Is tetrahedral H42+ a minimum? Anomalous behavior of popular basis sets with the standard p exponents on hydrogen
✍ Scribed by Mikhail N. Glukhovtsev; Paul Von Ragué Schleyer; Nicolaas J.R. van Eikema Hommes; Jose Walkimar De M. Carneiro; Wolfram Koch
- Publisher
- John Wiley and Sons
- Year
- 1993
- Tongue
- English
- Weight
- 829 KB
- Volume
- 14
- Category
- Article
- ISSN
- 0192-8651
No coin nor oath required. For personal study only.
✦ Synopsis
The nature of the tetrahedral H42+ stationary point (minimum or triply degenerate saddle) depends remarkably upon the theoretical level employed. Harmonic vibrational analyses with, e.g., the &31G** (and 6-31 + + G**) and Dunning's [4sZpld;2slp] [D95(dp)] basis sets using the standard p exponent suggest (erroneously!) that the Td geometry is a minimum at both the HF and MP2 levels. This is not the case at definitive higher levels. The C3H42+ structure with an apical H is another example of the failure of the calculations with the 6-31G**, 6-311G**, and D95(dp) basis sets. Even at MP2/6-31G** and MP2/cc-pVDZ levels, the C3, structure has no negative eigenvalues of the Hessian. Actually, this form is a second-order saddle point as shown by the MP2/ 6-31G** calculation with the optimized exponent. The D4,, methane dication structure is also an example of the misleading performance of the 6-31G** basis set. In all these cases, energy-optimized hydrogenp exponents give the correct results, i.e., those found with more extended treatments. Optimized values of the hydrogen polarization function exponents eliminate these defects in 6-31G** calculations. Species with higher coordinate hydrogens may also be calculated reliably by using more than one set of p functions on hydrogen [e.g., the 6-31G(d,Zp) basis set]. Not all cases are critical. A survey of examples, also including some boron compounds, provides calibration.