Results of ab initio self-consistent-field (SCF) and density functional theory (DFT) calculations of the gas-phase structure,acidity (free energy of deprotonation,G0) and aro-maticity of tetraselenosquaric acid (3,4-diselenyl-3-cy-dobutene-1,2-diselenone,H2C4Se4) are reported.The global minimum found on the potential energy surface of tetraselenosquaric acid presents a planar conformation.The ZZ iso-mer was found to have the lowest energy among the three planar conformers and the ZZ and ZE isomers are very dose in energy.The optimized geometric parameters exhibit a bond length equalization relative to reference compounds,cyclobu-tanediselenone,and cydobutenediselenol.The computed aromatic stabilization energy (ASE) by homodesmotic reaction is -77.4 (MP2(fu)/6 - 311 + G //RHF/6 - 311 + G) and - 54.8 kJ/mol (B3LYP/6 - 311 + G //B3LYP/6 -311 + G).The aromaticity of tetraselenosquaric add is indicated by the calculated diamagnetic susceptibility exaltation (A) - 19.13 (CSGT(IGAEM) - RHF/6 - 311 + G// RHF/6- Results of ab initio self-consistent-field (SCF) and density functional theory (DFT) calculations of the gas-phase structure, acidity (free energy of deprotonation, G0) and aro-maticity of tetraselenosquaric acid (3,4- diselenyl- 3-cy-dobutene-1,2-diselenone, H2C4Se4) are reported. The global minimum found on the potential energy surface of tetraselenosquaric acid presents a planar conformation. The ZZ iso-mer was found to have the lowest energy among the three planar conformers and the ZZ and isomers are very dose in energy. The optimized geometric parameters exhibit a bond length equalization relative to reference compounds, cyclobu-tanediselenone, and cydobutenediselenol. The computed aromatic stabilization energy (ASE) by homodesmotic reaction is -77.4 (MP2 (fu / 6-311 + G // RHF / 6-311 + G) and - 54.8 kJ / mol (B3LYP / 6-311 + G // B3LYP / 6-311 + G). The around of tetraselenosquaric add is indicated by the calculated diamagnetic susceptibility exaltation (A) - 19.13 (CSGT (IGAEM) - RHF / 6 - 311 + G // RHF / 6-