10 results on '"Aucar, Ignacio Agustín"'
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2. Relativistic relationship between nuclear-spin-dependent parity-violating NMR shielding and nuclear spin-rotation tensors
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Aucar, Ignacio Agustín, Jofré, Mariano Tomás Colombo, and Aucar, Gustavo Adolfo
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Physics - Chemical Physics - Abstract
It is known that the nuclear-spin-dependent parity-violating contributions to the NMR shielding and the nuclear spin-rotation tensors (${\bf \sigma}^{PV}$ and ${\bf M}^{PV}$, respectively) are formally related each other within the non-relativistic (NR) regime. Such a formal relationship is not any longer valid within the relativistic domain. A new more general formal relationship, that is valid within the relativistic framework is shown here, being developed through the use of the LRESC model. The formalism of polarization propagators is applied to write the different contributions to both properties within both regimes, relativistic and NR. In the relativistic regime the Dirac-Coulomb Hamiltonian was selected as the unperturbed Hamiltonian. Theoretical developments together with results of calculations performed on the H$_2X_2$ series of molecules ($X =$ $^{17}$O, $^{33}$S, $^{77}$Se, $^{125}$Te and $^{209}$Po) show that also within the relativistic regime there is a close relationship between the parity-violation contributions to both properties. In particular, spin-dependent contributions are the most important in the four-component calculations of electroweak effects on the isotropic values of both tensors, ${\bf \sigma}^{PV}$ and ${\bf M}^{PV}$, being also responsible for the breakdown of the previously mentioned NR relationship among them. This last relationship is still fulfilled when the scalar-relativistic effects are considered., Comment: 17 pages, 16 figures
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- 2022
3. The DIRAC code for relativistic molecular calculations
- Author
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Saue, Trond, Bast, Radovan, Gomes, Andre Severo Pereira, Jensen, Hans Jørgen Aagaard, Visscher, Lucas, Aucar, Ignacio Agustın, Di Remigio, Roberto, Dyall, Kenneth G., Eliav, Ephraim, Faßhauer, Elke, Fleig, Timo, Halbert, Loıc, Hedegård, Erik Donovan, Helmich-Paris, Benjamin, Iliaš, Miroslav, Jacob, Christoph R., Knecht, Stefan, Laerdahl, Jon K, Vidal, Marta L., Nayak, Malaya K, Olejniczak, Małgorzata, Olsen, Jógvan Magnus Haugaard, Pernpointner, Markus, Senjean, Bruno, Shee, Avijit, Sunaga, Ayaki, and van Stralen, Joost N. P.
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Physics - Chemical Physics - Abstract
DIRAC is a freely distributed general-purpose program system for 1-, 2- and 4-component relativistic molecular calculations at the level of Hartree--Fock, Kohn--Sham (including range-separated theory), multiconfigurational self-consistent-field, multireference configuration interaction, coupled cluster and electron propagator theory. At the self-consistent-field level a highly original scheme, based on quaternion algebra, is implemented for the treatment of both spatial and time reversal symmetry. DIRAC features a very general module for the calculation of molecular properties that to a large extent may be defined by the user and further analyzed through a powerful visualization module. It allows the inclusion of environmental effects through three different classes of increasingly sophisticated embedding approaches: the implicit solvation polarizable continuum model, the explicit polarizable embedding, and frozen density embedding models. DIRAC was one of the earliest codes for relativistic molecular calculations and remains a reference in its field., Comment: The following article has been submitted to The Journal of Chemical Physics. After it is published, it will be found at https://aip.scitation.org/toc/jcp/current
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- 2020
- Full Text
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4. Relativistic study of parity-violating nuclear spin-rotation tensors.
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Aucar, Ignacio Agustín and Borschevsky, Anastasia
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WEAK interactions (Nuclear physics) , *SPIN-orbit interactions , *ELECTROWEAK interactions , *DIHEDRAL angles , *PARITY (Physics) - Abstract
We present a four-component relativistic approach to describe the effects of the nuclear spin-dependent parity-violating (PV) weak nuclear forces on nuclear spin-rotation (NSR) tensors. The formalism is derived within the four-component polarization propagator theory based on the Dirac–Coulomb Hamiltonian. Such calculations are important for planning and interpretation of possible future experiments aimed at stringent tests of the standard model through the observation of PV effects in NSR spectroscopy. An exploratory application of this theory to the chiral molecules H2X2 (X = 17O, 33S, 77Se, 125Te, and 209Po) illustrates the dramatic effect of relativity on these contributions. In particular, spin-free and spin–orbit effects are even of opposite signs for some dihedral angles, and the latter fully dominate for the heavier nuclei. Relativistic four-component calculations of isotropic nuclear spin-rotation constants, including parity-violating electroweak interactions, give frequency differences of up to 4.2 mHz between the H2Po2 enantiomers; on the nonrelativistic level of theory, this energy difference is 0.1 mHz only. [ABSTRACT FROM AUTHOR]
- Published
- 2021
- Full Text
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5. The DIRAC code for relativistic molecular calculations:The Journal of Chemical Physics
- Author
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Saue, Trond, Bast, Radovan, Gomes, André Severo Pereira, Jensen, Hans Jørgen Aa., Visscher, Lucas, Aucar, Ignacio Agustín, Di Remigio, Roberto, Dyall, Kenneth G., Eliav, Ephraim, Fasshauer, Elke, Fleig, Timo, Halbert, Loïc, Hedegård, Erik Donovan, Helmich-Paris, Benjamin, Iliaš, Miroslav, Jacob, Christoph R., Knecht, Stefan, Laerdahl, Jon K., Vidal, Marta L., Nayak, Malaya K., Olejniczak, Małgorzata, Olsen, Jógvan Magnus Haugaard, Pernpointner, Markus, Senjean, Bruno, Shee, Avijit, Sunaga, Ayaki, and van Stralen, Joost N. P.
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Physics::Chemical Physics - Abstract
DIRAC is a freely distributed general-purpose program system for one-, two-, and four-component relativistic molecular calculations at the level of Hartree–Fock, Kohn–Sham (including range-separated theory), multiconfigurational self-consistent-field, multireference configuration interaction, electron propagator, and various flavors of coupled cluster theory. At the self-consistent-field level, a highly original scheme, based on quaternion algebra, is implemented for the treatment of both spatial and time reversal symmetry. DIRAC features a very general module for the calculation of molecular properties that to a large extent may be defined by the user and further analyzed through a powerful visualization module. It allows for the inclusion of environmental effects through three different classes of increasingly sophisticated embedding approaches: the implicit solvation polarizable continuum model, the explicit polarizable embedding model, and the frozen density embedding model.
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- 2020
6. The DIRAC code for relativistic molecular calculations
- Author
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Saue, Trond, primary, Bast, Radovan, additional, Gomes, André Severo Pereira, additional, Jensen, Hans Jørgen Aa., additional, Visscher, Lucas, additional, Aucar, Ignacio Agustín, additional, Di Remigio, Roberto, additional, Dyall, Kenneth G., additional, Eliav, Ephraim, additional, Fasshauer, Elke, additional, Fleig, Timo, additional, Halbert, Loïc, additional, Hedegård, Erik Donovan, additional, Helmich-Paris, Benjamin, additional, Iliaš, Miroslav, additional, Jacob, Christoph R., additional, Knecht, Stefan, additional, Laerdahl, Jon K., additional, Vidal, Marta L., additional, Nayak, Malaya K., additional, Olejniczak, Małgorzata, additional, Olsen, Jógvan Magnus Haugaard, additional, Pernpointner, Markus, additional, Senjean, Bruno, additional, Shee, Avijit, additional, Sunaga, Ayaki, additional, and van Stralen, Joost N. P., additional
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- 2020
- Full Text
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7. Recent Developments in Absolute Shielding Scales for NMR Spectroscopy
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Aucar, Gustavo Adolfo and Aucar, Ignacio Agustín
- Subjects
Física Atómica, Molecular y Química ,RAMSEY–FLYGARE ,Ciencias Físicas ,RELATIVISTIC EFFECTS ,SPIN–ROTATION ,CIENCIAS NATURALES Y EXACTAS - Abstract
The absolute value of the nuclear magnetic shielding tensor, σ, cannot be directly measured as such, though at the moment experimental devices are available to obtain them by extrapolation to zero-density limit. This is the reason why since the beginning of NMR spectroscopy to obtain the experimental values of this parameter required the interplay betweentheory and experiments.Early this century it was experimentally probed that the Ramsey-Flygare model, largely used to obtain the most accurate semi-experimental values of shieldings, does not work well when heavy-atom containing molecules are considered. Then, the search for a relativistic relationship like Ramsey-Flygare became highly necessary. This search has taken some yearsfor being successful.Within the research program that focused on such a goal, the first important breakthrough was the finding of the relativistic Hamiltonian for spin-rotations interactions. Once this crucial step was overcome it was possible to develop some relativistic models, being the most accurate the so called M-V. We shall show here some of the most important stepsthat were given in the search for a relativistic relationship between σ and the spin-rotation tensor, M . We will describe the bottom-up procedure that make it possible, together with relativistic (four-component) theories of shieldings and spin-rotations that were published recently.Why it was important to find that new relationship? There are now experimental developments that permits direct measurements of absolute shieldings in gas-phase NMR spectroscopy, and so, to compare with. It is also possible to accurately calculate them by using four-component methodologies, from which very small effects like nuclear charge distribu-tion, Breit/Gaunt electron-electron interactions and QED effects are included. All of them will be highlighted in some detail.We at least shall stress the new understandings that arise from these developments together with some new challenges. Fil: Aucar, Gustavo Adolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Instituto de Modelado e Innovación Tecnológica. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Instituto de Modelado e Innovación Tecnológica; Argentina Fil: Aucar, Ignacio Agustín. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Instituto de Modelado e Innovación Tecnológica. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Instituto de Modelado e Innovación Tecnológica; Argentina
- Published
- 2019
8. Foundations of the LRESC model for response properties and some applications
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Aucar, Gustavo A., primary, Melo, Juan I., additional, Aucar, Ignacio Agustín, additional, and Maldonado, Alejandro F., additional
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- 2017
- Full Text
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9. Efectos de átomos pesados en parámetros magnéticos moleculares
- Author
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Aucar, Ignacio Agustín, Ruiz de Azúa, Martín C., and Gómez, Sergio Santiago
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Apantallamiento magnético nuclear ,Relación de Flygare ,Química cuántica relativista ,Física molecular ,Constante de acoplamiento espín nuclear-rotación molecular ,Modelo lRESC ,Hamiltoniano molecular ,Ecuación de Dirac ,Propiedades magnéticas ,Parámetros espectroscópicos moleculares - Abstract
Fil: Gómez, Sergio S. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Nordeste. Instituto de Modelado e Innovación Tecnológica. ; Argentina. Durante las últimas décadas se consideró como válida la relación de Flygare, que vincula las expresiones teóricas de dos parámetros espectroscópicos moleculares, deducida mediante un formalismo no relativista. Uno de ellos proviene de la espectroscopia de resonancia magnética nuclear (RMN), el apantallamiento magnético nuclear, mientras el otro se obtiene mediante técnicas de espectroscopia rotacional, y es la llamada constante de acoplamiento espín nuclear-rotación molecular. En esta Tesis se presenta un estudio teórico de la relación entre las formulaciones relativistas de ambas propiedades moleculares. Se propone una expresión teórica para el tensor de acoplamiento espín nuclear-rotación molecular (SR) en el que por primera vez se considera la dinámica electrono-ca dentro del marco relativista de Dirac; la dinámica nuclear, por su parte, se estudia según la formulación de Schrödinger. Para ello, se considera el Hamiltoniano molecular de un rotor rígido en el sistema de laboratorio, y los efectos de la rotación molecular se introducen a partir de los términos de la descomposición de Born-Oppenheimer que acoplan las dinámica electrónica y nuclear. Se analizan los efectos relativistas de retardo en las interacciones electrón-núcleo y electrón-electrón, que dan origen a las contribuciones de Breit. La diferencia entre las expresiones teóricas de ambos parámetros, deducidas en un marco relativista, constituye una primera demostración formal de que la relación de Flygare pierde vigencia en este contexto. Utilizando un modelo perturbativo que permite describir efectos relativistas, denominado respuesta lineal mediante la eliminación de pequeñas componentes (LRESC), se estudia en detalle la extensión de la validez de la relación no relativista. El modelo LRESC permite distinguir los efectos relativistas comunes a ambas propiedades espectroscópicas de aquellos que describen exclusivamente al apantalla-miento magnético. Se muestra explícitamente que el tensor SR es menos afectado por efectos relativistas que el tensor de apantallamiento magnético nuclear. Aún cuando en un contexto relativista la relación de Flygare pierde validez general, el análisis de las contribuciones relativistas de ambas propiedades a partir del modelo LRESC permite distinguir los mecanismos electrónicos involucrados en cada parámetro y establecer la razón por la cual existen casos particulares en que la relación sigue siendo válida. Se presentan los primeros resultados de cálculos numéricos de las constantes SR relativista, según la formulación de 4-componentes [J. Chem. Phys. 136, 204119 (2012)]. Se toman los haluros de hidrógeno, HX (X=H, F, Cl, Br, I), como sistemas de estudio. Una comparación entre la mejor estimación de cálculo y resultados experimentales muestra diferencias menores al 5 % en todos los casos. Los cálculos se llevaron a cabo implementando en el código DIRAC la formulación desarrollada. Se obtuvieron resultados siguiendo el formalismo de respuesta lineal relativista, al nivel RPA de aproximación. Para los halógenos, se muestra que los efectos de correlación en el régimen no relativista son de magnitud similar y signo contrario a los efectos relativistas. Para el hidrógeno, la aproximación de respuesta lineal mediante la eliminación de pequeñas componentes pone de manifiesto que el efecto relativista está completamente determinado por el operador espín-´orbita combinado con el operador de contacto de Fermi. Se analizó, formal y numéricamente, la contribución de los efectos de las inter-acciones de Breit electrón-núcleo, que acoplan las dinámicas electrónica y nuclear, al tensor SR. Dado que esta es netamente relativista, resulta necesario establecer su importancia relativa. La expansión perturbativa de esta contribución, en función de 1/c, indica que el orden de su primer término no nulo y el de las correcciones relativistas más importantes al tensor SR son equivalentes. Por completitud, se considera también el efecto de la interacción interelectrónica de Breit, mostrando que en todas las moléculas de la serie estudiada aporta correcciones muy pequeñas a las constantes SR de ambos núcleos, H y X. Los resultados de este estudio sugieren fuertemente que, considerando la precisión experimental en el estudio teórico del tensor SR, se pueden despreciar los efectos de Breit electrón-núcleo y electrón-electrón. El formalismo teórico desarrollado permite también postular una expresión original del tensor rotacional molecular relativista, válida para compuestos que contienen átomos pesados. En esta formulación, se consideran aquellos términos relevantes del Hamiltoniano molecular, lineales y bilineales en el momento angular de rotación nuclear y en un campo magnético externo uniforme, para analizarlos en el marco de teoría de perturbaciones relativista de primero y segundo orden. Se analizaron los efectos relativistas utilizando el modelo de respuesta lineal mediante la eliminación de pequeñas componentes. Se obtuvieron resultados cuan-titativos del factor g para los sistemas modelo HX (X=F, Cl, Br, I, At), XF (X=Cl, Br, I), y YH+ (Y=Ne, Ar, Kr, Xe, Rn), según los niveles de aproximación RPA y DFT. Los efectos relativistas son pequeños para esta propiedad molecular. La relación entre las expresiones teóricas de los tensores rotacional molecular y de susceptibilidad magnética, formulada en un contexto no relativista, pierde validez en un marco relativista. Sin embargo, la aplicación a sistemas que contienen elementos de la quinta fila de la tabla periódica, como HI, IF, y XeH+, muestra que la relación no relativista mantiene vigencia, con un 2 % de error. Sólo para sistemas que contienen elementos de la sexta fila, como el HAt y el RnH+, se encuentra una desviación significativa, de entre 6 y 7 %, de esta relación. Fil: Aucar, Ignacio Agustín. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas y Naturales y Agrimensura; Argentina. Fil: Aucar, Ignacio Agustín. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Nordeste. Instituto de Modelado e Innovación Tecnológica; Argentina. Fil: Ruiz de Azúa, Martín C. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina. Fil: Ruiz de Azúa, Martín C. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires; Argentina. Fil: Gómez, Sergio Santiago. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas y Naturales y Agrimensura; Argentina.
- Published
- 2015
10. Foundations of the LRESC model for response properties and some applications.
- Author
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Aucar, Gustavo A., Melo, Juan I., Aucar, Ignacio Agustín, and Maldonado, Alejandro F.
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NUCLEAR magnetic resonance ,QUANTUM chemistry ,MAGNETIC shielding ,ELECTRONIC structure ,NUCLEAR spin - Abstract
Accurate calculations of some response properties, like the NMR spectroscopic parameters, are quite exigent for the theoretical quantum chemistry models together with the computational codes that are written from them. They need to include a very good description of the electronic density in regions close to the nuclei. When heavy-atom containing systems are studied, those requirements become even higher. Given that relativistic effects must be included in one way or another on the calculation of response properties of heavy-atoms and heavy-atom containing molecules, different schemes were developed during the past decades to include them in as good as possible way. There are some four-component models, which include relativistic effects in a very compact way, although calculations have large time-consumption; one also needs to deal with new and unusual four-component operators. There are also two-component models, which in general may be less accurate, although their application to property calculations on medium-size and large-size molecules are feasible, and they maintain the application of usual operators. In this review, we give the fundamentals of the two-component linear response elimination of small component formalism, LRESC, together with some applications to few selected response properties. New physical insights do appear when the LRESC model is used to analyze the effect of the environment on magnetic shieldings, and when one search for the relativistic extension of well-known nonrelativistic relationships like Flygare's relation among the NMR magnetic shielding and the nuclear spin-rotation constant. A similar relationship is found for the g-tensor and the susceptibility tensor. [ABSTRACT FROM AUTHOR]
- Published
- 2018
- Full Text
- View/download PDF
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