Open shell hydrocarbons, AM1 ROHF versus UHF

[<BETTENS.,at,.MPS.OHIO-STATE.EDU>]

 Dear Netters,
 Thanks to those of you who responded to my last posting on the 14th of
 November regarding spin contamination.  Before summarizing these
 responses I would like to ask one further question which is related to
 my last question in the previous posting.  I will then compile all
 responses and post a summary.  My question concerns ROHF versus UHF
 calculations using the AM1 semiempirical model.  Is anybody aware of
 any published work which recommends the use of either in calculating
 heats of formation, specifically for hydrocarbon open shell species?
 I have examined the original paper of Dewar, M.J.S.; Zoebisch, E.G.;
 Healy, E.F. and Stewart, J.J.P., J. Am. Chem. Soc., 107 (1985) 3902
 where AM1 was introduced.  In this work the heats of formation of 6
 hydrocarbon radicals (all doublets) where compared with the
 experimental values.  I could find no indication as to whether the
 reported calculations used the UHF or ROHF approaches.  This is
 important because the calculated heats for each approach produces
 significantly different results.  I consequently repeated the
 calculations on the 6 hydrocarbon radicals given in the above paper
 and found the following heats of formation (kcal mol^{-1}).  I also
 give the values of S^2 and the reported heats as well as the current
 experimental values (from J. Phys. Chem. Ref. Data, 17 (1988) Sup. 1)
 for comparison below.
            AM1           AM1    Reported
            UHF    S^2    ROHF      AM1     Exp.     Exp. - Calc.(ROHF)
 CH3       29.95  0.7613  31.25    31.25   34.8(3)       3.6
 C2H3      60.46  0.8589  64.78    64.78   63.4(10)     -1.4
 C2H5      15.49  0.7619  18.14    15.48   28           10
 CH2CHCH2  30.20  0.9300  38.58    38.58   39            0
 (CH3)2CH   3.61  0.7622   6.80    10.07   22.3(6)      15.5
 (CH3)3C   -6.14  0.7623  -2.66    -2.66   11.0(6)      13.7
 For C2H5 the authors seem to have reported the UHF value.  I was not
 able to reproduce the reported result for (CH3)2CH with either the
 ROHF or the UHF result.  I was able to reproduce the calculated heat
 of formation for this species given by Dewar, M.J.S. and Thiel, W., J.
 Am. Chem. Soc., 99 (1977) 4907, using the MNDO Hamiltonian.  In this
 earlier work the authors explicitly stated that they used ROHF heats
 of formation calculated with MNDO.  It is noted that for the remaining
 species the reported AM1 heats of formation also appear to be the ROHF
 values.  While agreement with the experimental values are not terrific
 and appear particularly bad for the larger saturated hydrocarbon
 species, they are adequate for my purposes given that these errors can
 be taken as typical for calculated heats of formation of analogous but
 larger open shell hydrocarbon radicals.
 Does anybody know of any further studies, using the AM1 Hamiltonian,
 on radicals where the calculated heats of formation have been compared
 with experiment?  It seems to me, from the above table, that the UHF
 results are not useful in comparison with the ROHF results, and that
 the extent of spin contamination tends to increase with molecular size
 and unsaturation, being close to 0.75 for the most saturated species.
 For instance, I repeated the calculation on triplet linear C19 given
 by Novoa et al., Inorganica Chemica Acta 198-200 (1992) 133, using UHF
 and ROHF.  I obtained the same heat of formation as these authors with
 no assumed symmetry in the linear chain and using the UHF AM1
 Hamiltonian.  However, I obtained an S^2 of 4.356 (should be 2) and a
 ROHF heat of formation of 678.72 kcal mol^{-1}, which is greater than
 the UHF value by 24.17 kcal mol^{-1}!  I would very much appreciate
 any helpful comments or literature references which deals with any of
 the above issues I've raised here about AM1 and its applicability of
 open shell systems.
 Regards,
 Ryan Bettens,
 OSU Physics Department,
 BETTENS.,at,.MPS.OHIO-STATE.edu