that the quality of the description of the ground- and excited states depends on the STO basis set used in the calculation. In order to obtain reliable excitation energies, special attention has been paid to the construction of well-balanced basis sets in which functions necessary for adequate description of both the ground state and excited states under consideration are included. Therefore, in spite of the moderate size of the orbital basis, our results compare quite well in terms of the total energy as well as in energy differences with results of other methods which, in some cases, use significantly larger basis sets.