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 fluoroformyloxyl radical, FC(O)O, using ab initio methods is complicated by orbital symmetry breaking in the Hartree-Fock wave function. While symmetry 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 adequately mixes the Hartree-Fock determinants leading to the symmetry breaking. 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 constructed in two different ways-unrestricted (UHF) and quasirestricted (QRHF) Hartree-Fock wave functions-in combination with a coupled cluster [CCSD and CCSD(T)] electron correlation treatment. We find that both methods must be employed with some caution, but in combination they provide reliable prediction of the vibrational spectrum of the FC(O)O radical. (C) 1995 American Institute of Physics.