The vibrational spectra of furoxan and dichlorofuroxan: A comparative theoretical study using density functional theory and local electron correlation methods

The vibrational spectra of furoxan and dichlorofuroxan have been studied using the local quadratic configuration interaction method including single and double excitations (LQCISD) and two density exchange-correlation functionals (B3LYP and B97r). The vibrational spectra of these molecules are very sensitive to electron correlation effects and are therefore well suited as benchmark systems for investigating the impact of local approximations at levels beyond second-order Moller-Plesset perturbation theory (MP2). Effects due to anharmonicity, basis set deficiencies, and lacking electron-correlation contributions are accounted for by scaling the force constants in internal coordinates with empirical factors, using the the scaled quantum mechanical (SQM) force fields procedure. For comparison, also the correlation-corrected vibrational SCF approach (cc-VSCF) has been applied to account for anharmonicity effects. Using the DFT and LQCISD results the experimental vibrational spectra are analyzed and assigned. The impact of the local approximations in the LQCISD method on the vibrational spectra has also been tested for related heterocyclic systems. As found earlier for local MP2, the effect of these approximations is in general small.