Ab initio SCF calculations of conformations of squaric acid have been reported. The ZZ isomer is found to be the most stable in the three planar isomers. An isodesmic energy analysis has been carried out at the HF 6-31G^*^*//6-31G^*^* level for the three planar isomers. The three isomers have positive isodesmic energies or small negative ones, suggesting that squaric acid is not aromatic. Vibrational frequencies have been calculated for the three isomers.

Key words: ISOMER, AB INITIO CALCULATION, FREQUENCY OF VIBRATION, SQUARIC ACID