While ion implantation is commonly carried out for p-type doping of 4H-SiC, high dose Al implantation leads to the formation of point defects that can harm the mobility of carriers in the channel of 4H-SiC MOSFETs [1,2]. For this reason, it is important to investigate species other than Al, for p-type doping of SiC. In this study, we present an ab-initio study of the electronic properties of acceptor impurities, with valency two or three, in both 4H-SiC bulk and in SiO2/4H-SiC interfaces. Ab initio calculations were performed on a 128-atom 4H-SiC supercell and a 100 atom SiO2/4H-SiC slab. For the latter, a 20 Å vacuum was set along the c-direction and dangling bonds were passivated by H. We used Troullier-Martins pseudopotentials and the Perdew-Burke-Ernzerhof (PBE) form of the generalized gradient approximation (GGA) for the exchange-correlation potential. A double- plus polarization was used as atomic orbital basis set. Both systems were relaxed until the forces on the atoms were <0.02 eV/Å. Formation energies (Eform) were estimated using the methodology described in Ref. [3]. In fig.1, we show the Eform of the Mg, Zn and Sc substitutional impurities sitting at Si(k) sites and bonded to either single (N), double (O) or triple (Cl) donor impurities. For Mg (fig.1(a)), the isolated substitutional impurity gives rise to two acceptor levels, at 0.6 eV and 1.14 eV above the valence band edge (EV). When a complex with N is formed (MgSiNC), a single donor level arises (EV+0.2 eV). The same occurs for MgSiOC, while for MgSiClC the (3-/2-) level arises close to the edge of the conduction band (EC). A similar effect can be seen when ZnSi bonds to N, O or Cl (fig.1(b)), with the difference that, unlike MgSi, ZnSi shows the presence of a donor level (EV+0.3 eV). On the other hand, ScSi (fig.1(c)) shows an amphoteric behavior, with a donor (EV+0.50 eV) and an acceptor (EV+0.77 eV) level in the bandgap. Its complexes with donors give rise to shallow double donor levels close to EV and also shallow acceptor levels close to EC. Estimation of the binding energy of the studied complexes is in progress. Since we observed that both Mg and Zn have a similar electronic behavior, we focused on the electronic properties of Mg and Sc in an abrupt SiO2/4H-SiC interface. First, we observe that the main contributions to the density of states (DOS) on the valence and conduction bands are due to the 2p orbital of C atoms and 3p of Si atoms. Although not shown in here, when MgSi is present in the oxide, no states can be seen in the bandgap (Egap) while ScSi gives rise to a state at midgap. On the other hand, if MgSi or ScSi are at the interface, new states arise in the proximity of EV, due to the formation of C dangling bonds. When analyzing the MgSi-CO complex (fig.2(a)), we observe three states at EV+0.1 eV, EV+0.8 eV and EV+2,0 eV due to the dangling bonds of the Mg first neighbors (C atoms). When MgSi is in the oxide (fig.2(b)), the MgSi-CO complex gives rise to a level at EV+1.8 eV. Regarding the ScSi-CO complex, when ScSi is at the interface (fig.3(a)) or in the SiO2 (fig.3(b)), one state at EV+1.2 eV arises, together with shallow states close to EV. To conclude, we analyzed the electronic properties of substitutional Mg, Zn and Sc atoms in bulk 4H-SiC and at the SiO2/4H-SiC interface, by first-principles calculations. We found that while Mg behaves as a deep acceptor, Zn and Sc show an amphoteric behavior. Such a behavior is also found when the impurities bind to single, double and triple donors. When Mg or Sc are incorporated at the interface or in the oxide, this results in the formation of C dangling bonds which lead to the rise of shallow states close to the valence band. This suggests that the presence of impurities such as Mg, Sc or Zn during 4H-SiC MOSFET processing can be detrimental for the mobility of charge carriers in the device channel. [1] T. Kobayashi et al., APL 108, ,152108 (2013). [2] T. Kobayashi et al., JAP 121, 145703 (2017). [3] C. Freysoldt et al., PRB 93, 165206 (2016) and references therein.