Consistent Double Zeta Basis Set Research Articles

A Theoretical Study of <i>β</i>-Amino Acid Conformational Energies and Solvent Effect

The conformations of four β-amino acids in a model peptide environment were investigated using Hartree-Fock (HF) and density functional theory (DFT) methods in gas phase and with solvation. Initial structures were obtained by varying dihedral angles in increments of 45° in the range 0° - 360°. Stable geometries were optimized at both levels of theory with the correlation consistent double-zeta basis set with polarization functions (cc-pVDZ). The results suggest that solvation generally stabilizes the conformations relative to the gas phase and that intramolecular hydrogen bonding may play an important role in the stability of the conformations. The β3 structures, in which the R-group of the amino acid is located on the carbon atom next to the N-terminus, are somewhat more stable relative to each other than the β2 structures which have the R-group on the carbon next to the carbonyl.

Benchmark calculations with correlated molecular wave functions. III. Configuration interaction calculations on first row homonuclear diatomics

Using correlation consistent basis sets from double through quintuple zeta quality, potential energy functions have been calculated for the electronic ground states of the first row homonuclear diatomic molecules B2, C2, N2, O2, and F2 using single and double excitation configuration interaction (HF+1+2, GVB+1+2, and CAS+1+2) wave functions. Spectroscopic constants have been calculated for each species and compared to experiment. The dependence of the calculated spectroscopic constants on systematic extensions of the one-particle basis set are, in general, found to be very regular. By fitting the directly calculated values with a simple exponential function, accurate estimates of the complete basis set (CBS) limit for Ee, De, and re have been obtained for each level of theory. The estimated CBS limits are compared to the available experimental results, and the intrinsic errors associated with each theoretical method are discussed. In addition, the accuracy of the internally contracted CAS+1+2 method is compared to conventional uncontracted calculations using large basis sets. For B2, a full CI calculation have been carried out for De with the correlation consistent double zeta basis set and is compared to the CAS+1+2 method using both a supermolecule and separated atom approach for the dissociated limit.

Ab initio studies of cyclic water clusters (H2O)n, n=1–6. I. Optimal structures and vibrational spectra

The optimal structures and harmonic vibrational frequencies of cyclic water clusters, (H2O)n, have been determined at the Hartree–Fock (for n=2–6) and second order perturbation theory (for n=2–4) levels of theory with an augmented correlation consistent double zeta basis set. At the MP2 level this basis set yields very accurate results for the structure, dipole moment, and polarizability of the water monomer as well as results of comparable accuracy for the structure, binding energy, and harmonic vibrational frequencies of the water dimer. The optimal structure of (H2O)4 and the harmonic frequencies of (H2O)3,4 are the first ones reported at a correlated level for these species. Analysis of the structural trends reveals that the separation between neighboring oxygen atoms decreases exponentially with increasing cluster size. The predicted R0(O–O) for the ring hexamer is less than 0.02 Å shorter than the interoxygen separation in ice Ih. Furthermore, the trends in the harmonic vibrational frequencies suggest that, for large clusters, the intramolecular bends are blue shifted by ∼70 cm−1 with respect to the monomer frequency. The frequencies corresponding to the ‘‘free’’ OH stretches show little (≤50 cm−1) shift to the blue for n=2–6, whereas the ones corresponding to the ‘‘bridge’’ hydrogens are shifted to the red by ∼500 cm−1 with respect to the average of the harmonic stretching frequencies in water.