Torsional vibrations and the equilibrium structure of the B2F4 molecule

The equilibrium structure of the B2F4 molecule was calculated and the depen dence of the total energy on the torsional dihedral angle was studied at th e Hartree-Fock, MP2, and B3LYP approximation levels using the 6-31G, 6-311G , and cc-pV(D,T)Z standard basis sets, if necessary, augmented by polarizat ion and diffuse functions. The contributions of the kinetic and potential e nergy to the total energy of the molecule were analyzed, and correlation en ergy was shown to play an important role in the formation of the torsional barrier. The density functional method could not be used to unambiguously d etermine the equilibrium torsional angle value if the torsional barrier was low. At all approximation levels, varying the torsional angle from 0 to 90 degrees with simultaneous geometry optimization shortened the B-B bond len gth by 0.009 Angstrom and increased the B-F bond length by 0.003 Angstrom a nd the BBF angle by 0.1 degrees to 0.25 degrees independently of the potent ial energy surface minimum position. The inclusion of polarization function s increased basis set flexibility necessary for correctly describing the cr oss section of the potential energy surface alone the torsional coordinate.