Ab initio and equilibrium bond angles. Structures of HNO and H2O2

The possibility of calculating accurate ab initio bond angles is examined u sing a sample of 29 molecules (35 independent angles) containing only first row atoms and whose equilibrium structures are known. Three different corr elated methods are compared: MP2, CCSD(T), and DFT, using the hybrid functi onal B3LYP. The convergence of Dunning's correlation consistent polarized v alence basis sets, cc-pVnZ is also studied. It is found that the CCSD(T) me thod is consistently the most accurate; the DFT/B3LYP being slightly less r eliable than MP2. It is shown that when convergence of the basis set is ach ieved (which is dependent on the kind of bonding) and when the effect of di ffuse functions on electronegative atoms is taken into account, a high accu racy may be obtained: 0.03 degrees for the median of absolute deviations or 0.07 degrees for the mean absolute deviation. It does not exclude the poss ibility that the ab initio method may fail in some particular case, for ins tance when a large amplitude motion is involved. The MP2/cc-pVQZ method giv es a mean absolute deviation of 0.22 degrees to be compared with the 0.07 d egrees of the CCSD(T) method. To obtain these results, it was necessary to reanalyze the structure of a few molecules, particularly, a new and more ac curate structure is proposed for nitroxyl, HNO and hydrogen peroxide, H2O2. (C) 2000 Elsevier Science B.V. All rights reserved.