Field evaporation of LaB6 single crystals resulting in a large variety of ionized clusters of LaBnm+ type, where n=1,2,...,6 and m=1, 2, 3, 4, is investigated in a collaborative experimental and theoretical study. The aim of the present work was to compare the measured appearance probability of various clusters in a flux with the calculated energetic stability of corresponding free clusters. The experiments were carried out with a time-of-flight atom probe. The appearance probability of cluster ions in the atom probe spectra was estimated by calculating its rating in many selected spectra. The methods for determining such ratings have been developed and discussed. The theoretical calculations are based on ab initio quantum chemical Hartree-Fock self-consistent field and configuration interaction methods. A double zeta basis set was used to determine the cluster geometries, energies, and relative stability. For the inner-shell electrons of lanthanum we have used relativistic compact effective potentials. The geometry optimization was carried out using the analytical gradient method. The experimentally obtained ratings of some ionic species were compared with the energetic stability of free clusters calculated as described previously. The main conclusion of the present study is that unstable clusters, e.g., LaB3+, or almost unstable LaBnm+ clusters were preferably observed in the ion flux. The physical explanation of this phenomenon lies in the possibility of a stable cluster to redistribute its interatomic bonds and thus to remain attached to the surface. An unstable cluster does not have this characteristic and as a result this must leave the surface.