THE VIBRATIONAL-SPECTRUM OF FC(O)O RADICAL - A CHALLENGING CASE FOR SINGLE-REFERENCE ELECTRON CORRELATION METHODS

Calculation of the structure and vibrational spectrum of the fluorofor myloxyl radical, FC(O)O, using ab initio methods is complicated by orb ital symmetry breaking in the Hartree-Fock wave function. While symmet ry breaking is most directly corrected with multiconfiguration Hartree -Fock methods, the problem can also be attacked with single reference methods when combined with an electron correlation treatment that adeq uately mixes the Hartree-Fock determinants leading to the symmetry bre aking. In this work the structure and vibrational spectrum of both the ground (X B-2(2)) and second excited (B (2)A(1)) states of the FC(O)O radical are calculated using single-reference wave functions construc ted in two different ways-unrestricted (UHF) and quasirestricted (QRHF ) Hartree-Fock wave functions-in combination with a coupled cluster [C CSD and CCSD(T)] electron correlation treatment. We find that both met hods must be employed with some caution, but in combination they provi de reliable prediction of the vibrational spectrum of the FC(O)O radic al. (C) 1995 American Institute of Physics.