Excited states with pair coupled cluster doubles tailored coupled cluster theory.

It is known that some non-dynamic effects of electron correlation can be included in coupled cluster theory using a tailoring technique that separates the effects of non-dynamic and dynamic correlations. Recently, the simple pCCD (pair coupled cluster doubles) wavefunction was shown to provide good results for some non-dynamic correlation problems, such as bond-breaking, in a spin-adapted way with no active space selection. In this paper, we report a study of excited states using "tailored coupled cluster singles and doubles," to attempt to use pCCD as a kernel for more complete coupled-cluster singles and doubles (CCSD) results for excited states. Several excited states are explored from those primarily due to single excitations to those dominated by doubly excited states and from singlet-triplet splittings for some diradical states. For the first two situations, tailored pCCD-TCCSD offers no improvement over equation of motion-CCSD. However, when we explore the singlet-triplet gap of diradical molecules that are manifestly multi-reference, a pCCD kernel provides improved results, particularly with generalized valence bond orbitals.

Density functionals for core excitations.

The core excitation energies and related principal ionization energies are obtained for selected molecules using several density functionals and compared with benchmark equation-of-motion coupled cluster (EOM-CC) results. Both time-dependent and time-independent formulations of excitation spectra in the time-dependent density functional theory and the EOM-CC are employed to obtain excited states that are not always easily accessible with the time-independent method. Among those functionals, we find that the QTP(00) functional, which is only parameterized to reproduce the five IPs of water, provides excellent core IPs and core excitation energies, consistently yielding better excitation and ionization energies. We show that orbital eigenvalues of KS density functional theory play an important role in determining the accuracy of the excitation and photoelectron spectra.

The intermediate state approach for doubly excited dark states in EOM-coupled-cluster theory.

Solution of dark, doubly excited states using equation-of-motion coupled-cluster (EOM-CC) usually equires at least triple excitations or even quadruples beyond the standard singles and doubles (EOM-CCSD) for an appropriate treatment. A new route to obtain these doubly excited states using EOM-CCSD is demonstrated. Traditionally, EOM-CC is performed on a closed shell reference state that has a well-described single reference CC wavefunction. In this Communication, we attempt to use low spin open-shell states such as the MS = 0 triplet and open-shell singlet as a reference state. Using this intermediate excited state as a reference state provides us with the benefit of obtaining a doubly excited state, as a single excitation at the cost of EOM-CCSD.

Equation of motion coupled-cluster study of core excitation spectra II: Beyond the dipole approximation.

We present the time-independent (TI) and time-dependent (TD) equation of motion coupled-cluster (EOM-CC) oscillator strengths not limited to those obtained by the dipole approximation. For the conventional TI-EOM-CC, we implement all the terms in the multipole expansion through second order that contributes to the oscillator strength. These include contributions such as magnetic dipole, electric quadrupole, electric octupole, and magnetic quadrupole. In TD-EOM-CC, we only include the quadrupole moment contributions. This augments our previous work [Y. C. Park, A. Perera, and R. J. Bartlett, J. Chem. Phys. 151, 164117 (2019)]. The inclusion of the quadrupole contributions (and all the other contributions through second order in the case of TI-EOM-CCSD) enables us to obtain the intensities for the pre-edge transitions in the metal K-edge spectra, which are dipole inactive. The TI-EOM-CCSD and TD-EOM-CCSD spectra of Ti4+ atoms are used to showcase the implementation of the second-order oscillator strengths. The origin of 1s → e and 1s → t2 in core spectra from iron tetrachloride and titanium tetrachloride is discussed and compared with the experiment.

Index of multi-determinantal and multi-reference character in coupled-cluster theory.

A full configuration interaction calculation (FCI) ultimately defines the innate molecular orbital description of a molecule. Its density matrix and the natural orbitals obtained from it quantify the difference between having N-dominantly occupied orbitals in a reference determinant for a wavefunction to describe N-correlated electrons and how many of those N-electrons are left to the remaining virtual orbitals. The latter provides a measure of the multi-determinantal character (MDC) required to be in a wavefunction. MDC is further split into a weak correlation part and a part that indicates stronger correlation often called multi-reference character (MRC). If several virtual orbitals have high occupation numbers, then one might argue that these additional orbitals should be allowed to have a larger role in the calculation, as in MR methods, such as MCSCF, MR-CI, or MR-coupled-cluster (MR-CC), to provide adequate approximations toward the FCI. However, there are problems with any of these MR methods that complicate the calculations compared to the uniformity and ease of application of single-reference CC calculations (SR-CC) and their operationally single-reference equation-of-motion (EOM-CC) extensions. As SR-CC theory is used in most of today's "predictive" calculations, an assessment of the accuracy of SR-CC at some truncation of the cluster operator would help to quantify how large an issue MRC actually is in a calculation, and how it might be alleviated while retaining the convenient SR computational character of CC/EOM-CC. This paper defines indices that identify MRC situations and help assess how reliable a given calculation is.

Prevalence of scabies in long-term care hospitals in South Korea.

BACKGROUND: Scabies is a common contagious skin disease. With the economic growth in South Korea, the incidence of scabies has decreased. However, with the recent advancements in medical facilities, mainly the establishment of long-term care hospitals (LTCHs), scabies is now considered an emerging public health problem. METHODOLOGY/PRINCIPAL FINDINGS: To examine the prevalence and management of scabies in LTCHs in South Korea, we contacted all 1,336 LTCHs registered at the Health Insurance Review and Assessment Service in South Korea in 2018. A total of 110 LTCHs completed a questionnaire, and we analyzed their responses. In the last 5 years, 71.8% (79/110) of LTCHs had a high incidence of scabies (suspected/confirmed cases). Usually, patients aged older than 80 years (45.5%) were diagnosed with the disease, with more women being affected than men. Only 30.0% of the patients were transferred to scabies-restricted rooms, and very few LTCHs (7.0%) had special departments for scabies. Fifty-five (61.1%) of 90 LTCHs reported contact between scabies patients and nurses, nurse aides, caregivers, and other employees (hereinafter, referred to as primary exposure), with 29 (32.2%) LTCHs reporting infections due to primary exposure. The most common challenges in managing scabies were patient isolation (47.8%), diagnosis (31.1%), management of individuals exposed to an individual with scabies (17.8%), lack of staff for managing the patients (16.7%), and treatment (11.1%). CONCLUSIONS: The incidence rate of scabies in LTCHs in South Korea has increased. Regular and enhanced staff training is needed, considering that most hospitals rarely focused on the handling of equipment and furniture used by scabies patients and on educating their healthcare staff. These findings can be used to develop various strategies to reduce the prevalence of scabies.

Equation of motion coupled-cluster for core excitation spectra: Two complementary approaches.

This paper presents core excitation spectra from coupled-cluster (CC) theory obtained from both a time-independent and a new time-dependent formalism. The conventional time-independent CC formulation for excited states is the equation-of-motion (EOM-CC) method whose eigenvalues and eigenvectors describe the core excited states. An alternative computational procedure is offered by a time-dependent CC description. In that case, the dipole transition operator is expressed in the CC effective Hamiltonian form and propagated with respect to time. The absorption spectrum is obtained from the CC dipole autocorrelation function via a Fourier transformation. Comparisons are made among the time-dependent results obtained from second-order perturbation theory, to coupled cluster doubles and their linearized forms (CCD and LCCD), to CC singles and doubles (CCSD) and the linearized form (LCCSD). In the time-independent case, considerations of triples (EOM-CCSDT) and quadruples (EOM-CCSDTQ) are used to approach sub-electron volt accuracy. A particular target is the allyl radical, as an example of an open-shell molecule. As the results have to ultimately be the same, the two procedures offer a complementary approach toward analyzing experimental results.

Vibronic structure and predissociation dynamics of 2-methoxythiophenol (S1): The effect of intramolecular hydrogen bonding on nonadiabatic dynamics.

Vibronic spectroscopy and the S-H bond predissociation dynamics of 2-methoxythiophenol (2-MTP) in the S1 (ππ*) state have been investigated for the first time. Resonant two-photon ionization and slow-electron velocity map imaging (SEVI) spectroscopies have revealed that the S1-S0 transition of 2-MTP is accompanied with the planar to the pseudoplanar structural change along the out-of-plane ring distortion and the tilt of the methoxy moiety. The S1 vibronic bands up to their internal energy of ∼1000 cm-1 are assigned from the SEVI spectra taken via various S1 vibronic intermediate states with the aid of ab initio calculations. Intriguingly, Fermi resonances have been identified for some vibronic bands. The S-H bond breakage of 2-MTP occurs via tunneling through an adiabatic barrier under the S1/S2 conical intersection seam, and it is followed by the bifurcation into either the adiabatic or nonadiabatic channel at the S0/S2 conical intersection where the diabatic S2 state (πσ*) is unbound with respect to the S-H bond elongation coordinate, giving the excited (Ã) or ground (X̃) state of the 2-methoxythiophenoxy radical, respectively. Surprisingly, the nonadiabatic transition probability at the S0/S2 conical intersection, estimated from the velocity map ion images of the nascent D fragment from 2-MTP-d1 (2-CH3O-C6H4SD) at the S1 zero-point energy level, is found to be exceptionally high to give the X̃/à product branching ratio of 2.03 ± 0.20, which is much higher than the value of ∼0.8 estimated for the bare thiophenol at the S1 origin. It even increases to 2.33 ± 0.17 at the ν45 2 mode (101 cm-1) before it rapidly decays to 0.69 ± 0.05 at the S1 internal energy of about 2200 cm-1. This suggests that the strong intramolecular hydrogen bonding of S⋯D⋯OCH3 in 2-MTP at least in the low S1 internal energy region should play a significant role in localizing the reactive flux onto the conical intersection seam. The minimum energy pathway calculations (second-order coupled-cluster resolution of the identity or time-dependent-density functional theory) of the adiabatic S1 state suggest that the intimate dynamic interplay between the S-H bond cleavage and intramolecular hydrogen bonding could be crucial in the nonadiabatic surface hopping dynamics taking place at the conical intersection.

Low scaling EOM-CCSD and EOM-MBPT(2) method with natural transition orbitals.

A low-scaling method is presented for the equation-of-motion coupled-cluster theory with single and double (EOM-CCSD) excitations and its second-order many-body perturbation theory [EOM-MBPT(2)] approximations. For a simple description of an excited state, the particular orbitals, ϕ I and ϕ à , are selected from the natural transition orbitals (NTOs, ϕ ), where I and à refer to NTO occupied and virtual orbital indices. They are chosen based on the largest eigenvalues of the transition density matrix. We expect the ϕ I and ϕ à pair to be dominant in representing excited states in EOM calculations. Therefore, the double excitation vector, R 2 which scale as ∼O 2 V 2, can be modified to keep only a few dominant excitations. Our work indicates that the most important contributions of the R 2 vector define smaller subspaces that scale as ∼OV, ∼O 2 V, and ∼OV 2, where O and V refer to the occupied and virtual orbitals in the NTO basis. Thus, the scaling for the EOM part becomes ∼M 5. The energy changes due to R 2 truncation are small (the mean average deviation from untruncated EOM-CCSD is ∼0.03 eV). We show that this approach works relatively well with various types of NTOs, ranging from configuration singles to time-dependent density functional theory making ∼M 5 scaling calculations possible with the use of MBPT(2) as the reference state.

Self-Consistent Constricted Variational Theory RSCF-CV(∞)-DFT and Its Restrictions To Obtain a Numerically Stable ΔSCF-DFT-like Method: Theory and Calculations for Triplet States.

In this paper, the relaxed self-consistent field infinite order constricted variational density functional theory (RSCF-CV(∞)-DFT) for triplet calculations is presented. Here, we focus on two main features of our implementation. First, as an extension of our previous work by Krykunov and Ziegler ( J. Chem. Theory Comput. 2013 , 9 , 2761 ), the optimization of the transition matrix representing the orbital transition is implemented and applied for vertical triplet excitations. Second, restricting the transition matrix, we introduce RSCF-CV(∞)-DFT-based numerically stable ΔSCF-DFT-like methods, the most general of them being SVD-RSCF-CV(∞)-DFT. The reliability of the different methods, RSCF-CV(∞)-DFT and its restricted versions, is examined using the benchmark test set of Silva-Junior et al. ( J. Chem. Phys. 2008 , 129 , 104103 ). The obtained excitation energies validate our approach and implementation for RSCF-CV(∞)-DFT and also show that SVD-RSCF-CV(∞)-DFT mimics very well ΔSCF-DFT, as the root-mean-square deviations between these methods are less than 0.1 eV for all functionals examined.

Dynamic Symmetry Breaking Hidden in Fano Resonance of a Molecule: S1 State of Diazirine Using Quantum Wave Packet Propagation.

Fano resonance in the predissociation of the S1 state of diazirine was studied by applying a time-dependent wave packet propagation method, and dynamic symmetry breaking (DSB) around the stationary structure of S1 was disclosed in a detailed analysis of this theoretical result. The DSB was found to originate in coupling between the asymmetric C-N2 stretching and CH2 wagging modes, suggesting that there is a slight time gap between ring opening and the concurrent dragging of two H atoms of the CH2 moiety. Although the depth of the double well due to DSB is just 0.011 eV, its presence noticeably affects the early time dynamics and observed spectrum.

Constricted variational density functional theory for spatially clearly separated charge-transfer excitations.

Constricted Variational Density Functional Theory (CV-DFT) is known to be one of the successful methods in predicting charge-transfer excitation energies. In this paper, we apply the CV-DFT method to the well-known model systems ethylene-tetrafluoroethylene (C2H4 × C2F4) and the zincbacteriochlorin-bacteriochlorin complex (ZnBC-BC). The analysis of the CV-DFT energies enables us to understand the -1/R charge-transfer behaviour in CV-DFT for large separation distances R. With this we discuss the importance of orbital relaxations using the relaxed version of CV(∞)-DFT, the R-CV(∞)-DFT method. Possible effects of the optimization of the transition matrix for the relaxed self-consistent field version of CV(∞)-DFT, RSCF-CV(∞)-DFT in the case of large fragment separations are shown and we introduce two possible gradient restrictions to avoid the unwanted admixing of other transitions.

Constricted Variational Density Functional Theory Approach to the Description of Excited States.

We review the theoretical foundation of constricted variational density functional theory and illustrate its scope through applications.

Rapid Dye Regeneration Mechanism of Dye-Sensitized Solar Cells.

During the light-harvesting process of dye-sensitized solar cells (DSSCs), the hole localized on the dye after the charge separation yields an oxidized dye, D(+). The fast regeneration of D(+) using the redox pair (typically the I(-)/I3(-) couple) is critical for the efficient DSSCs. However, the kinetic processes of dye regeneration remain uncertain, still promoting vigorous debates. Here, we use molecular dynamics simulations to determine that the inner-sphere electron-transfer pathway provides a rapid dye regeneration route of ∼4 ps, where penetration of I(-) next to D(+) enables an immediate electron transfer, forming a kinetic barrier. This explains the recently reported ultrafast dye regeneration rate of a few picoseconds determined experimentally. We expect that our MD based comprehensive understanding of the dye regeneration mechanism will provide a helpful guideline in designing TiO2-dye-electrolyte interfacial systems for better performing DSSCs.

Nuclear motion captured by the slow electron velocity imaging technique in the tunnelling predissociation of the S1 methylamine.

Predissociation dynamics of methylamines (CH(3)NH(2) and CH(3)ND(2)) on the first electronically excited states are studied using the slow-electron velocity imaging method to unravel the multi-dimensional nature of the N-H(D) chemical bond dissociation reaction which occurs via tunnelling. The nearly free internal rotation around the C-N bond axis is found to be strongly coupled to the reaction pathway, revealing nuclear motions actively involved in the tunnelling process on the S(1) potential energy surfaces. The vibrational state-resolved energy and angular distributions of photoelectron, ejected from the ionization mediated by the metastable intermediate S(1) state provide a unique way for mapping the predissociative potential energy surfaces.

Ambient carbon dioxide capture by boron-rich boron nitride nanotube.

Carbon dioxides (CO(2)) emitted from large-scale coal-fired power stations or industrial manufacturing plants have to be properly captured to minimize environmental side effects. From results of ab initio calculations using plane waves [PAW-PBE] and localized atomic orbitals [ONIOM(wB97X-D/6-31G*:AM1)], we report strong CO(2) adsorption on boron antisite (B(N)) in boron-rich boron nitride nanotube (BNNT). We have identified two adsorption states: (1) A linear CO(2) molecule is physically adsorbed on the B(N), showing electron donation from the CO(2) lone-pair states to the B(N) double-acceptor state, and (2) the physisorbed CO(2) undergoes a carboxylate-like structural distortion and C═O π-bond breaking due to electron back-donation from B(N) to CO(2). The CO(2) chemisorption energy on B(N) is almost independent of tube diameter and, more importantly, higher than the standard free energy of gaseous CO(2) at room temperature. This implies that boron-rich BNNT could capture CO(2) effectively at ambient conditions.

Accurate ab initio binding energies of the benzene dimer.

Accurate binding energies of the benzene dimer at the T and parallel displaced (PD) configurations were determined using the single- and double-coupled cluster method with perturbative triple correction (CCSD(T)) with correlation-consistent basis sets and an effective basis set extrapolation scheme recently devised. The difference between the estimated CCSD(T) basis set limit electronic binding energies for the T and PD shapes appears to amount to more than 0.3 kcal/mol, indicating the PD shape is a more stable configuration than the T shape for this dimer in the gas phase. This conclusion is further strengthened when a vibrational zero-point correction to the electronic binding energies of this dimer is made, which increases the difference between the two configurations to 0.4-0.5 kcal/mol. The binding energies of 2.4 and 2.8 kcal/mol for the T and PD configurations are in good accord with the previous experimental result from ionization potential measurement.

Epidemiological survey of Paragonimus westermani in Ulchin County, Kyungpook Province, Korea.

In order to determine the epidemiological pattern of the Paragonimus westermani, the infestation rates of the cercarial and metacercarial larvae of digenetic trematodes in the snail and crayfish host, and the prevalence of Paragonimus westermani among the residents in the vicinity of the stream Namdae, Wyangpi, and Kwang, in the Ulchin county, Kyungpook province were studied from March to October in 1984. The population density of the snails per square meter of the habitats ranged form 5 to 25, with average of 15. Among the seven habitats, one, Ducheon, had snails infested with the cercariae of Paragonimus westermani, and the proportion of infested snails was very low, the average being 0.152 per thousand. Of six hundred and four crayfish examined, 113 or 18.7 per cent harboured the encysted larvae of Paragonimus westermani. A high infestation rate for the metacercariae in the crayfish was found in two habitats; 39.0 per cent in the Ducheon and 21.6 per cent in the Sokwang. The prevalence of Paragonimus westermani among the residents in Ulchin county was relatively high, 25.8 per cent by the Paragonimus intradermal tests and the difference in the rate of infection between males and females was found to be significant(t>2). The results obtained in this study indicate that endemic foci of Paragonimus westermani exist in Ulchin county and the prevalence of this lung fluke among the residents is relatviely high.