A Density Matrix Renormalization Group Study of the Low-Lying Excited States of a Molybdenum Carbonyl-Nitrosyl Complex.

A density matrix renormalization group-self consistent field (DMRG-SCF) study has been carried out to calculate the low-lying excited states of CpMo(CO)2 NO, a molybdenum complex containing NO and CO ligands. In order to automatically select an appropriate active space, a novel procedure employing the maximum single-orbital entropy for several states has been introduced and shown to be efficient and easy-to-implement when several electronic states are simultaneously considered. The analysis of the resulting natural transition orbitals and charge-transfer numbers shows that the lowest five excited electronic states are excitation into metal-NO antibonding orbitals, which offer the possibility for nitric oxide (NO) photorelease after excitation with visible light. Higher excited states are metal-centered excitations with contributions of metal-CO antibonding orbitals, which may serve as a gateway for carbon monoxide (CO) delivery. Time-dependent density functional theory calculations done for comparison, show that the state characters agree remarkably well with those from DMRG-SCF, while excitation energies are 0.4-1.0 eV red-shifted with respect to the DMRG-SCF ones.

The generality of the GUGA MRCI approach in COLUMBUS for treating complex quantum chemistry.

The core part of the program system COLUMBUS allows highly efficient calculations using variational multireference (MR) methods in the framework of configuration interaction with single and double excitations (MR-CISD) and averaged quadratic coupled-cluster calculations (MR-AQCC), based on uncontracted sets of configurations and the graphical unitary group approach (GUGA). The availability of analytic MR-CISD and MR-AQCC energy gradients and analytic nonadiabatic couplings for MR-CISD enables exciting applications including, e.g., investigations of π-conjugated biradicaloid compounds, calculations of multitudes of excited states, development of diabatization procedures, and furnishing the electronic structure information for on-the-fly surface nonadiabatic dynamics. With fully variational uncontracted spin-orbit MRCI, COLUMBUS provides a unique possibility of performing high-level calculations on compounds containing heavy atoms up to lanthanides and actinides. Crucial for carrying out all of these calculations effectively is the availability of an efficient parallel code for the CI step. Configuration spaces of several billion in size now can be treated quite routinely on standard parallel computer clusters. Emerging developments in COLUMBUS, including the all configuration mean energy multiconfiguration self-consistent field method and the graphically contracted function method, promise to allow practically unlimited configuration space dimensions. Spin density based on the GUGA approach, analytic spin-orbit energy gradients, possibilities for local electron correlation MR calculations, development of general interfaces for nonadiabatic dynamics, and MRCI linear vibronic coupling models conclude this overview.

Controlling the Excited-State Dynamics of Nuclear Spin Isomers Using the Dynamic Stark Effect.

Stark control of chemical reactions uses intense laser pulses to distort the potential energy surfaces of a molecule, thus opening new chemical pathways. We use the concept of Stark shifts to convert a local minimum into a local maximum of the potential energy surface, triggering constructive and destructive wave-packet interferences, which then induce different dynamics on nuclear spin isomers in the electronically excited state of a quinodimethane derivative. Model quantum-dynamical simulations on reduced dimensionality using optimized ultrashort laser pulses demonstrate a difference of the excited-state dynamics of two sets of nuclear spin isomers, which ultimately can be used to discriminate between these isomers.

Enhancing intersystem crossing in phenotiazinium dyes by intercalation into DNA.

Phenothiazinium dyes are used as photosensitizers in photodynamic therapy. Their mode of action is related to the generation of triplet excited states by intersystem crossing. Therefore, rationalizing the factors that influence intersystem crossing is crucial to improve the efficacy of photodynamic therapy. Here we employ quantum mechanics/molecular mechanics calculations to investigate the effect of aqueous and nucleic acid environments on the intersystem crossing mechanism in methylene blue. We find that the mechanism by which the triplet states are generated depends strongly on the environment. While intersystem crossing in water is mediated exclusively by vibronic spin-orbit coupling, it is enhanced in DNA due to a second pathway driven by electronic spin-orbit coupling. Competing charge-transfer processes, which are also possible in the presence of DNA, can therefore be suppressed by a suitable structural functionalization, thereby increasing the efficacy of photodynamic therapy.

Discrimination of nuclear spin isomers exploiting the excited state dynamics of a quinodimethane derivative.

Despite the concept of nuclear spin isomers (NSIs) exists since the early days of quantum mechanics, only few approaches have been suggested to separate different NSIs. Here, a method is proposed to discriminate different NSIs of a quinodimethane derivative using its electronic excited state dynamics. After electronic excitation by a laser field with femtosecond time duration, a difference in the behavior of several quantum mechanical operators can be observed. A pump-probe experimental approach for separating these different NSIs is then proposed.

OpenMolcas: From Source Code to Insight.

In this Article we describe the OpenMolcas environment and invite the computational chemistry community to collaborate. The open-source project already includes a large number of new developments realized during the transition from the commercial MOLCAS product to the open-source platform. The paper initially describes the technical details of the new software development platform. This is followed by brief presentations of many new methods, implementations, and features of the OpenMolcas program suite. These developments include novel wave function methods such as stochastic complete active space self-consistent field, density matrix renormalization group (DMRG) methods, and hybrid multiconfigurational wave function and density functional theory models. Some of these implementations include an array of additional options and functionalities. The paper proceeds and describes developments related to explorations of potential energy surfaces. Here we present methods for the optimization of conical intersections, the simulation of adiabatic and nonadiabatic molecular dynamics, and interfaces to tools for semiclassical and quantum mechanical nuclear dynamics. Furthermore, the Article describes features unique to simulations of spectroscopic and magnetic phenomena such as the exact semiclassical description of the interaction between light and matter, various X-ray processes, magnetic circular dichroism, and properties. Finally, the paper describes a number of built-in and add-on features to support the OpenMolcas platform with postcalculation analysis and visualization, a multiscale simulation option using frozen-density embedding theory, and new electronic and muonic basis sets.

Efficient and Flexible Computation of Many-Electron Wave Function Overlaps.

A new algorithm for the computation of the overlap between many-electron wave functions is described. This algorithm allows for the extensive use of recurring intermediates and thus provides high computational efficiency. Because of the general formalism employed, overlaps can be computed for varying wave function types, molecular orbitals, basis sets, and molecular geometries. This paves the way for efficiently computing nonadiabatic interaction terms for dynamics simulations. In addition, other application areas can be envisaged, such as the comparison of wave functions constructed at different levels of theory. Aside from explaining the algorithm and evaluating the performance, a detailed analysis of the numerical stability of wave function overlaps is carried out, and strategies for overcoming potential severe pitfalls due to displaced atoms and truncated wave functions are presented.

Theory of ultrafast nonresonant multiphoton transitions in polyatomic molecules: basics and application to optimal control theory.

A systematic approach is presented to describe nonresonant multiphoton transitions, i.e., transitions between two electronic states without the presence of additional intermediate states resonant with the single-photon energy. The method is well suited to describe femtosecond spectroscopic experiments and, in particular, attempts to achieve laser pulse control of molecular dynamics. The obtained effective time-dependent Schrodinger equation includes effective couplings to the radiation field which combine powers of the field strength and effective transition dipole operators between the initial and final states. To arrive at time-local equations our derivation combines the well-known rotating wave approximation with the approximation of slowly varying amplitudes. Under these terms, the optimal control formalism can be readily extended to also account for nonresonant multiphoton events. Exemplary, nonresonant two- and three-photon processes, similar to those occurring in the recent femtosecond pulse-shaping experiments on CpMn(CO)(3), are treated using related ab initio potential energy surfaces.