Re-examination of the partition function and related thermodynamic functions of translation in a cubical box of very small volume fails to confirm the recent conclusion of Slanina about unlimited growth of the internal energy. On the contrary the translational internal energy for very small dimensions of the cube approaches zero in the limiting case. The source of the discrepancy is the zero point energy which should be taken into account when the partition function and some of related thermodynamic functions are calculated. The partition function calculation technique is also discussed. Simple and exact double formula expressions are suggested instead of direct summation.