Basis Sets For Second-row Atoms Research Articles

Generation, contraction, and polarisation of Gaussian basis sets for atomic and molecular calculations using the generator coordinate method with polynomial discretisation: atoms from Na through Cl

The pGCHF basis sets for second-row atoms were generated using the CG method based on polynomial integral expansion to discretise the GWHF equations. These new basis sets can achieve competitive accuracy while describing atomisation energies.

Core-valence correlation consistent basis sets for second-row atoms (Al–Ar) revisited

The augmented tight-d cc-pV(n + d)Z (where n = D, T, Q, 5) basis sets are now the recommended “standard” correlation consistent basis set for second-row atoms. These revised sets, however, do not have a suitable corresponding core–valence basis set series to enable an assessment of core–valence corrections. This is particularly important when such effects are assessed and are used as an additive effect, as is done in composite methods. Thus, there is a need for a new “standard” core–valence series of basis sets for second-row atoms that builds systematically upon the cc-pV(n + d)Z sets. In this study, we develop the cc-pCV(n + d)Z basis set series and demonstrate their usefulness through molecular benchmark calculations for a series of second-row systems. These revised core–valence basis sets provide greater consistency in the description of core–valence effects with respect to change in basis set, enabling greater utility of the sets, even for the lower values of n.

An improved determination of the equilibrium structure and molecular properties of XBS and XCP (X = H, F, Cl) molecules from ab initio calculations

The availability of core-valence basis sets for second-row atoms together with highly accurate coupled-cluster calculations allow us to present an improved determination of the equilibrium structure of the XBS and XCP (X = H, F, Cl) molecules. Core correlation effects and basis set incompleteness have been taken into account in order to obtain best estimates of their equilibrium geometries: results which can be quantitatively compared to experiment are reported. In addition, two important molecular properties, namely the dipole moment and the nuclear quadrupole coupling constants, have been investigated. The molecular dipole moment has been calculated at coupled-cluster level using bases of different quality including diffuse functions and taking into account both core correlation effects and basis set incompleteness. The quadrupole coupling constants, evaluated from the electric field gradient at the quadrupolar nuclei, have been computed at the multi reference configuration interaction, Møller–Plesset perturbation to second order, and coupled-cluster levels of theory employing large core-valence basis sets.

Effects of Basis Set Choice upon the Atomization Energy of the Second-Row Compounds SO2, CCl, and ClO2 for B3LYP and B3PW91

The atomization energies of the species SO2, ClO2, and CCl have been calculated using the hybrid density functional approaches B3LYP and B3PW91. These functionals have been used in combination with the correlation-consistent basis sets cc-pVxZ and aug-cc-pVxZ (where x = D(2), T(3), Q(4), or 5). The impact of the newly revised correlation-consistent basis sets for second-row atoms (cc-pV(x+d)Z, aug-cc-pV(x+d)Z) upon the description of these systems has been assessed. Smooth convergence toward the Kohn−Sham limit is observed. The impact of the tight d function is particularly significant in the description of atomization energies for ClO2 and SO2 at the double-ζ and triple-ζ levels of basis sets, improving the energies by 13−17 and 9−10 kcal/mol, respectively.

Definitive ab initio studies of model SN2 reactions CH(3)X+F- (X=F, Cl, CN, OH, SH, NH(2), PH(2)).

The energetics of the stationary points of the gas-phase reactions CH(3)X+F(-)-->CH(3)F+X(-) (X=F, Cl, CN, OH, SH, NH(2) and PH(2)) have been definitively computed using focal point analyses. These analyses entailed extrapolation to the one-particle limit for the Hartree-Fock and MP2 energies using basis sets of up to aug-cc-pV5Z quality, inclusion of higher-order electron correlation [CCSD and CCSD(T)] with basis sets of aug-cc-pVTZ quality, and addition of auxiliary terms for core correlation and scalar relativistic effects. The final net activation barriers for the forward reactions are: E (b/F,F)=-0.8, E (b/F, Cl)=-12.2, E (b/F,OH)=+13.6, E b/F,OH=+16.1, E b/F,SH=+2.8, Eb/F, NH=+32.8, and E b/F,PH =+19.7 kcal x mol(-1). For the reverse reactions E b/F,F= -0.8, Eb/Cl,F =+18.3, E b/CN,F=+12.2, E b/OH,F =-1.8, E b/SH,F =+13.2, E b/NH(2),=-1.5, and E b/PH(2) =+9.6 kcal x mol(-1). The change in energetics between the CCSD(T)/aug-cc-pVTZ reference prediction and the final extrapolated focal point value is generally 0.5-1.0 kcal mol(-1). The inclusion of a tight d function in the basis sets for second-row atoms, that is, utilizing the aug-cc-pV(X+d)Z series, appears to change the relative energies by only 0.2 kcal x mol(-1). Additionally, several decomposition schemes have been utilized to partition the ion-molecule complexation energies, namely the Morokuma-Kitaura (MK), reduced variational space (RVS), and symmetry adapted perturbation theory (SAPT) techniques. The reactant complexes fall into two groups, mostly electrostatic complexes (FCH(3).F(-) and ClCH(3).F(-)), and those with substantial covalent character (NCCH(3).F(-), CH(3)OH.F(-), CH(3)SH.F(-), CH(3)NH(2).F(-) and CH(3)PH(2).F(-)). All of the product complexes are of the form FCH(3).X(-) and are primarily electrostatic.

Accurate Gaussian basis sets for second-row atoms and ions generated with the improved generator coordinate Hartree–Fock method

The improved generator coordinate Hartree–Fock (IGCHF) method is used to generate accurate Gaussian basis sets for the second-row neutral atoms, singly charged cations from Na + through Ar +, and stable singly charged anions from Na − through Cl −. For all second-row atoms and ions studied, the ground-state energies obtained with the triply-optimized Gaussian basis sets (generated with the IGCHF method) are always better than those obtained with the original generator coordinate Hartree–Fock method, and do not differ from the corresponding numerical Hartree–Fock results by more than 0.8 millihartree.

Small gaussian basis sets for second-row atoms

6s-type and 4p-type gaussian basis sets are obtained for the second row atoms by fitting, using a least squares criterion, to 12s-type and 9p-type gaussian basis sets which are close to the self-consistent field atomic orbital wave functions. The small gaussian expansions are considered to be more suited for molecular calculations using double basis sets. The differences between these sets and the 10s-type, 6p-type and 9s-type, 5p-type are analysed. For molecular calculations using single gaussian basis sets the 10s-type and 6p-type would seem to be the best compromise.