Metallofullerenes with two erbium atoms encapsulated in IPR C-82 cage isomers C-s-6 (I), C-2v-9 (II), and C-3v-8 (III) were investigated using density functional theory. The calculations suggest that erbium atoms assume a trivalent state with Er (4f(11)) valence electronic configuration in Er-2@C-82 and Er(2)C(2)pC(82), where two electrons (6s(2)) per Er atom are transferred to the cage carrying four negative charges (C-82(4-)), while the third electron is promoted from the 4f to the 5d shell, becoming involved in covalent bonding to near atoms. Experimentally, Er3+-like emission from I-4(13/2) to I-4(15/2) was observed, and our calculations indicate that the Er Er covalent metal bond in Er-2@C-82, and the Er-C/C-2 covalent bonds in Er2C2@C-82, can account for the observed photoluminescence despite the cage with C-82(4-). Such existence is the reason that the C-2 unit was found to be neutral on the basis of MEM-Rietveld X-ray measurements, although formally it should be described as C-2(2-). Our prediction for isomer photoluminescence intensity agrees with the experimentally determined order (III > I > II), where the most pronounced activity of isomer III in Er2C2@C-82 stems from its higher charge of formal Er3+ and its largest HOMO-LUMO gap.