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.

We present a new implementation of the second-order polarization propagator approximation (SOPPA) using a direct linear transformation approach, in which the SOPPA equations are solved iteratively. This approach has two important advantages over its predecessors. First, the direct linear transformation allows for more efficient calculations for large two particle–two hole excitation manifolds. Second, the operation count for SOPPA is lowered by one order, to N5. As an application of the new implementation, we calculate the excitation energies and oscillator strengths of the lowest singlet and triplet transitions for benzene and naphthalene. The results compare well with experiment and CASPT2 values, calculated with identical basis sets and molecular geometries. This indicates that SOPPA can provide reliable values for excitation energies and response properties for relatively large molecular systems.

Various approximate methods for the calculation of vibrational second hyperpolarizabilities are assessed on the basis of results for CH4, NH3, H2O, HF, and CO2. Three specific variations of the perturbation-theoretic approach are analyzed. These are defined by the types of terms retained in expansions which involve electrical and mechanical anharmonicities. The lowest-order approximation being the double-harmonic one and the next higher one the relaxation method. For the dynamic hyperpolarizabilities, the replacement of the optical frequencies (ω) by ω→∞ (the infinite-frequency approximation) is also explored. On the evidence at hand it is concluded that the relaxation/infinite-frequency method is a satisfactory approximation for the nonlinear optical vibrational hyperpolarizabilities.

Molecular static and dynamic polarizabilities for thirteen small molecules have been calculated using four “black box” ab initio methods, the random phase approximation, RPA, the second-order polarization propagator approximation, SOPPA, the second-order polarization propagator approximation with coupled cluster singles and doubles amplitudes, SOPPA(CCSD), and the coupled cluster singles and doubles linear response function method, CCSDLR. The frequency dependence of the polarizabilities is given in terms of the dipole oscillator strength sum rules or Cauchy moments S(−4) and S(−6). Two basis sets were employed, Sadlej's medium size polarized basis set and Dunning's correlation consistent basis set of triple-ζ quality augmented by two diffuse functions of each angular momentum (daug-cc-pVTZ). The results are compared to other theoretical results as well as to experimental values for the static polarizabilities, polarizability anisotropies, and Cauchy moments. Frequency-dependent polarizabilities and polarizability anisotropies, calculated at the CCSDLR level using the daug-cc-pVTZ basis set, are presented for five typical laser frequencies.

We have calculated static and dynamic longitudinal polarizabilities, ␣ zz (Ϫ;), for the polyyne series, C 2n H 2 , using both the uncorrelated random phase approximation and the correlated second-order polarization propagator approximation. The calculated polarizabilities are extrapolated to the value for the infinitely long chain using seven different extrapolation techniques. We employ both conventional schemes, such as the fitting of simple polynomials, as well as new schemes, such as the fitting of a Padé approximant, or purely mathematically motivated nonlinear sequence transformations which have not previously been used in connection with this property. For the direct fits, where the number of parameters is the same as the number of points, we find that the most stable and reliable extrapolation schemes are to be found among the latter.