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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 comprehensive measurements of the C (carbon) K edge near-edge X-ray absorption (NEXAFS) spectra of all 20 amino acids commonly occurring in nature. Qualitative trends among the spectra of amino acids with similar chemical character are identified and spectral features are compared with extensive ab initio calculations. The contributions of individual units and substitutional groups have been determined to explore their fingerprinting character using the building block concept. Several such units are found. Two that give particularly clear features in the C 1s NEXAFS spectra are the carboxyl group (which can be clearly identified by a pronounced structure due to the C 1s→π*C  O transition with maximum at 288.65(5) eV) and modified phenol rings in aromatic amino acids (which give sharp C 1s→π*C  C structures). The latter transitions are located around 285 eV, and their shape is specific for each aromatic amino acid. Other building blocks, such as the CNH n group and the CH, CC, CO, CN pair bonds, are also identified, although their characteristic features are less pronounced in the C K edge spectra than the carboxylic and aromatic structures. This study provides the basis for rigorous assignment of the NEXAFS spectra of the amino acids, and will be helpful in developing X-ray absorption spectroscopy for quantitative analysis of proteins.

We present density-functional theory for linear and nonlinear response functions using an explicit exponential parametrization of the density operator. The response functions are derived using two alternative variation principles, namely, the Ehrenfest principle and the quasienergy principle, giving different but numerically equivalent formulas. We present, for the first time, calculations of dynamical hyperpolarizabilities for hybrid functionals including exchange-correlation functionals at the general gradient-approximation level and fractional exact Hartree–Fock exchange. Sample calculations are presented of the first hyperpolarizability of the para-nitroaniline molecule and of a porphyrin derived push–pull molecule, showing good agreement with available experimental data.

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