Breakdown of equipartition of energy for vibrational heat capacity of diatomic molecular gas due to nonvanishing bond length
When the theorem of equipartition of energy applies to the vibrational degree of freedom within diatomic molecular gas, the bond length is usually taken as zero so that the theorem is valid. Once the bond length is taken into consideration, calculations show that the mean energy of the vibrational heat capacity will significantly deviate from the standard value near the high temperature which breaks down the bond.
cond-mat.stat-mech↗