Force field comparisons of the heat capacity of carbon nanotubes

The force fields Tersoff, CHARMM, COMPASS, CVFF and PCFF are compared using molecular calculations and simulations of SWNT thermal properties. The heat capacity results from the force fields vary significantly in the low (room) temperature range. The COMPASS force field best reproduces the phonon frequencies calculated from density functional theory and is consistent with the Raman scattering results. The temperature dependent behavior of SWNT heat capacity is investigated using harmonic and quasi-harmonic dynamics theories. The impact of quasi-harmonic analysis is not significant in the low and intermediate temperature range (below 500 K). Thus, force field comparisons based on the harmonic approximation are valid in that temperature range. Above 500 K, heat capacity results based on the Tersoff force field using a quasi-harmonic approximation are further investigated.