Kenneth Ruud

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.

Recent advances in wave function-based methods of molecular-property calculations.

Recent Advances in Wave Function-Based Methods of Molecular-Property Calculations Trygve Helgaker,* Sonia Coriani, Poul Jørgensen, Kasper Kristensen, Jeppe Olsen, and Kenneth Ruud Centre for Theoretical and Computational Chemistry, Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway Dipartimento di Scienze Chimiche e Farmaceutiche, Universit a degli Studi di Trieste, Via Giorgieri 1, I-34127 Trieste, Italy Lundbeck Center for Theoretical Chemistry, Department of Chemistry, Aarhus University, 8000 Aarhus C, Denmark Centre for Theoretical and Computational Chemistry, Department of Chemistry, University of Tromsø, N-9037 Tromsø, Norway

Multiconfigurational self‐consistent field calculations of nuclear shieldings using London atomic orbitals

Nuclear shielding calculations are presented for multiconfigurational self‐consistent field wave functions using London atomic orbitals (gauge invariant atomic orbitals). Calculations of nuclear shieldings for eight molecules (H2O, H2S, CH4, N2, CO, HF, F2, and SO2) are presented and compared to corresponding individual gauges for localized orbitals (IGLO) results. The London results show better basis set convergence than IGLO, especially for heavier atoms. It is shown that the choice of active space is crucial for determination of accurate nuclear shielding constants.

Perturbation‐dependent atomic orbitals for the calculation of spin‐rotation constants and rotational g tensors

Spin‐rotation constants and rotational g tensors can be evaluated as second derivatives of the energy with respect to the rotational angular momentum and nuclear spin or angular momentum and external magnetic field, respectively. To overcome problems with the slow basis set convergence and the unphysical (gauge‐)origin dependence in quantum chemical calculations of these two properties, we suggest the use of perturbation dependent atomic orbitals (rotational London orbitals), which depend explicitly on the angular momentum and the external magnetic field and are a generalization of the conventional London orbitals (also known as gauge‐including atomic orbitals). It is shown that calculations of spin‐rotation constants and rotational g tensors based on rotational London orbitals are closely related to London‐orbital computations of nuclear shieldings and magnetizabilities. Test calculations at the Hartree–Fock self‐consistent‐field level for HF, N2, CO, and CH2O demonstrate the superior convergence to the ...

An efficient approach for calculating vibrational wave functions and zero-point vibrational corrections to molecular properties of polyatomic molecules

We have recently presented a formalism for calculating zero-point vibrational corrections to molecular properties of polyatomic molecules in which the contribution to the zero-point vibrational correction from the anharmonicity of the potential is included in the calculations by performing a perturbation expansion of the vibrational wave function around an effective geometry. In this paper we describe an implementation of this approach, focusing on computational aspects such as the definition of normal coordinates at a nonequilibrium geometry and the use of the Eckart frame in order to obtain accurate nonisotropic molecular properties. The formalism allows for a black-box evaluation of zero-point vibrational corrections, completed in two successive steps, requiring a total of two molecular Hessians, 6K–11 molecular gradients, and 6K–11 property evaluations, K being the number of atoms. We apply the approach to the study of a number of electric and magnetic properties—the dipole and quadrupole moments, the...

Hartree–Fock limit magnetizabilities from London orbitals

Molecular magnetizabilities have been calculated at the Hartree–Fock level for a series of diamagnetic molecules: H2O, NH3, CH4, PH3, H2S, CO2, CSO, CS2, and C3H4. Gauge invariance is imposed by the use of London atomic orbitals. The results are compared to those obtained with the IGLO (individual gauge for localized orbitals) method and are found to converge faster to the basis set limit. Magnetizabilities obtained from basis sets of different quality never differ by more than 4% for the London method, compared to 20% for IGLO. The Hartree–Fock limit may be approached using London basis sets of moderate size, in contrast to calculations of molecular polarizabilities which require large basis sets to be reliable. Comparison with experiment shows that the Hartree–Fock method overestimates experimental susceptibilities by 5%–10%.

Gauge‐origin independent multiconfigurational self‐consistent‐field theory for vibrational circular dichroism

Multiconfigurational self‐consistent‐field (MCSCF) theory is presented for the gauge‐origin independent calculation of vibrational circular dichroism. Origin independence is attained by the use of London atomic orbitals (LAO). MCSCF calculations on ammonia and its isotopomer NHDT demonstrate that atomic axial tensors and vibrational rotational strengths converge fast with the size of the basis set when LAOs are used. The correlation effects are significant both for the atomic tensors and the vibrational rotational strengths even for the single configuration dominated NHDT molecule.

Vibrational Raman optical activity calculations using London atomic orbitals

Ab initio calculations of Raman differential intensities are presented at the self-consistent field (SCF) level of theory. The electric dipole–electric dipole, electric dipole–magnetic dipole and electric dipole–electric quadrupole polarizability tensors are calculated at the frequency of the incident light, using SCF linear response theory. London atomic orbitals are employed, imposing gauge origin invariance on the calculations. Calculations have been carried out in the harmonic approximation for CFHDT and methyloxirane.

Optical rotation studied by density-functional and coupled-cluster methods

We describe the implementation of a gauge-origin independent, time-dependent linear-response formalism for the calculation of optical rotation using London atomic orbitals and density-functional theory. We test the accuracy of density-functional methods for studying optical rotation on difficult systems by modeling the optical rotation as a function of the dihedral angle. We also report the first linear response coupled-cluster singles-and-doubles results of optical rotation. The B3LYP functional gives reliable results for the optical rotation, even for molecules with nearly degenerate excited electronic states of opposite polarization.

Vibrational corrections to indirect nuclear spin-spin coupling constants calculated by density-functional theory

At the present level of electronic-structure theory, the differences between calculated and experimental indirect nuclear spin–spin coupling constants are typically as large as the vibrational contributions to these constants. For a meaningful comparison with experiment, it is therefore necessary to include vibrational corrections in the calculated spin–spin coupling constants. In the present paper, such corrections have been calculated for a number of small molecular systems by using hybrid density-functional theory (DFT), yielding results in good agreement with previous wave-function calculations. A set of empirical equilibrium spin–spin coupling constants has been compiled from the experimentally observed constants and the calculated vibrational corrections. A comparison of these empirical constants with calculations suggests that the restricted-active-space self-consistent field method is the best approach for calculating the indirect spin–spin coupling constants of small molecules, and that the secon...