for the low lying electronic states. The data for the C 2 molecule has been obtained from the book by Huber and Herzberg. The effects on thermochemistry are studied by considering the following cases; (i) solely the ground state, (ii) ground state and low lying electronic states with the same harmonic and rotational frequencies for all the levels, done when factorizing the partition function into translation, vibration, rotation and electronic modes, and (iii) ground state and low lying electronic states with the harmonic and rotational frequencies computed for each state separately. ] for the ground state and the first 4 excited electronic states. The calculations are done using the W1 and EOM-CCSD method for the ground state and excited electronic states, respectively.