Thermodynamic properties of the most stable gaseous small silicon-carbon clusters in their ground states

The most stable structures of gaseous Si _m C _n (3 n+m 6) clusters in their ground electronic states are determined with the high level electronic correlation method QCISD(T)/g3large. Thermodynamic properties on heat capacity (C _p,m ^Θ ), entropy (S _m ^Θ ), Gibbs energy function (-[G ^Θ -H ^Θ(T _r )]/T) and enthalpy function (H ^Θ-H ^Θ(T _r )) are predicted with standard statistical thermodynamics using the structure parameters and vibrational frequencies obtained with B3PW91/6-31G(d) method combined with the electronic excitation energies determined with time dependent density functional (TD DFT) method at B3PW91/6-31G(d) level. The electronic energies are calculated with the accurate model chemistry method at G3(QCI) level of theory and the Δ _f H _m ^Θ (0 K), Δ _f H _m ^Θ (298.15 K) and Δ _f G _m ^Θ (298.15 K) values are predicted. The heat capacities C _p,m ^Θ (T) as a function of temperature within 298.15-2000 K are fitted into analytical equations. The thermodynamic functions at higher temperatures are determined classically by using these equations. Most of the results obtained in this work are consistent with the available experiments.