We use a contracted $\text{basis}+\text{Lanczos}$ eigensolver approach to compute vinylidene-like vibrational states of the acetylene-vinylidene system. To overcome problems caused by loss of orthogonality of the Lanczos vectors we reorthogonalize Lanczos vector and use a coupled two-term approach. The calculations are done in CC–HH diatom-diatom Jacobi coordinates which make it easy to compute states one irreducible representation at a time. The most costly parts of the calculation are parallelized and scale well. We estimate that the vinylidene energies we compute are converged to $\sim 1{\mathrm{cm}}^{-1}$ .

Two different sets of approaches for the density-functional calculation of the spin-orbit contributions to zero-field splitting (ZFS) parameters of high-spin systems have been implemented within the same quantum chemistry code ReSpect and have been validated and compared for a series of model systems. The first approach includes spin-orbit coupling variationally in a two-component calculation, using either an all-electron Douglas-Kroll-Hess ansatz or two-component relativistic pseudopotentials. The ZFS parameters are computed directly from energy differences between different relativistic states. Additionally, an approximate second-order perturbation theory approach has been implemented, based on nonrelativistic or scalar relativistic wave functions. For a series of group 16 triplet diatomics and for the octet GdH3 molecules, two-component density functional calculations underestimate the zero-field splitting D systematically by a factor of 2. This may be rationalized readily by the incomplete description of states with ∣MJ∣<J by a single-determinantal wave function built from two-component spinors. In the case of two 3d transition metal complexes and for GdH3, the results depend furthermore sensitively on exchange-correlation functional. Results of the alternative one-component approach agree strikingly with the two-component data for systems with small spin-orbit effects and start to deviate from them only for heavier systems with large spin-orbit effects. These results have fundamental implications for the achievable accuracy of one-component density-functional approaches used widely to compute ZFS parameters in the field of molecular magnetism. Possible refinements of both one-and two-component approaches are discussed.

Correlated, multielectron dynamics of “open” electronic systems within the fixed-nuclei approximation are treated here within explicitly time-dependent configuration-interaction schemes. Specifically, we present simulations of laser-pulse driven excitations of selected electronic states of LiCN in the presence of energy and phase relaxation. The evolution of the system is studied using open-system density matrix theory, which embeds naturally in the time-dependent configuration-interaction singles (doubles) formalism. Different models for dissipation based on the Lindblad semigroup formalism are presented. These models give rise to lifetimes for energy relaxation ranging from a few hundreds of femtoseconds to several nanoseconds. Pure dephasing is treated using a Kossakowski-like Gaussian model, proceeding on similar time scales. The pulse lengths employed range from very short (tens of femtoseconds) to very long (several nanoseconds). To make long-time propagations tractable, the quasiresonant approximation is used. The results show that despite the loss of efficiency, selective dipole switching can still be achieved in the presence of dissipation when using appropriately designed laser pulses.