The dynamics of the guest molecules CO2 and CH4 in p-tert-butylcalix[4] arene (TBC4) motion were studied from 100 K to 400 K using molecular simulation techniques. The rattling motion of the CH4 guest molecules, characterized by velocity autocorrelation functions, exhibit an increase in the Einstein frequency with increasing temperature. This has been observed experimentally and in simulations for some rare gas clathrate systems. The Einstein frequency for the CH4 molecule shows an increase from 75 cm(-1) at 100 K to 91 cm(-1) at 400 K. The frequency corresponding to the CO2 rattling motion exhibits a single peak and less temperature dependence going from 76 cm(-1) at 100 K to 72 cm(-1) at 400 K. The rotational spectra for CO2 exhibit two peaks at 100 K with the higher frequency peak shifting to lower frequency with increasing temperature. The computed rotational Einstein frequencies go from 86 cm(-1) at 100 K to 71 cm(-1) at 400 K for CO2. The temperature dependent Gibbs absorption free energies of the guest molecules CO2 and CH4 in TBC4 have also been studied at 1 bar using thermodynamic integration from 10 K to 400 K. As expected, the simulated absorption free energy for CO2 is greater at all temperatures than for CH4. At 10 K, the simulated values of -11.4 kcal/mol and -8.4 kcal/mol for CO2 and CH4, respectively, while at 300 K the values are -5.4 kcal/mol and -4.4 kcal/mol. Published by Elsevier B.V.