Different approaches to the theoretical study of the local centres in solids are discussed. Particular attention is paid to the embedded-cluster models. There are given the mathematical formulation, the computational scheme and applications for some local centres on/in graphite monolayers of the moderately large embedded cluster and the large-unit-cell-embedded-cluster (LUC-EC) models. It is shown that both models are based on related approximation and give comparable results for the centres considered. At the same time the LUC-EC scheme is, in practice, realised more easily and enables one to study the charged centres. A study of some charged states of adsorbed hydrogen and substitutional boron and nitrogen atoms on/in graphite monolayers is used to illustrate the LUC-EC model applications.