DFT calculations for anharmonic force field and spectroscopic constants of YC and its C isotopologues.

The construction of the complete third and the semi-diagonal quartic force fields including the anharmonicity of the ground state (X˜A) for yttrium dicarbide (YC) is carried out employing the vibrational second-order perturbation theory (VPT2) in combination with the density functional theory (DFT). The equilibrium geometries optimization, anharmonic force field and vibrational spectroscopic constants of YC are calculated by B3LYP, B3PW91 and B3P86 methods. Aug-cc-pVnZ (n=D, T, Q) and cc-pVnZ-PP (n=D, T, Q) basis sets are chosen for C and Y atoms, respectively. The calculated geometry parameters of YC agree well with the corresponding experimental and previous theoretical results. The bonding characters of YC or CC are discussed. Based on the optimized equilibrium geometries, the spectroscopic constants and anharmonic force field of YC are calculated. Comparing with the spectroscopic constants of YC derived from the experiment, the calculated results show that the B3PW91 and B3P86 methods are superior to B3LYP for YC. The Coriolis coupling constants, cubic and quartic force constants of YC are reasonably predicted. Besides, the spectroscopic constants and anharmonic force field of YC (X˜A) and YCC (X˜A) are calculated for the first time, which are expected to guide the high resolution experimental work for YC and its C isotopologues.