Ordering of the O–O stretching vibrational frequencies in ozone

The ordering of ν1 and ν3 for O3 is incorrectly predicted by most theoretical methods, including some very high level methods. The first systematic electron correlation method based on one-reference configuration to solve this problem is the coupled cluster single and double excitation (CCSD) method. However, a relatively large basis set, triple zeta plus double polarization (TZ+2P), is required. Comparison with other theoretical methods is made.