ISSN:
0192-8651
Keywords:
ab initio
;
anharmonicity
;
force field
;
Morse potential
;
vibrational frequency
;
Chemistry
;
Theoretical, Physical and Computational Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Computer Science
Notes:
Four alternatives are compared for estimating vibrational anharmonicity constants without explicitly calculating the expensive fourth derivatives of the potential curves. In the first, semiempirical approach, fourth derivatives for 53 diatomic molecules are estimated from ab initio second and third derivatives by using the Morse model potential. Vibrational anharmonicities ωexe are then computed from the third and fourth derivatives. The second approach invokes a purely empirical linear correlation between ωexe and the harmonic frequencies ωe. The third and fourth empirical approaches suppose that the effective harmonic and anharmonic force constants are proportional (with an additive constant in the fourth approach). Experimental values for ωexe are compared with empirical predictions and with semiempirical estimates based upon Hartree-Fock (HF), Møller-Plesset (MP2), and local, nonlocal, and hybrid density-functional theories (DFT), using the small 6-31G* basis set. Ab initio values of ωe and bond lengths re are also compared against experiment. The (U)MP2 results are the worst and include several anomalies. The other semiempirical methods yield results of comparable accuracy for ωexe of hydrides, although the DFT methods are markedly better for ωe and re and for ωexe of nonhydrides. The empirical estimates are nearly as good as the semiempirical ones. We conclude that: (1) both empirical and semiempirical approximations are useful for predicting stretching anharmonicity constants ωexe to precisions of σ≈5 cm-1 for hydrides and σ≈1.5 cm-1 for nonhydrides; and (2) MP2 theory is relatively unreliable for such calculations. In addition, we find the following tests to be useful when evaluating the reliability of vibrational constants calculated at the UMP2 level: (a) the calculated values of ωe and ωexe should not deviate substantially from the empirical relations; (b) harmonic frequencies and intensities calculated at the MP2 level should be smaller than those calculated at the corresponding HF level; (c) a large distance-dependence of the spin contamination, d〈S2〉/dR≳0.05 Å-1, suggests that calculated constants are too large. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1315-1324, 1998
Additional Material:
6 Tab.
Type of Medium:
Electronic Resource
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