The dependence of the results of single-reference state coupled cluster (CC) methods on the choice of orbitals defining the reference-state determinant was studied for the broadly used H4 model. It is demonstrated that the topology of the energy surfaces defined by varying orbital sets is entirely different for the CCD and CCSD approaches, which is first of all reflected in the different character of the stationary points. The CCD energy has a global minimum attained for orbitals close to the Brueckner orbitals. For the CCSD energy a saddle point has been found which corresponds to orbitals very different from standard orbitals employed in quantum chemistry. Various types of Brueckner orbitals related to the CC method are discussed and employed in calculations of the CC energy.