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.

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We have investigated different models for parameterizing the frequency-dependent molecular polarizability. The parameterization is based on an electrostatic model for interacting atoms and includes atomic polarizabilities, atom-type parameters describing the damping of the electric fields and the frequency dependence. One set of parameters has been used for each element. The investigation has been carried out for 115 molecules with the elements H, C, N, O, F, and Cl, for which the frequency-dependent polarizability tensor has been calculated with ab initio methods. We find that the static polarizability of aliphatic and aromatic compounds can be described with the same set of parameters. The conclusion is that a simple electrostatic model to a good degree can model the essential behavior of the frequency-dependent molecular polarizability.

We present an extension of the molecular response method of solvated compounds. The extension treats in a proper fashion the effects of nonequilibrium solvation on the molecular properties of the solvated compound. The molecular properties evaluated within the response formalism are properties that are measured using high-frequency electromagnetic fields. We consider the changes in the molecular properties due to nonequilibrium solvation. The nonequilibrium solvation arises from changes in the molecular charge distribution induced by the high-frequency perturbation. The optical polarization vector is at all times assumed to be in equilibrium with the charge distribution of the solute, contrary to the inertial polarization, which is not necessarily in equilibrium with the charge distribution of the solute. We calculate shifts of excitation energies and frequency-dependent polarizabilities as a function of the optical and static dielectric constants.

A dipole interaction model (IM) for calculating the molecular second hyperpolarizability, γ, of aliphatic and aromatic molecules has been investigated. The model has been parametrized from quantum chemical calculations of γ at the self-consistent field (SCF) level of theory for 72 molecules. The model consists of three parameters for each element p:  an atomic polarizability, an atomic second hyperpolarizability, and an atomic parameter, Φp, describing the width of the atomic charge distribution. The Φp parameters are used for modeling the damping of the interatomic interactions. Parameters for elements H, C, N, O, F, and Cl were determined, and typical differences between the molecular γ derived from quantum chemical calculations and from the IM are below 30% and on average around 10%. As a preliminary test, the dipole interaction model was applied to the following molecular systems not included in the training set:  the urea molecule, linear chains of urea molecules, and C60. For these molecules deviations of the IM result for the molecular γ from the corresponding SCF value were at most around 30% for the individual components, which in all cases is a better performance than obtained with semiempirical methods.