Active Space Configuration Interaction Method Research Articles

InteraChem: Exploring Excited States in Virtual Reality with Ab Initio Interactive Molecular Dynamics.

InteraChem is an ab initio interactive molecular dynamics (AI-IMD) visualizer that leverages recent advances in virtual reality hardware and software, as well as the graphical processing unit (GPU)-accelerated TeraChem electronic structure package, in order to render quantum chemistry in real time. We introduce the exploration of electronically excited states via AI-IMD using the floating occupation molecular orbital-complete active space configuration interaction method. The optimization tools in InteraChem enable identification of excited state minima as well as minimum energy conical intersections for further characterization of excited state chemistry in small- to medium-sized systems. We demonstrate that finite-temperature Hartree-Fock theory is an efficient method to perform ground state AI-IMD. InteraChem allows users to track electronic properties such as molecular orbitals and bond order in real time, resulting in an interactive visualization tool that aids in the interpretation of excited state chemistry data and makes quantum chemistry more accessible for both research and educational purposes.

Molecular Design Principle for Efficient Singlet Fission Based on Diradical Characters and Exchange Integrals: Multiple Heteroatom Substitution Effect on Anthracenes

We theoretically investigate significant impacts of diradical characters and exchange integrals on the first singlet (S1) and triplet (T1) excitation energies related to singlet fission (SF), from which a series of C2h-symmetrically diazadibora-substituted anthracenes [(BN)2-anthracenes] are newly proposed as potential efficient SF chromophores. From the analytical expressions of electronic structures for a model system with two electrons in two orbitals, a tuning of exchange integrals represented by localized natural orbitals is shown to be effective to vary E(S1) and E(T1), in addition to the control of diradical characters, both of which are expected to result in SF isothermal or exothermic [E(S1) – 2E(T1) ≈ 0 or > 0]. The applicability of this strategy to realistic molecules is verified for multiple heteroatom substitution to anthracene, namely, (BN)2-anthracenes, by the complete active space configuration interaction method and the time-dependent density functional theory method. Our calculations show that (BN)2-anthracenes exhibit a wide range of diradical characters and enhanced exchange integrals, enabling flexible tunings of E(S1) and E(T1). Accordingly, several (BN)2-anthracenes are found to be promising candidates for efficient SF. Such a behavior of diradical character can be understood from resonance structures associated with the captodative effect. Moreover, a concrete strategy of tuning exchange integrals is demonstrated in terms of frontier molecular orbital distributions. The present results open up novel molecular design guidelines for efficient SF based on the tuning of exchange integrals in addition to diradical characters.

Spin-Forbidden Transitions between Electronic States in the Active Site of Rubredoxin.

Rubredoxin is a small iron-sulfur protein involved in biological electron transfer, which is accomplished by changing the oxidation state of the iron atom in the active site. We investigate the possibility of spin-forbidden transitions between the lowest energy electronic states with different spin multiplicities in the rubredoxin active site models [Fe(SCH3)4]n (n = 2-, 1-, 0) using nonadiabatic transition state theory (NA-TST). The equilibrium structures, minimum energy crossing point structures and Hessians were obtained with density functional theory. The spin-orbit coupling (SOC) was calculated with the complete active space configuration interaction method using the two-electron spin-orbit Breit-Pauli Hamiltonian. We found several crossings between the lowest energy spin states associated with the changes in Fe coordination. However, only triplet/quintet crossings in [Fe(SCH3)4]2- and [Fe(SCH3)4]0, as well as a quartet/sextet crossing in [Fe(SCH3)4]- are characterized by nonzero first-order SOC responsible for transitions between these spin states. The rates of spin-forbidden transitions in the [Fe(SCH3)4]2- complex are 1 and 2 orders of magnitude higher than the rates in the [Fe(SCH3)4]- and [Fe(SCH3)4]0 complexes, respectively. These rate differences are related to a large variation of the SOC between the complexes with different charges, which in turn comes from different molecular orbitals involved in the spin-flip transitions. Finally, we demonstrate that the differences between the NA-TST rates and the rates calculated under the assumption of completely spin-allowed transitions could be as large as 4 orders of magnitude. This means that even in qualitative discussions of the reaction mechanisms involving changes in spin states the partially spin-forbidden nature of the transitions between these states must be taken into account.

First-Principles Study of Nonradiative Recombination in Silicon Nanocrystals: The Role of Surface Silanol

The photophysical properties of emissive silicon nanomaterials depend strongly on the chemical composition and structure of their surfaces, and the development of a causal, microscopic understanding of this relationship is highly desirable. One surface-dependent property of interest is the propensity for nonradiative recombination (NRR). In this work, we apply ab initio theoretical methods to investigate the mechanism of NRR in a silicon nanocrystal with a single surface silanol group. Ab initio multiple spawning simulations of an electronically excited cluster model (Si7H11OH) indicate ultrafast nonradiative decay to the electronic ground state. A multireference electronic structure study demonstrates that this nonradiative decay occurs near conical intersections between the ground and first excited electronic states of the cluster. These intersections are accessed after stretching of the bond between the silanol silicon atom and an adjacent silicon atom. The presence of this intersection in a true nanomaterial is confirmed by optimization of a similar conical intersection in a silicon nanocrystal (oblate, major diameter 1.4 nm, minor diameter 1.0 nm) with a silanol group on the surface (Si44H45OH). This intersection was identified using a graphics processing unit accelerated implementation of the configuration interaction singles natural orbital complete active space configuration interaction method. All intersections identified in this work are predicted to be at least 4.3 eV above the ground state minimum energy. This confirms the widely held view that silanol groups do not introduce efficient pathways for nonradiative recombination of excitons created upon absorption of visible light. That such an assignment is made entirely from first-principles underscores the value of conical intersection optimization as a tool for elucidating semiconductor photophysics.

Nonadiabatic Dynamics of Photoinduced Proton-Coupled Electron Transfer in a Solvated Phenol–Amine Complex

Photoinduced concerted electron-proton transfer (EPT), denoted photo-EPT, is important for a wide range of energy conversion processes. Transient absorption and Raman spectroscopy experiments on the hydrogen-bonded p-nitrophenylphenol-t-butylamine complex, solvated in 1,2-dichloroethane, suggested that this complex may undergo photo-EPT. The experiments probed two excited electronic states that were interpreted as an intramolecular charge transfer (ICT) state and an EPT state. Herein mixed quantum mechanical/molecular mechanical nonadiabatic surface hopping dynamics is used to investigate the relaxation pathways following photoexcitation. The potential energy surface is generated on the fly with a semiempirical floating occupation molecular orbital complete active space configuration interaction method for the solute molecule and a molecular mechanical force field for the explicit solvent molecules. The free energy curves along the proton transfer coordinate illustrate that proton transfer is thermodynamically and kinetically favorable on the lower-energy excited state but not on the higher-energy excited state, supporting the characterization of these states as EPT and ICT, respectively. The nonadiabatic dynamics simulations indicate that the population decays from the ICT state to the EPT state in ∼100 fs and from the EPT state to the ground state on the slower time scale of ∼1 ps, qualitatively consistent with the experimental measurements. For ∼54% of the trajectories, the proton transfers from the phenol to the amine in ∼400 fs on the EPT state and then transfers back to the phenol rapidly upon decay to the ground state. Thus, these calculations augment the original interpretation of the experimental data by providing evidence of proton transfer on the EPT state prior to decay to the ground state. The fundamental insights obtained from these simulations are also relevant to other photo-EPT processes.

Time-dependent restricted active space Configuration Interaction theory applied to the photoionization of neon

We give a presentation on the time-dependent restricted active space Configuration Interaction method, which has been recently introduced [D. Hochstuhl, M. Bonitz, PRA, in press] to efficiently tackle the few-particle nonequilibrium many-body problem. Particularly, we apply it to the calculation of photoionization cross sections and angular resolved ionization yields of groundstate neon.

Time-dependent restricted-active-space configuration-interaction method for the photoionization of many-electron atoms

We introduce the time-dependent restricted active space Configuration Interaction method to solve the time-dependent Schr\"odinger equation for many-electron atoms, and particularly apply it to the treatment of photoionization processes in atoms. The method is presented in a very general formulation and incorporates a wide range of commonly used approximation schemes, like the single-active electron approximation, time-dependent Configuration Interaction with single-excitations, or the time-dependent R-matrix method. We proof the applicability of the method by calculating the photoionization cross sections of Helium and Beryllium, as well as the X-ray--IR pump-probe ionization in Beryllium

Ab initio floating occupation molecular orbital-complete active space configuration interaction: An efficient approximation to CASSCF

We have implemented a complete active space configuration interaction method (CASCI) based on floating occupation molecular orbitals (FOMOs) at the ab initio level. The performance of this FOMO-CASCI method was investigated for potential applications in photochemistry and photodynamics. We found that FOMO-CASCI often represents a good approximation to the state-averaged complete active space self-consistent field (SA-CASSCF) method. FOMO-CASCI is therefore an attractive alternative for use in ab initio photodynamics. The method is more efficient and more stable than SA-CASSCF. We also discuss some problematic cases for the FOMO-CASCI approach. Possible extensions of the FOMO-CASCI approach are discussed briefly.

Molecular applications of analytical gradient approach for the improved virtual orbital-complete active space configuration interaction method

The improved virtual orbital-complete active space configuration interaction (IVO-CASCI) method is extended to determine the geometry and vibrational frequencies for ground and excited electronic states using an analytical total energy gradient scheme involving both first and second order analytical derivatives. Illustrative applications consider the ground state geometries of the benzene (C(6)H(6)), biphenyl (C(12)H(10)), and alanine dipeptide (CH(3)CONHCHCH(3)CONHCH(3)) molecules. In addition, the IVO-CASCI geometry optimization has been performed for the first excited singlet ((1)B(2u)) and triplet states ((3)B(1u)) of benzene to assess its applicability for excited and open-shell systems. The D(6h) symmetry benzene triplet optimization produces a saddle point, and a descent along the unstable mode produces the stable minimum. Comparisons with Hartree-Fock, second order Moller-Plesset perturbation theory, complete active space self-consistent field (CASSCF), and density functional theory demonstrate that the IVO-CASCI approach generally fares comparable to or better for all systems studied. The vibrational frequencies of the benzene and biphenyl molecules computed with the analytical gradient based IVO-CASCI method agree with the experiment and with other accurate theoretical estimates. Satisfactory agreement between our results, other benchmark calculations, and available experiment demonstrates the efficacy and potential of the method. The close similarity between CASSCF and IVO-CASCI optimized geometries and the greater computational efficiency of the IVO-CASCI method suggests the replacement of CASSCF treatments by the IVO-CASCI approach, which is free from the convergence problems that often plague CASSCF treatments.

Relativistic configuration-interaction study of the nuclear-spin-dependent parity-nonconserving electron-nucleus interaction constantWAin BaF

A fully relativistic restricted active space configuration-interaction method is employed to compute the $P$-odd electron-nucleus interaction constant ${W}_{\text{A}}$ for the ground $({^{2}\ensuremath{\Sigma}}_{1/2})$ state of BaF, which yields the result of ${W}_{\text{A}}=160\text{ }\text{Hz}$. Our present estimated result of the $P$-odd interaction constant ${W}_{\text{A}}$ is in reasonable agreement with previous calculations. This result will be useful for the interpretation of the nuclear-spin-dependent parity-nonconservation experiment on BaF.

The improved virtual orbital-complete active space configuration interaction method, a “packageable” efficient ab initio many-body method for describing electronically excited states

We describe a computationally efficient ab initio many-body method that can be used as a “packageable approximation” for computing excited state properties for small to large molecular systems, including those of multiconfigurational character. The method is based on first order multi-reference many-body perturbation theory (MR-MBPT), where the unoccupied valence orbitals are obtained by using an extension of Huzinaga’s improved virtual orbital (IVO) generation technique. Because the method employs a complete active space (CAS) which contains singly, doubly, and higher excited state configurations with respect to the zeroth order ground state configuration, the approach (IVO-CASCI) is capable of providing a more accurate description of the excited states than the widely used packageable configuration interaction with singles (CIS) at a fraction of computational labor. Moreover, unlike the CASSCF approach this IVO-CASCI method does not require iterations and therefore is more computationally efficient and free of the convergence problems that sometimes plague CASSCF calculations with increasing size of the CAS. Excited state energies are compared with energies from the widely used CIS, MCSCF, and CASSCF methods for the C2H+, C2H, CaOH, cyclic-C3H, and porphin molecules. The computed IVO-CASCI transition energies are generally more accurate than the CASSCF. For example, our energies are comparable to CIS energies for CaOH and porphin, while the C2H+, C2H, and C3H IVO-CASCI transition energies are more accurate than the CASSCF and CIS energies.