Multireference approaches for excited states of molecules

Understanding the properties of electronically excited states is a challenging task that becomes increasingly important for numerous applications in chemistry, molecular physics, molecular biology, and materials science. A substantial impact is exerted by the fascinating progress in time-resolved spectroscopy, which leads to a strongly growing demand for theoretical methods to describe the characteristic features of excited states accurately. Whereas for electronic ground state problems of stable molecules the quantum chemical methodology is now so well developed that informed nonexperts can use it efficiently, the situation is entirely different concerning the investigation of excited states. This review is devoted to a specific class of approaches, usually denoted as multireference (MR) methods, the generality of which is needed for solving many spectroscopic or photodynamical problems. However, the understanding and proper application of these MR methods is often found to be difficult due to their complexity and their computational cost. The purpose of this review is to provide an overview of the most important facts about the different theoretical approaches available and to present by means of a collection of characteristic examples useful information, which can guide the reader in performing their own applications

Externally Corrected CCSD with Renormalized Perturbative Triples (R-ecCCSD(T)) and the Density Matrix Renormalization Group and Selected Configuration Interaction External Sources

We investigate the renormalized perturbative triples correction together with the externally corrected coupled-cluster singles and doubles (ecCCSD) method. We use the density matrix renormalization group (DMRG) and heat-bath CI (HCI) as external sources for the ecCCSD equations. The accuracy is assessed for the potential energy surfaces of H₂O, N₂, and F₂. We find that the triples correction significantly improves upon ecCCSD, and we do not see any instability of the renormalized triples with respect to dissociation. We explore how to balance the cost of computing the external source amplitudes against the accuracy of the subsequent CC calculation. In this context, we find that very approximate wave functions (and their large amplitudes) serve as an efficient and accurate external source. Finally, we characterize the domain of correlation treatable using the ecCCSD and renormalized triples combination studied in this work via a well-known wave function diagnostic

Can Density Matrix Embedding Theory with the Complete Activate Space Self-Consistent Field Solver Describe Single and Double Bond Breaking in Molecular Systems?

Density matrix embedding theory (DMET) [Phys. Rev. Lett.2012, 109, 186404] has been demonstrated as an efficient wave-function-based embedding method to treat extended systems. Despite its success in many quantum lattice models, the extension of DMET to real chemical systems has been tested only on selected cases. Herein, we introduce the use of the complete active space self-consistent field (CASSCF) method as a correlated impurity solver for DMET, leading to a method called CAS-DMET. We test its performance in describing the dissociation of a H-H single bond in a H10 ring model system and an N=N double bond in azomethane (CH3-N=N-CH3) and pentyldiazene (CH3(CH2)4-N=NH). We find that the performance of CAS-DMET is comparable to CASSCF with different active space choices when single-embedding DMET corresponding to only one embedding problem for the system is used. When multiple embedding problems are used for the system, the CAS-DMET is in a good agreement with CASSCF for the geometries around the equilibrium, but not in equal agreement at bond dissociation.Comment: 28 pages, 9 figures, TOC graphi

Design of Small Intramolecular Singlet Fission Chromophore: An Azaborine Candidate and General Small Size Effects

We report the first attempt to design small intramolecular singlet fission chromophores, with the aid of quantum chemistry and explicitly simulating the time evolution of state populations using quantum dynamics method. We start with three previously proposed azaborine-substituted intermolecular singlet fission chromophores. Through analyzing their frontier orbital amplitudes, we select a BN-substituted azulene as the building block. Covalently connecting two such monomers and tuning their relative configuration, we examine three dimers. One dimer is found to be an eminent candidate: the triplet-pair state is quickly formed within 1 ps, and the two triplets are ready to be disentangled. We elucidate the general small size effects in intramolecular singlet fission and focus on specific aspects which should be taken care of when manipulating the fission rate through steric hindrance

THRESHOLD IONIZATION SPECTROSCOPY AND SPIN-ORBIT COUPLING OF LANTHANIDE COMPLEXES

The C-C and C-H bonds have high bond strength and low polarization, which make many small hydrocarbons too inert to react with other molecules under ambient pressure and temperature conditions. Therefore, activation of these bonds is required to convert such small molecules into other value-added chemicals. Among various bond activation methods, metal activation is widely used and reported in the literature, because of its relatively mild reaction conditions and high selectivity. In this work, Ce atom reactions with several small hydrocarbons are carried out in a pulsed laser vaporization supersonic molecular beam source, and Ce -hydrocarbon species are observed with time-of-flight mass spectrometry and characterized by mass-analyzed threshold ionization (MATI) spectroscopy and theoretical calculations. The small hydrocarbon compounds include ethylene, propene, 2-butene, and iso-butene. In addition to these alkene molecules, ammonia is used to investigate the N-H bond activation and compare with the C-H activation of the alkene molecules. Ammonia reaction with La atom is also included in this work to help investigate effects of the Ce 4f1 electron on the Ce reactivity and MATI spectra of Ce-containing species. The theoretical calculations include quantum chemical computations and spectral simulations. The quantum chemical methods include density functional theory, electron correlation, and spin-orbit coupling, and the spectral simulations are based on multi-dimensional Frack-Condon factor calculations. Vibrationally-resolved MATI spectra are obtained for Ce(C2H2) formed through ethylene dehydrogenation, Ce(C3Hn) (n = 4 and 6) by the C-H and C-C bond activation of propene, Ce(C4H6) two isomers from the C-C bond coupling of ethylene and dehydrogenation of 2- and iso-butene, and LnNH (Ln= La and Ce) formed in the Ce and La reactions with ammonia. The MATI spectra of Ce-hydrocarbon and CeNH complexes consist of two or more vibronic band systems due to spin-orbit coupling between the Ce 4f and 6s electrons, while the spectrum of LaNH has only one vibronic band system. The ground valence electron configurations of all Ce-containing species are Ce 4f16s1, while that of LaNH is La 6s1. Ionization removes the Ce 6s1 or La 6s1 electron and produces doublet electronic states for the Ce-containing species and a singlet state for LaNH. The remaining two 5d electrons that are associated with bare Ce or La atom are spin paired in one or two molecular orbitals that are in binding combinations with ligand orbitals

The OpenMolcas Web: A Community-Driven Approach to Advancing Computational Chemistry

The developments of the open-source OpenMolcas chemistry software environment since spring 2020 are described, with a focus on novel functionalities accessible in the stable branch of the package or via interfaces with other packages. These developments span a wide range of topics in computational chemistry and are presented in thematic sections: electronic structure theory, electronic spectroscopy simulations, analytic gradients and molecular structure optimizations, ab initio molecular dynamics, and other new features. This report offers an overview of the chemical phenomena and processes OpenMolcas can address, while showing that OpenMolcas is an attractive platform for state-of-the-art atomistic computer simulations