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Is tetrahedral H 4 2+ a minimum? Anomalous behavior of popular basis sets with the standard p exponents on hydrogen
Author(s) -
Glukhovtsev Mikhail N.,
Schleyer Paul Von Ragué,
van Eikema Hommes Nicolaas J.R.,
De M. Carneiro Jose Walkimar,
Koch Wolfram
Publication year - 1993
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/jcc.540140305
Subject(s) - basis set , exponent , saddle point , degenerate energy levels , chemistry , hydrogen atom , tetrahedron , physics , atomic physics , crystallography , combinatorics , computational chemistry , mathematics , quantum mechanics , geometry , density functional theory , philosophy , linguistics , group (periodic table)
The nature of the tetrahedral H 4 2+ stationary point (minimum or triply degenerate saddle) depends remarkably upon the theoretical level employed. Harmonic vibrational analyses with, e.g., the 6‐31G** (and 6‐31 + +G**) and Dunning's [4 s 2 p 1 d ;2 s 1 p ] [D95( d , p )] basis sets using the standard p exponent suggest (erroneously) that the T d geometry is a minimum at both the HF and MP2 levels. This is not the case at definitive higher levels. The C 3 H 4 2+ structure with an apical H is another example of the failure of the calculations with the 6‐31G**, 6‐311G**, and D95( d , p ) basis sets. Even at MP2/6‐31G** and MP2/ cc‐ p VDZ levels, the C 3 v 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 D 4 h methane dication structure is also an example of the misleading performance of the 6‐31G** basis set. In all these cases, energy‐optimized hydrogen p 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 ,2 p ) basis set]. Not all cases are critical. A survey of examples, also including some boron compounds, provides calibration. © 1993 John Wiley & Sons, Inc.