Applications of Density Functional Theory Methods in Millimeter-Wave Spectroscopy

摘要

Density functional theory (DFT), using the most common functionals, and ab initio quantum chemistry methods are used to calculate the rotational constants and dipole moments of the astrophysically important molecules HCN, CH_3CN, CH_3CNH~+, HCCCN, and HCCNC. As far as millimeter-wave spectroscopy is of interest the DFT methods perofrmed well with most functionals, giving results within +-1% of experiments for rotational constants and +-3% for dipole moments. Analyzing the results obtained with all theoretical models, it may be concluded that the Becke's three-parameter exchange functional and the gradient-corrected functiona of Lee, Yang, and Paar (B3LYP) and Becke's three-parameter functional with Perdew-Wang correlational functional [B3P291/6-31G(d,p)] give the best performances. A detailed analysis of the electron correlation effects shows that HCCCN is more stable than in HCCNC, by 1.16 eV, with important contribution arising from triple excitations. This results is also compared with those obtained with DFT methods. Despite occasional difficulties, DFT with the currently available functionals are of great utility in quickly assessing spectroscopic parameters of astrophysical interest.