The ab initio limit quartic force field of BH₃

Abstract

The complete quartic force field of BH₃ has been converged to the ab initio limit by extrapolation of core‐valence correlation‐consistent basis set series (cc‐pCVXZ, X = T, Q, 5) of all‐electron CCSD(T) (coupled‐cluster singles and doubles with perturbative triples) energy points. Additional computations including full coupled‐cluster treatments through quadruple excitations (CCSDTQ), scalar relativistic effects, and diagonal Born–Oppenheimer corrections (DBOC) were concurrently executed. Within second‐order vibrational perturbation theory (VPT2) our quartic force field yields the fundamental frequencies ν₁ = 2502.3 cm⁻¹, ν₂ = 1147.2 cm⁻¹, ν₃ = 2602.1 cm⁻¹, and ν₄ = 1196.5 cm⁻¹, in excellent agreement with observed gas‐phase fundamentals, displaying a mean absolute error of only 0.3 cm⁻¹. Our converged prediction for the equilibrium bond length of BH₃ is r_e = 1.1867 Å. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 1106–1112, 2005