Entropic effects on the free energies of clusters in silane plasmas

The present work demonstrates the importance of entropic effects on the thermodynamics of branched open chain silicon hydride clusters (Si_nH_m and Si_nH_m^-). These clusters are important constituents in nanodusty silane plasmas. We include all categories of such single-bonded species for n = 5 and 6, namely silyl radicals, silyl anions, silylenes and silylene anions, and silanes. We calculated the statistical mechanical partition functions by employing Kohn-Sham density functional theory with the multistructural method for torsional anharmonicity to estimate thermodynamic quantities, in particular Gibbs free energy, enthalpy, entropy, and heat capacity. For each species we included contributions from all conformational structures of all possible isomers, and we calculated the thermodynamic propeties in three ways, namely by using the multistructural quasiharmonic approximation, the multistructural method with uncoupled torsional potential anharmonicity, and the multistructural method with coupled torsional potential anharmonicity. Our results show that the entropic effects are large and are primarily due to multistructural effects, although torsional potential anharmonicities are not negligible. We find that the multiple-structure effect, which is always greater than unity, is not only very large (as large as a factor of 592) but also very isomer-dependent, so that free energy differences between isomers can be greatly affected. The torsional potential effect is relatively smaller on average but certainly not negligible; it varies from a factor of 0.2 to 1.4.