Study on spectroscopic parameters and molecular constants of HCl(X¹∑⁺) molecule by using multireference configuration interaction approach

Equilibrium internuclear separations, harmonic frequencies and potential energy curves (PECs) of HCl(X^1\Sigma ^ + ) molecule are investigated by using the highly accurate valence internally contracted multireference configuration interaction (MRCI) approach in combination with a series of correlation-consistent basis sets in the valence range. The PECs are all fitted to the Murrell--Sorbie function, and they are used to accurately derive the spectroscopic parameters (D_\rm e, D_0, \omega_\rm e\chi_\rm e, \alpha_\rm e and B_\rm e). Compared with the available measurements, the PEC obtained at the basis set, aug-cc-pV5Z, is selected to investigate the vibrational manifolds. The constants D_0, D_\rm e, R_\rm e, \omega_\rm e, \omega_\rm e\chi_\rm e, \alpha_\rm e and B_\rm e at this basis set are 4.4006 eV, 4.5845 eV, 0.12757 nm, 2993.33 cm^ - 1, 52.6273 cm^ - 1, 0.2981 cm^ - 1 and 10.5841 cm^ - 1, respectively, which almost perfectly conform to the available experimental results. With the potential determined at the MRCI/aug-cc-pV5Z level of theory, by numerically solving the radial Schr?dinger equation of nuclear motion in the adiabatic approximation, a total of 21 vibrational levels are predicted. Complete vibrational levels, classical turning points, inertial rotation and centrifugal distortion constants are reproduced, which are in excellent agreement with the available Rydberg--Klein--Rees data. Most of these theoretical vibrational manifolds are reported for the first time to the best of our knowledge.