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 correlated calculations of the indirect nuclear spin-spin coupling constants of HD, HF, H 2 O, CH 4 , C 2 H 2 , BH, AlH, CO and N 2 at the level of the second-order polarization propagator approximation (SOPPA) and the second-order polarization propagator approximation with coupled-cluster singles and doubles amplitudes ± SOPPA(CCSD). Attention is given to the eect of the so-called 4 term, which has not been included in previous SOPPA spin-spin coupling constant studies of these molecules. Large sets of Gaussian basis functions, optimized for the calculation of indirect nuclear spin-spin coupling constants, were used instead of the in general rather small basis sets used in previous studies. We ®nd that for nearly all couplings the SOPPA(CCSD) method performs better than SOPPA.

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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.

Relativistic four-component random phase approximation ͑RPA͒ calculations of indirect nuclear spin-spin coupling constants in MH 4 (MϭC, Si, Ge, Sn, Pb) and Pb͑CH 3 ͒ 3 H are presented. The need for tight s-functions also in relativistic four-component calculations is verified and explained, and the effect of omission of ͑SS-LL͒ and ͑SS-SS͒ two-electron integrals is investigated. Already in GeH 4 we see a relativistic increase in the coupling constant by 12%, and for PbH 4 the effect is a 156% increase for the one-bond coupling. Large relativistic effects are also computed for the two-bonds couplings. We find that the relativistic effects on the one-bond couplings are mainly due to scalar relativistic factors rather than spin-orbit corrections.

The use of perturbation-dependent London atomic orbitals, also called gauge including atomic orbitals, has proven efficient for calculations of NMR shielding constants and other magnetic properties in the nonrelativistic framework. In this paper, the theory of London atomic orbitals for NMR shieldings is extended to the four-component relativistic framework and our implementation is described. The relevance of London atomic orbitals in four-component calculations as well as computational aspects are illustrated with test calculations on hydrogen iodide. We find that the use of London atomic orbitals is an efficient method for reliable calculations of NMR shielding constants with standard basis sets, also for four-component calculations with spin-orbit coupling effects included in the wave function optimization. Furthermore, we find that it is important that the small component basis functions fulfill the magnetic balance for accurate description of the diamagnetic shielding and that the role of London atomic orbitals in the relativistic domain is to provide atomic magnetic balance even in the molecular case, thus greatly improving basis set convergence. The Sternheim approximation, which calculates the diamagnetic contribution as an expectation value, leads to significant errors and is not recommended.

Two approximations to the normalized elimination of the small component are presented which enable the work of a relativistic calculation to be substantially reduced. The first involves fixing the ratio of the large and small components in atomic calculations, which corresponds to a basis set expansion in terms of positive energy atomic 4-spinors. The second involves the definition of a local, i.e., center-dependent, fine structure constant, which has the effect of making atoms with α=0 nonrelativistic. A series of test calculations on a variety of molecules and properties indicates that the errors incurred in the first approximation are negligible. In the second approximation, the errors are dependent on the property, the chemical environment and the atomic number. For the second period elements the errors in the approximation are for chemical purposes negligible. In the third period this is true for many properties, but for some, such as ligand-metal binding energies, there are discrepancies which may be a cause for concern in more accurate calculations. Beyond the third period it is usually necessary to treat atoms relativistically.

ABSTRACT:Various methods for the inclusion of relativistic effects in the calculation of NMR parameters are discussed. Benchmark values for the NMR shieldings and indirect nuclear spin᎐spin coupling tensors for the hydrogen halides are calculated using the four-component relativistic random phase approximation method. Apart from recovering the well-known trend of increasing hydrogen isotropic shielding going from HF to HI, we also find a large effect on the anisotropy that decreases along this series. Inclusion of spin-orbit coupling in a nonrelativistic formalism suffices to recover both effects on the hydrogen shieldings but fails to reproduce the much larger effect on the halogen shieldings. This effect can be explained by considering the relativistic mass-velocity operator that contains correction terms to the nonrelativistic magnetic field operators. We recommend routine inclusion of the one-electron spin-orbit correction in calculations of hydrogen shieldings for hydrogens bonded to heavy atoms. For the heavy nucleus shielding one should include an additional *Permanent address: