Double-zeta plus polarization geometries

Although there is no strict relationship between the basis sets developed for, and used in, conventional ah initio calculations and those applicable in DFT, the basis sets employed in molecular DFT calculations are usually the same or highly similar to those. For most practical purposes, a standard valence double-zeta plus polarization basis set (e.g. the Pople basis set 6-31G(d,p) [29] and similar) provides sufficiently accurate geometries and energetics when employed in combination with one of the more accurate functionals (B3LYP, PBEO, PW91). A somewhat sweeping statement is that the accuracy usually lies mid-way between that of M P2 and that of the CCSD(T) or G2 conventional wave-function methods. 

Calculation has been performed mainly at the HF level with the double-zeta plus polarization quality basis sets and ECP for Pd, P, Si, and Sn. The energetics for R = Me has been recalculated at the MPn (n = 2-4) level at the HF geometries. The overall reactions for various RCCH (R = CN, H, CH3, and OCH3) and H3SiSnH3 with the model catalyst Pd(PH3)2 have been studied. It has been shown that H3Si-SnH3 easily, with a few kilocalories per mole barrier, adds oxidatively to the catalyst Pd(PH3)2 and the resultant complex lies only 5.9 kcal/mol lower than the reactants. The next step, acetylene coordination to the catalyst, causes with the dissociation of... 

A relatively large basis set has to be used for a reasonable description of correlation effects. Minimal basis sets or split valence basis sets are not suitable for carrying out MP or other correlation corrected ab initio calculations. One needs at least a DZ + P(VDZ -I- P) or TZ + 2P basis set to get reasonable energies, geometries and first-order properties. For second-order properties, TZ -I- 2P or QZ + 3P basis sets are needed. (DZ -I- P = double-zeta plus polarization VDZ = valence DZ TZ = triple-zeta QZ = quadruple-zeta.)... 

Disrotatory, Woodward-Hofffnann rules, 360 Distance geometry methods, 343 Distributed Multipole Analysis (DMA), 222 Distribution functions in simulations, 378 Double Zeta plus Polarization (DZP) basis set, 153... 

The ab initio method used to obtain the potential surface employs many-body perturbation theory (MBPT) relative to an unrestricted Hartree-Fock (UHF) reference function. Correlation is treated to fourth order in MBPT and includes contributions from all single, double, and quadruple excitations. A Gaussian basis of double zeta plus polarization functions (DZP) is used. This model is computationally efficient and has been shown to provide accurate force fields and geometries and reliable thermochemical data for a wide variety of molecular systems.It has also been used to calculate reaction paths for some triatomic systems. 

The more recent 1992 Maslen et al. force field for benzene (153) was computed by evaluating analytic derivatives through quartic terms at the SCF level. The DZP basis set was used in these studies. The derivatives were evaluated at the equilibrium geometry determined at the MP2 (Moller-Plesset second-order pertubation theory) level using a larger basis set, TZ2P (triple-zeta plus double polarization). It is significant that a complete ab initio force field at the quartic level has been computed for benzene. 

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