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# Calculated product of the moments of inertia

The moment of inertia is a measure of how much torque it takes to spin the molecule. The product of the three moments of inertia is used in the calculation of the molecular Entropy. This comparison of products of moments of inertia illustrates how well a given method/basis set performs at reproducing the experimental geometry. The three rotational constants (A,B,C for polyatomic species, only B for linear species) are inversely related to the moments of inertia (IA,IB,IC). See the section on the rotational contribution to the partition function in the thermochemistry page (Page I.D.) for more information.

#### Rules for chemical formula

• Enter a sequence of element symbols followed by numbers to specify the amounts of desired elements (e.g., C6H6).
• Elements may be in any order.
• If only one of a given atom is desired, you may omit the number after the element symbol.
• Parentheses may be used to group atoms.
• Multiple specifications for an atom will be added. This means that CH3(CH2)4CH3 will be treated the same as C6H14.
• A comma delimited list of several species may be entered.
• Ions are indicated by placing + or - at the end of the formula (CH3+, BF4-, CO3--)

#### Species in the CCCBDB

• Mostly atoms with atomic number less than than 36 (Krypton), except for most of the transition metals. See section I.B.1 for a periodic table view.
• Six or fewer heavy atoms and twenty or fewer total atoms. Exception: Versions 8 and higher have a few substituted benzenes with more than six heavy atoms. Versions 12 and higher have bromine-containing molecules.