Unusually large effects of single excitations on the geometry of radical species and limiting spin-projection invariance of some correlated methods
✍ Scribed by J.M.L. Martin; J.P. François; R. Gijbels
- Publisher
- Elsevier Science
- Year
- 1990
- Tongue
- English
- Weight
- 569 KB
- Volume
- 166
- Category
- Article
- ISSN
- 0009-2614
No coin nor oath required. For personal study only.
✦ Synopsis
For a number of radical species with heavy spin contamination, including BO, CCH, CN, CO+, NO, N3, and BN, it is shown that single excitations have a large effect, up to several hundredths of an A, on calculated bond distances. The effect decreases with increasing basis set size. Harmonic frequencies may be affected by as much as 200 cm-'. It is also shown that CCSD, CISD, CID, and CCD energies are effectively invariant under single spin annihilation when no major higher contaminants are present. It can be concluded that for species with heavy spin contamination, CISD is presently the method to be preferred over MP2 and even spm-projected MP2.