Some observations about quantum chemistry software GAUSSIAN

In a cosmic object having molecules, kinetic temperature in general is very low; few tens of Kelvin. Thus, one is concerned with the rotational levels in the ground vibrational state and ground electronic state. The rotational and centrifugal distortion constants, electric dipole moment can be used for calculation of energies of rotational levels and radiative transition probabilities (Einstein A-coefficients) for radiative transitions between the levels. We have investigated some molecules where laboratory data are available and for the same molecules we have obtained the data with the help of GAUSSIAN also. We have found that for some molecules, the two sets of data are in good agreement whereas for some molecules, they differ very much. As the laboratory data are the most reliable, one would like to use the GAUSSIAN data only in absence of the laboratory data. Thus, an obvious question arises how to decide the reliability of GAUSSIAN data. We are aware of the fact that the frequencies of spectral lines obtained from the GAUSSIAN data are not as accurate as required by the astronomers. However, the GAUSSIAN data can play important role in getting qualitative results about a molecule. We could not succeed in running the CCSD and CCSD (T) methods for the GAUSSIAN, as the computer program broke down each time during the execution. Therefore, we have employed the functional B3LYP method, i.e., Becke’s three parameter exchange function B3 (Becke1) with Lee, Yang and Parr’s gradient corrected exchangecorrelation functional.2