For C 4 , C 4 H and other larger carbon clusters and hydrocarbons, we compute the thermodynamics using the W1 method from Gaussian 16 software package. All the excited electronic states are computed using the EOM-CCSD method. The paper is organised in two major sections. The first section involves the calculation of thermochemistry data for the ground state of carbon clusters and hydrocarbons including the study of the effect of excited electronic states. This data is then used to compare the equilibrium compositions of pure carbon and acetylene mixtures with state of the art thermodynamic tables used in hypersonics today. The second part is focused on studying the excited electronic structure of the C 3 , C 4 and C 4 H molecules. These molecules have been chosen on basis of their absorption features in the VUV region that closely match the emission features of atomic oxygen and nitrogen. An overview of the theory and computational methods of quantum mechanics used for the calculation of properties of various molecules is presented in Section 2. Subsection 2.2 discusses the quantum chemistry methods used to compute the excited electronic state energies and transition moments whereas subsection 2.1 discusses the tools from statistical mechanics used to compute the thermodynamic properties. The results are presented in section 3 followed by the concluding remarks in section 4.