30 research outputs found
State-Specific Configuration Interaction for Excited States
We introduce and benchmark a new systematically improvable route for
excited-state calculations, state-specific configuration interaction
(CI). Starting with a reference built from optimized configuration
state functions, separate CI calculations are performed for each targeted state
(hence state-specific orbitals and determinants). Accounting for single and
double excitations produces the CISD model, which can be improved with
second-order Epstein-Nesbet perturbation theory (CISD+EN2) or a
posteriori Davidson corrections (CISD+Q). These models were gauged
against a vast and diverse set of 294 reference excitation energies. We have
found that CI is significantly more accurate than standard
ground-state-based CI, whereas close performances were found between
CISD and EOM-CC2, and between CISD+EN2 and EOM-CCSD. For larger
systems, CISD+Q delivers more accurate results than EOM-CC2 and
EOM-CCSD. The CI route can handle challenging multireference problems,
singly- and doubly-excited states, from closed- and open-shell species, with
overall comparable accuracy, and thus represents a promising alternative to
more established methodologies.Comment: 10 pages, 2 figures (supplementary information available
Seniority and Hierarchy Configuration Interaction for Radicals and Excited States
Hierarchy configuration interaction (hCI) has been recently introduced as an
alternative configuration interaction (CI) route combining excitation degree
and seniority number, which showed to efficiently recover both dynamic and
static correlations for closed-shell molecular systems
[\href{https://doi.org/10.1021/acs.jpclett.2c00730}{\textit{J.~Phys.~Chem.~Lett.}~\textbf{2022},
\textit{13}, 4342}]. Here, we generalize hCI for an arbitrary reference
determinant, allowing calculations for radicals and for excited states in a
state-specific way. We gauge this route against excitation-based CI (eCI) and
seniority-based CI (sCI) by evaluating how different ground-state properties of
radicals converge to the full CI limit. We find that hCI outperforms or matches
eCI, whereas sCI is far less accurate, in line with previous observations for
closed-shell molecules. Employing the second-order Epstein-Nesbet (EN2)
perturbation theory as a correction significantly accelerates the convergence
of hCI and eCI. We further explore various hCI and sCI models to calculate
excitation energies of closed- and open-shell systems. Our results underline
that both the choice of the reference determinant and the set of orbitals drive
the fine balance between correlation of ground and excited states.
State-specific hCI2 and higher order models perform similarly to their eCI
counterparts, whereas lower orders of hCI deliver poor results, unless
supplemented by the EN2 correction, which substantially improves their
accuracy. In turn, sCI1 produces decent excitation energies for radicals,
encouraging the development of related seniority-based coupled-cluster methods.Comment: 18 pages, 5 figure
Espalhamento elåstico de elétrons por isÎmeros de C2H2CL2, C4H4N2 e C3H3NX (X=NH, O, S)
Orientador: Prof. Dr. MĂĄrcio Henrique Franco BettegaDissertação (mestrado) - Universidade Federal do ParanĂĄ, Setor de CiĂȘncias Exatas, Curso de PĂłs-Graduação em FĂsica. Defesa: Curitiba, 27/02/2012Bibliografia: fls. 95-103Resumo: Neste trabalho apresentamos seçÔes de choque de espalhamento elĂĄstico de elĂ©trons pelos isĂŽmeros C2H2Cl2, C4H4N2 e C3H3NX (onde X=NH, O, S). Os cĂĄlculos de espalhamento foram efetuados na aproximação de nĂșcleos fixos, com uso do mĂ©todo multicanal de Schwinger implementado com pseudopotenciais de Bachelet, Hamann e Schlšuter, e nas aproximaçÔes estĂĄtico-troca e estĂĄtico-troca mais polarização. Para os trĂȘs isĂŽmeros C2H2Cl2 apresentamos seçÔes de choque integrais, de trasferĂȘncia de momento e diferenciais. O procedimento Born-closure Ă© empregado a fim de levar em conta o efeito do momento de dipolo. As ressonĂąncias encontradas tiveram suas posiçÔes comparadas aos valores experimentais. Discutimos ainda o efeito isĂŽmero, que consiste em diferenças apresentadas pelas seçÔes de choque de cada molĂ©cula. Para os trĂȘs isĂŽmeros C4H4N2 apresentamos seçÔes de choque integrais. As ressonĂąncias sĂŁo identificadas e caracterizadas. Suas posiçÔes sĂŁo comparadas com os resultados encontrados na literatura. CĂĄlculos de estrutura eletrĂŽncia tambĂ©m sĂŁo efetuados para ajudar na interpretação dos resultados. Os seis sistemas C3H3NX, onde X=NH, O, S tiveram suas seçÔes de choque integrais apresentadas. As ressonĂąncias sĂŁo identificadas e comparadas ao resultado experimental existente. TambĂ©m discutimos as diferenças nas caracterĂsticas das ressonĂąncias de cada molĂ©cula. Para todos os sistemas estudados, nossos resultados concordam bem com os reportados experimentalmente. Foi desenvolvido ainda um novo esquema para a construção do espaço de configuraçÔes.Abstract: In this work we present cross sections for elastic electron scattering from C2H2Cl2, C4H4N2 and C3H3NX (where X=NH, O, S) isomers. The scattering calculations were done in the fixed-nuclei approximation, with the Schwinger multichannel method implemented with pseudopotentials of Bachelet, Hamann and Schlšuter, in the static-exchange and in the static-exchange plus polarization approximations. For the three C2H2Cl2 isomers, we present integral, momentum transfer and differential cross sections. The Born-closure procedure is employed in order to consider the dipole moment effect. The identified resonances had their positions compared to the experimental values. We also discuss the isomer effect, which consists in differences presented in the cross sections of each molecule. For the three C4H4N2 isomers, we present integral cross sections. The resonances are identified and characterized. Their positions are compared to the results found in the literature. Electronic structure calculations were done as well, in order to help in the interpretation of the results. The six systems C3H3NX, where X=NH, O, S had their integral cross sections presented. The resonances are identified and compared to the existing experimental result. We also discuss the differences in the characteristics of the resonances of each molecule. For all the studied systems, our results agree well with the ones reported experimentally. We also developed a new scheme for the construction of the configuration space. i
Excited States From State Specific Orbital Optimized Pair Coupled Cluster
The pair coupled cluster doubles (pCCD) method (where the excitation manifold
is restricted to electron pairs) has a series of interesting features. Among
others, it provides ground-state energies very close to what is obtained with
doubly-occupied configuration interaction (DOCI), but with polynomial cost
(compared with the exponential cost of the latter). Here, we address whether
this similarity holds for excited states, by exploring the symmetric
dissociation of the linear \ce{H4} molecule. When ground-state Hartree-Fock
(HF) orbitals are employed, pCCD and DOCI excited-state energies do not match,
a feature that is assigned to the poor HF reference. In contrast, by optimizing
the orbitals at the pCCD level (oo-pCCD) specifically for each excited state,
the discrepancies between pCCD and DOCI decrease by one or two orders of
magnitude. Therefore, the pCCD and DOCI methodologies still provide comparable
energies for excited states, but only if suitable, state-specific orbitals are
adopted. We also assessed whether a pCCD approach could be used to directly
target doubly-excited states, without having to resort to the
equation-of-motion (EOM) formalism. In our oo-pCCD model, excitation
energies were extracted from the energy difference between separate oo-pCCD
calculations for the ground state and the targeted excited state. For a set
comprising the doubly-excited states of \ce{CH+}, \ce{BH}, nitroxyl,
nitrosomethane, and formaldehyde, we found that oo-pCCD provides quite
accurate excitation energies, with root mean square deviations (with respect to
full configuration interaction results) lower than CC3 and comparable to
EOM-CCSDT, two methods with much higher computational cost.Comment: 12 pages, 4 figure
Reference Vertical Excitation Energies for Transition Metal Compounds
To enrich and enhance the diversity of the \textsc{quest} database of
highly-accurate excitation energies
[\href{https://doi.org/10.1002/wcms.1517}{V\'eril \textit{et al.},
\textit{WIREs Comput.~Mol.~Sci.}~\textbf{11}, e1517 (2021)}], we report
vertical transition energies in transition metal compounds. Eleven diatomic
molecules with singlet or doublet ground state containing a fourth-row
transition metal (\ce{CuCl}, \ce{CuF}, \ce{CuH}, \ce{ScF}, \ce{ScH}, \ce{ScO},
\ce{ScS}, \ce{TiN}, \ce{ZnH}, \ce{ZnO}, and \ce{ZnS}) are considered and the
corresponding excitation energies are computed using high-level coupled-cluster
(CC) methods, namely CC3, CCSDT, CC4, and CCSDTQ, as well as
multiconfigurational methods such as CASPT2 and NEVPT2. In some cases, to
provide more comprehensive benchmark data, we also provide full configuration
interaction estimates computed with the \textit{"Configuration Interaction
using a Perturbative Selection made Iteratively"} (CIPSI) method. Based on
these calculations, theoretical best estimates of the transition energies are
established in both the aug-cc-pVDZ and aug-cc-pVTZ basis sets. This allows us
to accurately assess the performance of CC and multiconfigurational methods for
this specific set of challenging transitions. Furthermore, comparisons with
experimental data and previous theoretical results are also reported.Comment: 17 pages, 3 figure
Ground- and Excited-State Dipole Moments and Oscillator Strengths of Full Configuration Interaction Quality
We report ground- and excited-state dipole moments and oscillator strengths
(computed in different ``gauges'' or representations) of full configuration
interaction (FCI) quality using the selected configuration interaction method
known as \textit{Configuration Interaction using a Perturbative Selection made
Iteratively} (CIPSI). Thanks to a set encompassing 35 ground- and excited-state
properties computed in 11 small molecules, the present near-FCI estimates allow
us to assess the accuracy of high-order coupled-cluster (CC) calculations
including up to quadruple excitations. In particular, we show that incrementing
the excitation degree of the CC expansion (from CCSD to CCSDT or from CCSDT to
CCSDTQ) reduces the average error with respect to the near-FCI reference values
by approximately one order of magnitude.Comment: 14 pages, 8 figures (supporting information available
Excited states of bromopyrimidines probed by vuv photoabsorption spectroscopy and theoretical calculations
Funding Information: Funding: J.P.âd.âS. acknowledges the Portuguese National Funding Agency FCTâMCTES through PhD grant PD/BD/142768/2018, together with R.R., J.A., M.M. and F.F.d.S. through the researcher grant PTDC/FISâAQM/31215/2017 and AIL through the researcher grant PTDC/FISâ AQM/31281/2017. This work was also supported by Radiation Biology and Biophysics Doctoral Training Programme (RaBBiT, PD/00193/2012); UIDB/04378/2020 (UCIBIO); and UIDB/00068/2020 (CEFITEC). The research leading to this result has been supported by the project CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020.We report absolute photoabsorption cross sections for gasâphase 2â and 5â bromopyrimidine in the 3.7â10.8 eV energy range, in a joint theoretical and experimental study. The measurements were carried out using highâresolution vacuum ultraviolet synchrotron radiation, with quantum chemical calculations performed through the nuclear ensemble approach in combination with timeâdependent density functional theory, along with additional FranckâCondon HerzbergâTeller calculations for the first absorption band (3.7â4.6 eV). The cross sections of both bromopyrimidines are very similar below 7.3 eV, deviating more substantially from each other at higher energies. In the 7.3â9.0 eV range where the maximum crossâsection is found, a single and broad band is observed for 5âbromopyrimidine, while more discernible features appear in the case of 2âbromopyrimidine. Several Ï* â Ï transitions account for the most intense bands, while weaker ones are assigned to transitions involving the nitrogen and bromine lone pairs, the antibonding Ï*Br orbital, and the lowerâlying Rydberg states. A detailed comparison with the available photoâabsorption data of bromobenzene is also reported. We have found significant differences regarding the main absorption band, which is more peaked in bromobenzene, becoming broader and shifting to higher energies in both bromopyrimidines. In addition, there is a significant suppression of vibrational structures and of Rydberg states in the pair of isomers, most noticeably for 2âbromopyrimidine.publishersversionpublishe
Electron Scattering from 1-Methyl-5-Nitroimidazole: Cross-Sections forModeling Electron Transport through Potential Radiosensitizers
In this study, we present a complete set of electron scattering cross-sections from 1-Methyl-5- Nitroimidazole (1M5NI) molecules for impact energies ranging from 0.1 to 1000 eV. This information is relevant to evaluate the potential role of 1M5NI as a molecular radiosensitizers. The total electron scattering cross-sections (TCS) that we previously measured with a magnetically confined electron transmission apparatus were considered as the reference values for the present analysis. Elastic scattering cross-sections were calculated by means of two different schemes: The Schwinger multichannel (SMC) method for the lower energies (below 15 eV) and the independent atom model-based screening-corrected additivity rule with interferences (IAM-SCARI) for higher energies (above 15 eV). The latter was also applied to calculate the total ionization cross-sections, which were complemented with experimental values of the induced cationic fragmentation by electron impact. Double differential ionization cross-sections were measured with a reaction microscope multi-particle coincidence spectrometer. Using a momentum imaging spectrometer, direct measurements of the anion fragment yields and kinetic energies by the dissociative electron attachment are also presented. Cross-sections for the other inelastic channels were derived with a self-consistent procedure by sampling their values at a given energy to ensure that the sum of the cross-sections of all the scattering processes available at that energy coincides with the corresponding TCS. This cross-section data set is ready to be used for modelling electron-induced radiation damage at the molecular level to biologically relevant media containing 1M5NI as a potential radiosensitizer. Nonetheless, a proper evaluation of its radiosensitizing effects would require further radiobiological experiments
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 Ï-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
Dynamics of halogenated biomolecules transient ions
Apresentamos resultados para o espectro de Ăąnions e para a dinĂąmica vibracional induzida pela captura eletrĂŽnica, para uma sĂ©rie de molĂ©culas halogenadas. Os estados do Ăąnion foram caracterizados por meio de cĂĄlculos de espalhamento elĂĄstico realizados na aproximação de nĂșcleos fixos, efetuados com o mĂ©todo multicanal de Schwinger com pseudopotenciais. CĂĄlculos de dinĂąmica quĂąntica do pacote de onda nuclear foram realizados a partir da propagação desse nas superfĂcies de energia potencial descritas na aproximação local. Foram alvo de estudo clorometano, cloroeteno, uracila, 5-fluorouracila, 5-clorouracila, 5-bromouracila, 5-iodouracila, 6-clorouracila, 2-tiouracila, adenina, 2-cloroadenina e 8-cloroadenina. Para o clorometano, calculamos seçÔes de choque de excitação vibracional do estiramento C-Cl, e mostramos que esse modo Ă© prontamente ativado pela presença da carga adicional. Para o cloroeteno, demonstramos que o mecanismo direto de dissociação Ă© muito ineficiente, e tambĂ©m revelamos a interessante topologia de suas superfĂcies de energia potencial complexas. Os derivados de uracila apresentaram ricos espectros aniĂŽnicos, contando com trĂȘs estados delocalizados p*, um estado s* localizado na ligação do ĂĄtomo substituĂdo com o anel, alĂ©m de um estado ligado por dipolo. No geral, as energias obtidas estĂŁo em Ăłtimo acordo com os valores experimentais. AnĂĄlises dos espectros aniĂŽnicos e das superfĂcies de energia potencial indicam mecanismos em que o Ăąnion Ă© formado numa ressonĂąncia p* de longo tempo de vida, que muda de carĂĄter para o estado dissociativo s*. Na 5-clorouracila em particular, esse acoplamento Ă© mediado pelo movimento do cloro para fora do plano molecular. Conforme os estados do Ăąnion progressivamente estabilizam com o aumento do nĂșmero atĂŽmico do halogĂȘnio, os acoplamentos tornam-se mais favorĂĄveis, o que responde pelas crescentes seçÔes de choque de dissociação. Nossos resultados explicam muitas das caracterĂsticas observadas na captura eletrĂŽnica dissociativa de halouracilas, fornecendo uma base teĂłrica para sua habilidade radiossensibilizadora. Nas cloroadeninas, encontramos uma ressonĂąncia s* e quatro ressonĂąncias p*. NĂłs sustentamos que elas tambĂ©m poderiam atuar como potenciais radiossensibilizadores.We present results on the anion spectrum and on the vibrational dynamics induced by electron capture, for a series of halogenated molecules. The anion states were characterized by means of elastic scattering calculations, in the fixed nuclei approximation, performed with the Schwinger multichannel method with pseudopotentials. Quantum dynamics calculations of the nuclear wavepacket were performed by the propagation on potential energy surfaces described in the local approximation. The target molecules comprise chloromethane, chloroethene, uracil, 5-fluorouracil, 5-chlorouracil, 5-bromouracil, 5-iodouracil, 6-chlorouracil, 2-thiouracil, adenine, 2-chloroadenine and 8-chloroadenine. For chloromethane we computed vibrational excitation cross sections for the C-Cl stretching, and showed this mode is promptly activated by the presence of the extra charge. For chloroethene, we demonstrated that the direct mechanism of dissociation is very inefficient, and also revealed the interesting topology of its complex potential energy surfaces. The derivatives of uracil presented rich anionic spectra, as each one has three p* delocalized states, a s* state located at the bond between the substituted atom and the ring, and a dipole bound state. Overall, the obtained energetics are in very good agreement with experimental data. Analysis of the anionic spectra and the potential energy surfaces indicate mechanisms in which the anion is formed in a long-lived p* resonance and changes its character to the dissociative s* state. For 5-chlorouracil in particular, this coupling is mediated by an out-of-plane movement of the chlorine atom. As the anion states progressively stabilize as the halogen atomic number increases, the couplings become more favorable, and account for the increasing dissociation cross sections. Our results explain many of the observed features of dissociative electron attachment to halouracils, providing a theoretical basis for its radiosensitizing ability. In chloroadenines, we found a s* resonance and four p* resonances. We support they could also act as potential radiosensitizers