The NV-center in diamond is one of the most well researched defects to date. Since its discovery in the 1960’s, a large body of experimental as well as theoretical work have been produced, investigating its properties and applications. The reason for the attention on this defect are its properties that are well suited for a number of applications. Some of those properties are: 1) photostable at room temperature; 2) long spin coherence time; 3) spin-flipping during the process of optical excitation and decay; 4) Optical readout of spin state. Some of the applications include qubits in quantum computers and sensors for single molecule properties. In order for the NV-center to function well, it is important to decouple its interaction with other defects in the diamond lattice or with the surface of the diamond, that could have a detrimental effect on the NV-center properties. In this work, we theoretically investigate how the NV-center properties are affected by some nearby defects. Those defects include: a nitrogen point defect in the diamond lattice, diamond surfaces, and an extended intrinsic stacking fault defect in the diamond lattice. It is the negative charge state of the NV-center that has the properties mentioned above, and therefore it is this charge state that is interesting for the applications. Here, we investigate our new theoretical method of charging the NV-center through an electron donor nitrogen in the diamond lattice. By instead charging with an explicit electron donor/acceptor, we avoid the complicated correction schemes associated with the tra-ditional theoretical method of introducing an artificial background charge density in a supercell for simulating charged defects. It can also be argued that our new method is a more physically correct method, as negatively charged NV-centers in diamond get their charge by accepting electrons from nearby nitrogens in the diamond lattice. In addition to the NV-center, we further test the method for other point defects in diamond.In this thesis, an introduction to the field is given and the current research questions are stated in chapter 1. Followed by chapters reviewing the current experimental and theoret-ical work regarding the NV-center, computational physics and density functional theory, and an overview of the software used in this work. The results presented in this thesis are obtained using density functional theory computations.Our results show that the method of charging the NV-center with a donor-nitrogen is viable for an NV-N distance of 7.5 ˚A or greater.When placing the NV-center in the vicinity to a terminated surface (F- H/O/OH- and N-terminated), its properties converge to bulk values already at 5 ˚A depth. This is great news when compared with the recent experimentally achieved distance of 1 nm, meaning that NV-centers could possibly be placed even closer to the surface without being affected. When placing the NV-center in the vicinity of an intrinsic stacking fault (ISF), our results show that the NV-center is not greatly affected down to a distance of 4.2 ˚A. However, when the NV-center is placed 3.8 ˚A or closer to the ISF, the ZPL is perturbed between 2.0 and 11.3 %. It is perturbed the most when placed inside the ISF glide plane. This is great news for the technical applications; some diamonds contain high densities of ISFs, and our results show that a NV-center can be placed really close to such an ISF without losing its sensitivity as a sensor of magnetic fields.We have also found that the excitation from the NV⁻ ground state into donor-N⁺ (one-photon process) requires 2.31 eV and lead to a meta-stable NV0 and donor-N0 charge state, both of which are electron spin resonance (ESR) active and, thus, this transition could be investigated experimentally. The excitation to the neutral state can also be achieved through a two-photon process with the first step at 2.19 eV and the second step at 0.81 eV.When placing the NV-center in the vicinity to two nitrogens (one neutrally charged, and one positively charged acting as electron donor), we find that it is almost unaffected, with changes in the ZPL of 1-8 meV when the distance to the nitrogens is 9.40-12.52 ˚A. This means that a nearby nitrogen, whether it is neutral or positively charged does not affect the NV-center in a detrimental way.Our tests on charging defects through electron donors/acceptors reveals that our method also works for the following defect-donor/acceptor pairs: NV⁻-P⁺, NV⁻-B⁺, N⁺-B⁻, SiV⁻-N⁺, Be⁻-O⁺, Be²⁻-N⁺-N⁺.