Calculation of vibrational parameters of an electride-like molecule Li$_{4}$C$_{4}$H$_{2}$N$_{2}$ and the pyridazine molecule C$_{4}$H$_{4}$N$_{2}$

The frequencies and intensities for fundamental transitions between vibrational states of an electride-like molecule Li$_{4}$C$_{4}$H$_{2}$N$_{2}$ that can be obtained from the pyridazine molecule C$_{4}$H$_{4}$N$_{2}$ by substituting lithium atoms for two hydrogen atoms and adding two other lithium atoms to nitrogen atoms are calculated. Spectral parameters of Li$_{4}$C$_{4}$H$_{2}$N$_{2}$ are calculated in the harmonic and anharmonic approximations using the MP2, CCSD, and QCISD methods with the atomic function sets aug-cc-pVDZ and aug-cc-pVTZ. For comparison the absorption spectrum of a pyridazine molecule was calculated in the same approximations. The calculations showed that new intense bands appear upon entering lithium atoms into the pyridazine molecule and the spectral parameters of bands present in the pyridazine spectrum are significantly changed. The results obtained may be useful for spectroscopic observation and identification of the new compound.