Split-valence plus polarization basis

IG and 6-3IG. These are commonly used split-valence plus polarization basis sets. These basis sets contain inner-shell functions, written as a linear combination of six Gaussians, and two valence shells, represented by three and one Gaussian primitives, respectively (noted as 6-3IG). When a set of six d-type Gaussian primitives is added to each heavy atom and a single set of Gaussian p-type functions to each hydrogen atom, this is noted as and... 

A DZP or split-valence plus polarization basis is a minimal basis for correlated calculations. 

A variation of G2 theory that uses reduced orders of Moller-Plesset perturbation theory in combination with a smaller basis set for the quadratic configuration correction is G2(MP2,SVP) theory.77 78 The SVP refers to the split-valence plus polarization basis, 6-31G(d), used in this QCISD(T) correction. In this theory the final energy is given by... 

Concerning the VEE, we have chosen, as standard references for comparison, the EOM-CCSD results from Stanton et al. [56] and from Comeau and Bartlett [57]. Results from other very efficient state-of-the-art methods such as Exponentially Generated Cl or the now widely used CASPT2 could also have been used. In general, these are methods from which mean errors in VEE less than 0.2 eV might be expected provided that the basis set used is at least of split-valence plus polarization quality and it is augmented in a well-conditioned way to account for Rydberg states. 

Optimization of the basis set for atoms inevitably introduces some bias towards the atomic description, although this effect is seldom noticeable in basis sets of the traditional types described in Sec. 4.2. Bias towards the atoms can appear, however, if very small ANO sets (or, presumably, cc sets) are used. An ANO set contracted to split-valence plus polarization, [3s 2p Id], is probably too contracted to provide a good description of molecular binding. In particular, the d NO may be quite different in shape from what is required to describe polarization and correlation in the molecule. At least two d orbitals should be included to properly allow for these effects. Hence ANO or cc basis sets smaller than, say, [4s 3p 2d] should probably not be used for molecular calculations. 

The protonation of thiirane and thiirene was studied systematically by using basis sets ranging from minimal to split valence plus polarization quality. The 6-31G optimized geometries for... 

SVP Split valence plus polarization. Synonym for a (-+) DZP basis set, where only the valence AOs are split into inner and outer functions, such as the (- ) cc-pVDZ basis set of Dunning or the (->) 6-31G basis set of Pople. 

Abbreviations used t(0)dzp - basis set is double-zeta polarization on all atoms except oxygen for which it is valence triple-zeta svp - basis set is split-valence plus polarization. B3LYP, BP86, and PW are all gradient-corrected density functionals. See the original papers for references. 

For most molecules studied, modest Hartree-Fock calculations yield remarkably accurate barriers that allow confident prediction of the lowest energy conformer in the S0 and D0 states. The simplest level of theory that predicts barriers in good agreement with experiment is HF/6-31G for the closed-shell S0 state (Hartree-Fock theory) and UHF/6-31G for the open-shell D0 state (unrestricted Hartree-Fock theory). The 6-31G basis set has double-zeta quality, with split valence plus d-type polarization on heavy atoms. This is quite modest by current standards. Nevertheless, such calculations reproduce experimental barrier heights within 10%. 

According to Davidson and Feller (1986) small split valence (e.g. 3-21G (...)) or split-valence plus polarisation (e.g. 6-31G ") basis sets perform adequately in SCF and correlated geometry optimizations of closed shell organic compounds, but (.. . ) very extended sets with flexible valence spaces and several sets of polarization functions are needed for energy differences accurate to 2-5 kcal mol (...) . Therefore in this work we report the RHF results of complete geometry optimisations of three systems containing a-hydroxy - amide/carboxylic moiety at 6-3IG, ... 

The ab initio search for adequate basis functions, after a decade, showed that the failure of the atomic orbital basis comes from the change in size and shape of the atomic orbitals in the high electrostatic fields of the other atoms. Use of a double-zeta plus polarization basis, such as the Dunning-Hay split-valence or one of the Pople x-yzG bases, allows flexibility for the atomic orbitals to adjust to the molecular environment. 

A speculative proposal was made thirty years ago by Schmid and Krenmayr77, namely that a nitrosyl ion solvated, but not covalently bonded, by a water molecule may be involved in these systems. This hypothesis was investigated theoretically in 1984 by Nguyen and Hegarty78 who carried out ab initio SCF calculations of structure and properties employing the minimal STO-3G basis set, a split-valence basis set plus polarization functions. Optimized geometries of six planar and two nonplanar forms were studied for the nitrosoacidium ion. The lowest minimum of molecular electrostatic potential... 

However, especially for core AOs, where an exponential function has a very sharp cusp at the nucleus, several Gaussian functions are required even to begin to represent an exponential function accurately. For example, a split-valence basis set, developed by John Pople s group and widely employed in current calculations, uses six Gaussian functions to represent the H core orbitals on first row atoms whereas, a set of only three Gaussians, plus one more independent Gaussian, is used to represent the valence 2s and 2p AOs. With 3d functions included, to serve as polarization functions for the valence orbitals on the heavy atoms, Pople denotes this Gaussian basis set as 6-31G(d) or, more commonly, 6-31G. " ... 

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