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In this work, the two-center Dirac equation is solved numerically using an extension of an adapted B-spline basis set method previously implemented in relativistic atomic calculations (Fischer, C. F.; Zatsarinny, O. Comput. Phys. Commun. 2009 , 180 , 879). The robustness of the chosen numerical method, which avoids the appearance of spurious states common in other approaches, allows us to investigate molecular photoionization within a relativistic framework by simply adapting those methods already available in the nonrelativistic case (Brosolo, M.; Decleva, P. Chem. Phys. 1992 , 159 , 185; Brosolo, M.; Decleva, P.; Lisini, A. Mol. Opt. Phys. 1992 , 25 , 3345). First, light diatomic molecules (i.e., H 2 + and HeH 2+ ) are investigated with the purpose of testing the validity and efficiency of the method. Then, a series of one-electron molecular hydrides (i.e., HF 9+ , HCl 17+ and HI 53+ ) is explored by computing the total photoionization cross sections, asymmetry β-parameters and partial phase shifts. The present methodology can be easily extended to treat N -electron molecules following previous approaches in nonrelativistic calculations (Plesiat, E.; Decleva, P.; Martin, F. Phys. Chem. Chem. Phys. 2012 , 14 , 10853). The inclusion of a second photon can be also accomplished just like in atomic investigations aiming at reproducing pump-probe experiments capable to extract the photoionization time-delays (Vinbladh, J.; Dahlstrom, J. M.; Lindroth, E. Phys. Rev A 2019 , 100 , 043424; Vinblach, J.; Dahlstrom, J. M.; Lindroth, E. Atoms 2022 , 10 , 80).

We introduce individual component maps of rotatory strength (ICM-RS) and rotatory strength density (RSD) plots as analysis tools of chiro-optical linear response spectra deriving from time-dependent density functional theory (TDDFT) simulations. ICM-RS and RSD allow one to visualize the origin of chiro-optical response in momentum or real space, including signed contributions and therefore highlighting cancellation terms that are ubiquitous in chirality phenomena, and should be especially useful in analyzing the spectra of complex systems. As test cases, we use ICM-RS and RSD to analyze circular dichroism spectra of selected (Ag-Au) 30 (SR) 18 monolayer-protected metal nanoclusters, showing the potential of the proposed tools to derive insight and understanding, and eventually rational design, in chiro-optical studies of complex systems.

We describe, in detail, a basis set approach to the multichannel scattering problem. The full set of linearly independent scattering states at each prefixed energy of the continuum spectrum can be obtained via a least-squares approach. To test the algorithm in a concrete setup, we report a parallel implementation of the close-coupling method in which the final states are treated within the configuration-interaction singles (CIS) approximation. The method requires, as input, a set of orthonormal orbitals, obtained from any quantum chemistry package. A one-center expansion (OCE) basis set consisting of products of radial B-splines and symmetry adapted angular functions is then used to expand the continuum electron wave function. To assess the quality of the CIS approximation, we compute total and partial cross sections and angular asymmetry parameters for the photoionization of a selection of closed-shell atoms (He, Ne, and Ar), H 2 , H 2 O, and ethylene. Results are compared with the experimental data and with theoretical predictions obtained with time-dependent density functional theory (TDDFT). It is seen that, generally, the photoionization observables obtained at the CIS level compare well with TDDFT predictions. The same basis can be employed to describe molecular multiphoton or strong field ionization.

The availability of an accurate representation of the potential energy surface (PES) is an essential prerequisite in an anharmonic vibrational calculation. At the same time, the high dimensionality of the fully coupled PES and the adverse scaling properties with respect to the molecular size make the construction of an accurate PES a computationally demanding task. In the past few years, our group tested and developed a series of tools and techniques aimed at defining computationally efficient, black-box protocols for the construction of PESs for use in vibrational calculations. This includes the definition of an adaptive density-guided approach (ADGA) for the construction of PESs from an automatically generated set of evaluation points. Another separate aspect has been the exploration of the use of derivative information through modified Shepard (MS) interpolation/extrapolation procedures. With this article, we present an assembled machinery where these methods are embedded in an efficient way to provide both a general machinery as well as concrete computational protocols. In this framework we introduce and discuss the accuracy and computational efficiency of two methods, called ADGA[2gx3M] and ADGA[2hx3M], where the ADGA recipe is used (with MS interpolation) to automatically define modest sized grids for up to two-mode couplings, while MS extrapolation based on, respectively, gradients only and gradients and Hessians from the ADGA determined points provides access to sufficiently accurate three-mode couplings. The performance of the resulting potentials is investigated in vibrational coupled cluster (VCC) calculations. Three molecular systems serve as benchmarks: a trisubstituted methane (CHFClBr), methanimine (CH2NH), and oxazole (C3H3NO). Furthermore, methanimine and oxazole are addressed in accurate calculations aiming to reproduce experimental results.

A new implementation of the vibrational self-consistent field (VSCF) method is presented on the basis of a second quantization formulation. A so-called active terms algorithm is shown to be a significant improvement over a standard implementation reducing the computational effort by one order in the number of degrees of freedom. Various types of screening provide even further reductions in computational scaling and absolute CPU time. VSCF calculations on large polyaromatic hydrocarbon model systems are presented. Further, it is demonstrated that in cases where distant modes are not directly coupled in the Hamiltonian, down to linear scaling of the required CPU time with respect to the number of vibrational modes can be obtained. This is illustrated with calculations on simple model systems with up to 1 million degrees of freedom.