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ABSTRACT. We have performed mPW1PW91 calculations to investigate the conformational characteristics and hydrogen bonds of p-tert-butylcalix [4]arene (1), p-tert-butylcalix [5]arene (2), calix [6]arene (3) and p-tertbutylcalix[6]arene (4). The structures of the different conformers of 1-3 were optimized by using mPW1PW91/ 6-31+G(d,p) method. The relative stability of the four conformers of 1 is in the following order: cone (most stable) > partial-cone > 1,2-alternate > 1,3-alternate. The relative stability of the conformers of 2 is in the following order: cone (most stable) > 1,2-alternate > partial-cone > 1,3-alternate. The relative stability of the various conformers of 3 is in the following order: cone (pinched: most stable) > partial-cone > cone (winged) 1 ,2-alternate ~ 1,2,3-alternate > 1,4-alternate > 1,3-alternate > 1,3,5-alternate. The structures of the various conformers of 4 were optimized by using the mPW1PW91/6-31G(d,p) method followed by single point calculation of mPW1PW91/6-31+G (d,p). The relative stability of the conformers of 4 is in the following order: cone (pinched) > 1,2-alternate > cone (winged) > 1,4-alternate ~ partial-cone > 1,2,3-alternate > 1,3,5-alternate > 1,3-alternate. The primary factor affecting the relative stabilities of the various conformers of the 1-4 are the number and strength of the intramolecular hydrogen bonds. The hydrogen-bond distances are discussed based on two different calculation methods (B3LYP and mPW1PW91).