Peter R. Taylor

The Dalton quantum chemistry program system

Dalton is a powerful general‐purpose program system for the study of molecular electronic structure at the Hartree–Fock, Kohn–Sham, multiconfigurational self‐consistent‐field, Møller–Plesset, configuration‐interaction, and coupled‐cluster levels of theory. Apart from the total energy, a wide variety of molecular properties may be calculated using these electronic‐structure models. Molecular gradients and Hessians are available for geometry optimizations, molecular dynamics, and vibrational studies, whereas magnetic resonance and optical activity can be studied in a gauge‐origin‐invariant manner. Frequency‐dependent molecular properties can be calculated using linear, quadratic, and cubic response theory. A large number of singlet and triplet perturbation operators are available for the study of one‐, two‐, and three‐photon processes. Environmental effects may be included using various dielectric‐medium and quantum‐mechanics/molecular‐mechanics models. Large molecules may be studied using linear‐scaling and massively parallel algorithms. Dalton is distributed at no cost from http://www.daltonprogram.org for a number of UNIX platforms.

THE ANHARMONIC FORCE FIELD OF ETHYLENE, C2H4, BY MEANS OF ACCURATE AB INITIO CALCULATIONS

UNIV INSTELLING ANTWERP,INST MAT SCI,DEPT CHEM,B-2610 WILRIJK,BELGIUM. NASA,AMES RES CTR,MOFFETT FIELD,CA 94035. SAN DIEGO SUPERCOMP CTR,SAN DIEGO,CA 92186.MARTIN, JML, LIMBURGS UNIV CENTRUM,DEPT SBG,UNIV CAMPUS,B-3590 DIEPENBEEK,BELGIUM.

An Accurate ab initio Quartic Force Field and Vibrational Frequencies for CH4 and Isotopomers

A very accurate ab initio quartic force field for CH4 and its isotopomers is presented. The quartic force field was determined with the singles and doubles coupled‐cluster procedure that includes a quasiperturbative estimate of the effects of connected triple excitations, CCSD(T), using the correlation consistent polarized valence triple zeta, cc‐pVTZ, basis set. Improved quadratic force constants were evaluated with the correlation consistent polarized valence quadruple zeta, cc‐pVQZ, basis set. Fundamental vibrational frequencies are determined using second‐order perturbation theory anharmonic analyses. All fundamentals of CH4 and isotopomers for which accurate experimental values exist and for which there is not a large Fermi resonance, are predicted to within ±6 cm−1. It is thus concluded that our predictions for the harmonic frequencies and the anharmonic constants are the most accurate estimates available. It is also shown that using cubic and quartic force constants determined with the correlation ...

Basis set convergence for geometry and harmonic frequencies. Are h functions enough

The geometries and harmonic frequencies of H2O and HF have been computed using a systematic sequence of ‘correlation consistent’ basis sets. Even basis sets of spdfgh quality fail to reproduce the ωe of HF unless they are augmented with special anion functions. CCSD(T) calculations with an ‘augmented’ spdfg basis set are in close agreement with experiment; addition of h functions does not substantially affect results, indicating convergence with respect to angular momentum. Core correlation increases stretching frequencies by 4–7 cm−1; accounting for residual inaccuracies in the correlation treatment and contraction error lowers them by about 6 and 3 cm−1, respectively. The remaining small errors are due to relativistic and nonadiabatic effects, in that order of importance. Calculations on BH indicate that, as expected, the augmenting functions can safely be omitted on elements that are not highly electronegative.

The geometry, vibrational frequencies, and total atomization energy of ethylene. A calibration study

Abstract The anharmonic part of a recently calculated ab initio quartic force field for ethylene has been combined with geometries and harmonic frequencies at higher levels of theory, including expansion to spdfg basis sets and inclusion of core correlation. Resulting fundamentals and ground-state rotational constants have been compared with experiment. Our best estimate for the re geometry is r e ( CC ) = 1.3307 (3) A , r e ( CH ) = 1.0809(3) A , θ e ( CCH ) = 121.44(3)° , whic reproduces the experimental rotational constants to 0.01%. The experimental fundamentals and main resonance partners are calculated with a mean absolute error of 2.3 cm−1. Our best calculated total atomization energy, 531.7(5) kcal/mol, falls within the error bar of the experimental value 531.9(3) kcal/mol.

OH + H2 → H2O + H. The importance of ‘exact exchange’ in density functional theory

Abstract The potential energy surface of the title reaction has been studied at Hue5f8O … Hue5f8H distances between 1.0 and 2.1 A along a C s reaction path with a variety of density functionals, both local and nonlocal, and compared to potential energy curves obtained at the Hartree-Fock, MP2 and CCSD(T) levels. Whereas the latter curves are repulsive from 2.1–1.4 A with energies above that of the reactants, nearly all the DFT curves are either essentially flat or attractive with energies below the reactants, giving an entirely incorrect picture of this important radical reaction. Only the ACM functional — which includes part of the exact Hartree-Fock exchange — gives a definite nonzero barrier.

C20: Fullerene, Bowl or Ring? New Results from Coupled-Cluster Calculations

Abstract Contrary to recent experimental evidence suggesting that the monocyclic ring is the most stable 20-atom carbon species, highly accurate calculations convincingly predict that the smallest fullerene, dodecahedral C20, has the lowest energy. A related corannulene-like bowl is nearly degenerate in energy to the fullerene. Thermodynamic considerations suggest that at formation temperatures of around 700 K the bowl should be the dominant species.

Accurate ab initio quartic force fields for the N2O and CO2 molecules

Abstract The quartic force fields of N 2 O and CO 2 have been computed ab initio using large basis sets of spdf and spdfg quality and augmented coupled-cluster (CCSD(T)) methods. The CCSD(T)/spdf frequencies for N 2 O, and the CCSD(T)/spdfg ones for CO 2 , are in excellent agreement with experiment. g functions appear to be important for cumulenic double bond stretches and bends. Improving the basis from spdf to spdfg appears to shorten double bonds by ≈0.003 A; the effect on single bonds is smaller and too unsystematic to be quantified. SCF level anharmonicities for N 2 O are qualitatively incorrect: this is not the case for CO 2 where reasonable agreement with experiment is reached even at this level. Of the various published experimental force fields for N 2 O and CO 2 , the ones obtained by algebraic contact transformation appear to be the most reliable.

Ab initio study of the spectroscopy and thermochemistry of the C2N and CN2 molecules

Abstract Several structures and electronic states of the C 2 N and CN 2 molecules have been studied using complete active space SCF (CASSCF), multireference configuration interaction (MRCI), and coupled cluster (CCSD(T)) methods. Both molecules are very stable. Our best computed total atomization energies Σ D e are 288.6±2 kcal/mol for CN 2 , and 294.1±2 kcal/mol for C 2 N. The CNC and CCN structures for C 2 N are nearly isoenergetic. CNN( 3 Π) lies about 30 kcal/mol above NCN( 3 Π g ), but has a high barrier towards interconversion and is therefore observed experimentally. Computed harmonic frequencies for CNN are sensitive to the correlation treatment: they are reproduced well using multireference methods as well as the CCSD(T) method. High spin contamination has a detrimental effect on computed harmonic frequencies at the CCSD(T) level.

Accurate ab initio quartic force fields for the ions HCO+ and HOC+

The quartic force fields of HCO+ and HOC+ have been computed using augmented coupled cluster methods and basis sets of spdf and spdfg quality. Calculations on HCN, CO, and N2 have been performed to assist in calibrating the computed results. Going from an spdf to an spdfg basis shortens triple bonds by about 0.004 A, and increases the corresponding harmonic frequency by 10–20 cm−1, leaving bond distances about 0.003 A too long and triple bond stretching frequencies about 5 cm−1 too low. Accurate estimates for the bond distances, fundamental frequencies, and thermochemical quantities are given. HOC+ lies 37.8±0.5 kcal/mol (0 K) above HCO+; the classical barrier height for proton exchange is 76.7±1.0 kcal/mol.