Regularities in molecular geometries, force fields, and vibrational spectra of the titanium subgroup tetrahalides MX₄ (M = Ti, Zr, Hf; X = F, Cl, Br, I): A nonempirical CISD + Q study

Nonempirical calculations of the equilibrium internuclear distances R_e, quadratic force constants f_ij, harmonic vibrational frequencies ω_i, and infrared intensities A_i were performed at the configuration interaction level of quantum theory, including all single and double excitations augmented with the Davidson correction for quadruple excitations. Regularities in the behavior of the R_e and f_ij parameters were established for the TiX₄ → ZrX₄ → HfX₄ and MF₄ → MCl₄ → MBr₄ → MI₄ molecular series.