Thomas Enevoldsen

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.

Full four‐component relativistic calculations of NMR shielding and indirect spin–spin coupling tensors in hydrogen halides

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 mass‐velocity correction. The relativistic effect on the indirect nuclear spin–spin coupling tensor is large and affects mainly the isotropic values; the effect on the anisotropy is small.u2003©1999 John Wiley & Sons, Inc.u2003J Comput Chem 20: 1262–1273, 1999

A new implementation of the second‐order polarization propagator approximation (SOPPA): The excitation spectra of benzene and naphthalene

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 4-component calculations of indirect nuclear spin-spin couplings in MH4(M=C,Si,Ge,Sn,Pb (CH3)3H

Relativistic four-component random phase approximation (RPA) calculations of indirect nuclear spin–spin coupling constants in MH4 (M=C,u200aSi,u200aGe,u200aSn,u200aPb) and Pb(CH3)3H 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–LL) and (SS–SS) two-electron integrals is investigated. Already in GeH4 we see a relativistic increase in the coupling constant by 12%, and for PbH4 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.

Interfacing relativistic and nonrelativistic methods. III. Atomic 4-spinor expansions and integral approximations

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 b...

Molecular relativistic calculations of the electric field gradients at the nuclei in the hydrogen halides

Electric field gradients at the position of the nuclei in the hydrogen halides are calculated using four-component relativistic methods. Benchmark values at the Dirac–Hartree–Fock level of theory are obtained by using large uncontracted basis sets. Electron correlation corrections are obtained by means of finite field MP2, CCSD, and CCSD(T) calculations in smaller basis sets. The importance of spin–orbit coupling and the so-called picture change effect are discussed.

Paramagnetism of closed shell diatomic hydrides with six valence electrons

We have investigated the potential temperature independent van Vleck paramagnetism of closed shell diatomic hydrides with six valence electrons. More specifically, we have studied the magnetizability of the first row hydrides BeH−, BH, CH+, of the second row hydrides MgH−, AlH, and SiH+, and of the third row hydride GeH+. The magnetizability was calculated using a gauge origin independent method at the uncorrelated SCF level within the random‐phase approximation (RPA) as well as at different correlated levels within the second‐order polarization propagator approximation (SOPPA) or various coupled cluster polarization propagator approximations (CCDPPA/CCSDPPA). We find that BH, CH+, and SiH+ are paramagnetic, MgH−, AlH, and GeH+ are diamagnetic and BeH− is a borderline case tilting towards paramagnetism. It is primarily variations in the diamagnetic contribution to the magnetizability that determine the overall sign of the magnetizability.

Gauge origin independent calculations of nuclear magnetic shieldings in relativistic four-component theory

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.

Vibrational and thermal averaging of the indirect nuclear spin-spin coupling constants of CH4, SiH4, GeH4 and SnH4

The effects of nuclear motion on the indirect nuclear spin-spin coupling constants of CH4, SiH4, GeH4 and SnH4 were calculated up to first order in the normal coordinates. We report the random-phase approximation, multiconfigurational linear response and second-order polarization propagator calculations of the zero-point rovibrational corrections, as well as the temperature dependence of both one-bond and two-bond coupling constants. We find that the random-phase approximation overestimates the zero-point corrections. The results demonstrate that rovibrational corrections are as important as the non-contact terms and must be included in accurate determinations of indirect nuclear spin-spin coupling constants of these molecules.

Relativistic calculations of the rotational g factor of the hydrogen halides and noble gas hydride cations

The rotational g factors of the hydrogen halides, HX (X=F,Cl,Br,I), and noble gas hydride cations, XH+ (X=Ne,Ar,Kr,Xe), have been calculated at the level of the random phase approximation (RPA) as relativistic four-component linear response functions as well as nonrelativistic linear response functions. In addition, using perturbation theory with the mass-velocity and Darwin operators as perturbations, the relativistic corrections have been estimated as quadratic response functions. It was found that the four-component relativistic calculations give in general a more negative electronic contribution to the rotational g factor than the nonrelativistic calculations with relativistic corrections ranging from 0.2% for HF and NeH+ to 2.9% for XeH+ and 3.5% for HI. The estimates of the relativistic corrections obtained by perturbation theory with the mass-velocity and Darwin operators are in good agreement with the four-component results for HF, HCl, NeH+, and ArH+, whereas for HI, KrH+, and XeH+ they have the ...