A systematic theoretical investigation of tetramethyldiarsane is presented to help understand its structure and photoelectron spectrum. Full potential energy curve along the C-As-As-C torsion was calculated at the Hartree-Fock (HF) level, and complete geometry optimizations were performed at HF, second-order Many-Body Perturbation Theory (MBPT(2)), and Coupled-Cluster Singles and Doubles (CCSD) levels of theory. Two conformers, anti and gauche, have been found in accordance with the experimental observations. The calculated geometries are in good agreement with the electron diffraction results. Ionization energies were computed by the Equation-of-Motion Coupled-Cluster (EOM-CC) method. The calculations predict a substantial lone-pair splitting for both conformers which contradicts the original assignment of the photoelectron spectrum by Cowley et al, According to the new assignment, the first and third bands belong to the lone pairs of the anti rotamer, the second band is attributed to the n(+) lone-pair combination in the gauche conformer, while the peak of the n(-) combination is merged with the intense first band previously ascribed exclusively to the anti rotamer. The conformer ratios calculated from the present assignment of the photoelectron spectrum are in good agreement with the quantum-chemical results. Since our assignment contradicts the theoretical reasoning, which the original assignment was based on, a new explanation is presented. We find that the s-character of the lone-pair orbitals increases in the order of nitrogen, phosphorus, arsenic, and antimony which explains not only the increasing pyramidalization of the Me2E moiety but also the increasing splitting of the energy of the lone-pair MOs.