Sheela Kirpekar

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.

Nuclear spin–spin coupling in the acetylene isotopomers calculated from ab initio correlated surfaces for 1J(C, H), 1J(C, C), 2J(C, H), and 3J(H, H)

Ab initio calculated coordinate and internal valence coordinate coefficients for each of the four spin–spin coupling surfaces of the acetylene molecule—1J(C,u200aH), 1J(C,u200aC), 2J(C,u200aH), and 3J(H,u200aH) are presented. Calculations were carried out at the SOPPA(CCSD) level using a large basis set. Couplings were calculated at 35 geometries (including equilibrium) giving 35 distinct sites on the 1J(C,u200aC) and 3J(H,u200aH) surfaces and 53 distinct sites on the 1J(C,u200aH) and 2J(C,u200aH) surfaces. The results were fitted to fourth order in Taylor series expansions and are presented to second order in the coordinates. All couplings are sensitive to geometry with the principal features being (a) an even steeper increase of J(C1,u200aH1) with CC bond stretching than with CH bond stretching—an example of “unexpected differential sensitivity” (or UDS), (b) very opposite variations of 2J(C1,u200aH2) with variations of the CC and C2H2 bond lengths, (c) very opposite variations of 1J(C,u200aC) with a CC stretch and a CH stretch and (d) very oppos...

Correlated calculations of indirect nuclear spin-spin coupling constants for XH4 (X = Si, Ge, and Sn)

Abstract We report multiconfigurational linear response calculations of the one-bond and geminal indirect nuclear spin-spin couplings constants in SiH4, GeH4, and SnH4. Results of a basis set study on SnH4 using 13 different basis sets on Sn are given, showing the effects on the individual contributions to the coupling constant. In addition to the more conventional choice of active orbital space (one correlating orbital for each strongly occupied valence orbital), calculations employing extended active spaces were carried out. The results are compared with available experimental data showing that correlation effects are very important. For the one-bond couplings good agreement with experiment, within 1–5%, is obtained, whereas the geminai proton-proton couplings show larger deviations. The calculations reported here are, to our knowledge, the first that take into consideration correlation effects in all four contributions to the coupling constants for molecules as large as GeH4 and SnH4. All X-H couplings are dominated by the Fermi contact term while the picture is more diverse for the geminal couplings.

Nuclear magnetic shielding in the acetylene isotopomers calculated from correlated shielding surfaces

Ab initio, symmetry-coordinate and internal valence coordinate carbon and hydrogen nuclear shielding surfaces for the acetylene molecule are presented. Calculations were performed at the correlated level of theory using gauge-including atomic orbitals and a large basis set. The shielding was calculated at equilibrium and at 34 distinct geometries corresponding to 53 distinct sites for each nucleus. The results were fitted to fourth order in Taylor series expansions and are presented to second order in the coordinates. The carbon-13 shielding is sensitive to all geometrical parameters and displays some unexpected features; most significantly, the shielding at a carbon nucleus (C1, say) is six times more sensitive to change of the C1C2H2 angle than it is to change of the H1C1C2 angle. In addition, for small changes, σ(C1) is more sensitive to the C2H2 bond length than it is to the C1H1 bond length. These, and other, examples of “unexpected differential sensitivity” are discussed. The proton shielding surfac...

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.

Spin–orbit corrections to the indirect nuclear spin–spin coupling constants in XH

Summary.u2002Using the quadratic response function at the ab initio SCF level of approximation we have calculated the relativistic corrections from the spin–orbit Hamiltonian, HSO, to the indirect nuclear spin–spin coupling constants of XH4 (X=C, Si, Ge, and Sn). We find that the spin–orbit contributions to JX–H are small, amounting only to about 1% for JSn–H. For the geminal H–H coupling constants the relativistic corrections are numerically smaller than for JX–H, but in some cases relatively larger compared to the actual magnitude of JH–H. We also investigate the use of an effective one-electron spin–orbit Hamiltonian rather than the full HSO in the calculation of these corrections.