The calculation of out-of-plane bending vibrations for π-bonded systems appears to be extraordinarily sensitive to the choice of a one-particle basis set. Ab initio predictions are reported for acetylene, an extreme example, at the self-consistent field (SCF), singles and doubles configuration interaction (CISD), nth order Mo/ller–Plesset perturbation theory (MPn,n=2–4), coupled-pair functional (CPF), and singles and doubles coupled cluster (CCSD) levels of theory. It is found that the addition of a set of f basis functions to the carbon atom changes the value of the SCF πg frequency by +45 cm−1, and the value of all correlated πg frequencies by more than +100 cm−1. Evidence is presented that this behavior is present in other π-bonded systems. It is concluded that basis sets consisting of triple zeta plus two sets of polarization functions plus one set of f functions (TZ2P+f ) can predict highly accurate (∼1% average error) harmonic frequencies with the MP2, CPF, and CCSD methods, for a large number of molecules.
High-resolution X-ray pair distribution functions for
molten and
glassy TeO2 reveal coordination numbers n
TeO ≈ 4. However, distinct from the known α-,
β-, and γ-TeO2 polymorphs, there is considerable
short-range disorder such that no clear cutoff distance between bonded
and nonbonded interactions exists. We suggest that this is similar
to disorder in δ-TeO2 and arises from a broad distribution
of asymmetric Te–O–Te bridges, something that we observe
becomes increasingly asymmetric with increasing liquid temperature.
Such behavior is qualitatively consistent with existing interpretations
of Raman scattering spectra, and equivalent to temperature-induced
coordination number reduction, for sufficiently large cutoff radii.
Therefore, TeO2 contains a distribution of local environments
that are, furthermore, temperature dependent, making it distinct from
the canonical single-oxide glass formers. Our results are in good
agreement with high-level ab initio cluster calculations.
Equilibrium geometries, harmonic vibrational frequencies, and infrared intensities are calculated analytically at the second-order Moller-Plesset level (MP2) with a DZP basis for the five-membered heterocyclic aromatics furan, pyrrole, and thiophene. The results are of an accuracy to show up misassignments in the original experimental interpretation of the spectra. They also give confidence that ab initio calculations including electron correlation and using flexible basis sets can describe accurately the quadratic part of the potential energy surface. For such systems, these ab initio studies will aid the spectroscopic determination of force constants.
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