Lorenzo Pisani - Academia.edu (original) (raw)
Papers by Lorenzo Pisani
Collana seminari interni 2012, Number 20120307.Viene presentato un modello geometrico, che lega l... more Collana seminari interni 2012, Number 20120307.Viene presentato un modello geometrico, che lega le proprietà di trasporto in un mezzo poroso alla sua porosità e ad un fattore di forma che ne caratterizza la struttura. Al variare del fattore di forma, si riesce a descrivere una gran varietà di mezzi porosi, da quelli non "tortuosi" a quelli molto tortuosi con limiti di percolazione, con innumerevoli applicazioni
In this work, a geometrical model is used to evaluate the sun radiation reflected from the helios... more In this work, a geometrical model is used to evaluate the sun radiation reflected from the heliostats toward the aim point on the tower top, by taking into account the shading and blocking between neighboring heliostats. This results in an analytical expression for the heliostats efficiency, which can provide very useful informations for the optimization of the heliostat field. In particular, we obtain a very simple and exact expression of the maximal energy collectable by the solar field and present effective strategies to reach such maximum.
Solar Energy, 2019
This paper presents an analysis of the effect of the land cost on the size and shape of the solar... more This paper presents an analysis of the effect of the land cost on the size and shape of the solar field. A parametric study has been performed, where the effect of the land cost is considered through the introduction of a parameter k, representing the ratio between the land and heliostats surface cost. Three different field layouts and two different geographical sites have been taken into consideration. The selected field layouts have been studied for several k-values within a relevant range. Results show that introducing a cost ratio k different from 0 has a strong influence on the shape of the resulting fields, significantly compressing the heliostats fields also for low land costs. For example, for , the area is reduced, with respect to = k 0, from 28% to 49%, following the considered cases; on the same time, the yearly collection efficiency is reduced by only about 1% in all cases.
AIP Conference Proceedings, 2018
This paper summarizes the collaborative work done by CENER, Fraunhofer ISE and CRS4 within the sc... more This paper summarizes the collaborative work done by CENER, Fraunhofer ISE and CRS4 within the scope of the STAGE-STE project related to heliostat field generation algorithms and their application to small heliostats (<10 m 2). Radially staggered heliostat field layouts have been commonly applied to real solar fields with known results. However, their use in conjunction with small heliostats or the use of brand new ways for heliostat allocation, such natural pattern based algorithms, is still unknown. Herein, the most promising heliostat field generation algorithms are studied and compared, in annual optical efficiency terms, for different scenarios. These efficiencies correspond to the best heliostat layouts found by the optimization process that each generation algorithm can create, for three scenarios. Results show that the fields from the selected algorithms lead to similar optical efficiencies. The slight differences are not enough evidence to nominate one of the algorithms as the best choice, taking into account the inherent error of the simulation tools, the optimization process and further requirements needed in commercial applications (e.g. access paths) not coped in this study.
Http Www Theses Fr, 1997
Le premier chapitre decrit les fondements physiques de la chimie quantique relativiste a partir d... more Le premier chapitre decrit les fondements physiques de la chimie quantique relativiste a partir de l'equation de dirac pour un electron dans un champ electromagnetique. Ensuite nous decrivons le calcul des proprietes des systemes comprenant plusieurs electrons dans l'approximation orbitale (dirac-fock), et enfin nous abordons le probleme de la correlation electronique en decrivant une methode originale pour l'estimation de la correlation dynamique, le k-functional. Dans le deuxieme chapitre nous decrivons en detail le programme morel pour le calcul moleculaire relativiste. Nous commencons par presenter la formulation originale choisie pour l'implementation des equations de dirac-fock dans un programme de calcul. Ensuite, nous decrivons quelques uns des aspects plus importantes du calcul comme le traitement des couches ouvertes, l'emploi d'une base contractee, la convergence du procede scf et la parallelisation. Les deux chapitres suivants presentent deux applications du code morel. La premiere application porte sur l'etude des hydrides des atomes du sixieme groupe. La deuxieme application s'interesse a l'etude de la reaction de l'ammoniac avec l'acide chlorhydrique.
Current Environmental Issues and Challenges, 2014
In this chapter, the key role of fuel cells and regenerative fuel cell technologies, in the futur... more In this chapter, the key role of fuel cells and regenerative fuel cell technologies, in the future energetic scenarios based on renewable energy sources, is described. The fuel cell technology is reviewed; its working principles are summarized and mathematically described by means of a simple analytical model. It is found that the limits of fuel cell competitiveness can be overcome only through a specific design of its key component materials: the electrolyte and the electrocatalyst. Examples of material design research achievements are reported for both fuel cells and regenerative fuel cells.
Encyclopedia of Electrochemical Power Sources, 2009
Proton-exchange membrane fuel cells are categorized into two classes defined by their specific el... more Proton-exchange membrane fuel cells are categorized into two classes defined by their specific electrical connection architecture that is used to increase the output voltage within application requirement ranges. For automotive and stationary applications, separate unit cells are stacked together in series to form an integrated three-dimensional structure that includes connections and manifolds to distribute reactants and coolant. For smaller portable applications, the serial connection is achieved on a mostly two-dimensional structure based preferably on a single proton-exchange membrane for manufacturing ease. Functions, materials, and designs for the prevalent components are discussed and include the bipolar plate, membrane/electrode assembly, and seal and compression mechanism. The advent of portable devices using printed circuit board or microelectromechanical system technologies has led to the appearance of components fulfilling many of the functions traditionally attributed to components designed for larger automotive and stationary applications.
Transport in Porous Media, 2011
In this article, we derive a simple expression for the tortuosity of porous media as a function o... more In this article, we derive a simple expression for the tortuosity of porous media as a function of porosity and of a single parameter characterizing the shape of the porous medium components. Following its value, a very large range of porous materials is described, from non-tortuous to high tortuosity ones with percolation limits. The proposed relation is compared with a widely used expression derived from percolation theory, and its predictive power is demonstrated through comparison with numerical simulations of diffusion phenomena. Application to the tortuosity of hydrated polymeric membranes is shown.
Journal of Power Sources, 2009
In this work, the conductivity limits of sulfonated membranes are investigated through a model an... more In this work, the conductivity limits of sulfonated membranes are investigated through a model analysis. A recent analytical conductivity model has been modified by reducing the number of variables to only three parameters, representing the hydration level, the ion exchange capacity and the morphology of the membrane. The effects of these parameters on the conductivity are investigated through a parametric
Journal of The Electrochemical Society, 2007
This work presents the development of an analytical model for the simulation of solid oxide fuel ... more This work presents the development of an analytical model for the simulation of solid oxide fuel cells (SOFCs). The unique features of the analytical code include Stefan-Maxwell-Knudsen diffusion of five-component gas mixtures (H 2 , CO, CO 2 , H 2 O, and CH 4 ) as fuel and of air as comburent, microdiffusion to the reaction sites, reaction kinetics described accurately beyond the linear limit, internal reforming reaction, shift reaction equilibrium, thermal sources evaluation. This results in a fast and accurate computer code, which can be used both as a stand-alone tool for one-dimensional simulations and as a user-supplied source term in the framework of fluid-dynamics three-dimensional platforms. The one-dimensional simulation model is validated against literature experimental data of SOFCs operating with hydrogen-carbon monoxide mixtures. A performance analysis on the effect of SOFC geometry variation has been reported. Analysis of the results indicates that the analytical model is an effective tool for SOFC design and optimization.
Journal of Computational Chemistry, 1994
A new code has been written to perform relativistic Dirac-Fock self-consistent field (SCF) calcul... more A new code has been written to perform relativistic Dirac-Fock self-consistent field (SCF) calculations on closed-shell molecules of any symmetry. The choice of the basis set allows us to work at different levels of approximation depending on the precision required. Calculations on the H2Po molecule show that accurate results on specific problems like geometry optimization can be obtained by evaluating the two-electron integrals on half the basis spinors. 0 1994 by John Wiley & Sons, Inc.
The Journal of Chemical Physics, 1994
Dirac–Fock (DF) and Hartree–Fock (HF) calculations have been performed for the ground state confi... more Dirac–Fock (DF) and Hartree–Fock (HF) calculations have been performed for the ground state configuration of the H2O, H2S, H2Se, H2Te, and H2Po molecules. Equilibrium geometries, atomization energies, and molecular orbitals energies are evaluated with both methods, compared and discussed with the help of population analysis and atomic orbital energies. Particular attention has been given to a qualitative understanding of the relativistic effects. Molecular spin–orbits corrections appear to be essential to a description of some in the sixth group hydrides set. A description of the relativistic computer program is presented elsewhere [L. Pisani and E. Clementi, J. Comput. Chem. (in press)].
The Journal of Chemical Physics, 1995
Dirac–Fock and Hartree–Fock calculations have been performed for the ground state of the HO, HS, ... more Dirac–Fock and Hartree–Fock calculations have been performed for the ground state of the HO, HS, HSe, HTe, and HPo molecules. Equilibrium geometries, atomization energies, and ionization potentials, with both methods, are evaluated, compared, and discussed. Calculations on the molecules H2M (M=O, S, Se, Te, and Po) have been already published [L. Pisani and E. Clementi, J. Chem. Phys. 101, 3079 (1994)], therefore, the results of the two series of molecules are compared. The effects of electronic correlation have been estimated by using the k-functional technique [L. Pisani, L. De Windt, and E. Clementi, Int. J. Quantum Chem. (in press)]. The agreement with the experimental data, available for low Z, is satisfactory.
Journal of Chemical Education, 1993
International Journal of Quantum Chemistry, 1995
The Coulomb-Hole-Hartree-Fock method introduced by E. Clementi in the early 1960s and reparametri... more The Coulomb-Hole-Hartree-Fock method introduced by E. Clementi in the early 1960s and reparametrized more recently by S. Chakraworty and E, Clementi to compute the correlated electronic energy in atomic systems, is here extended to compute molecules. The new parametrization is obtained empirically by fitting first and second atomic ionization potentials from He to Ca and a few diatomic molecules. The
International Journal of Quantum Chemistry, 1996
ABSTRACT Theoretical calculations of the potential energy surface (PES) for the [NH3 + HCl] syste... more ABSTRACT Theoretical calculations of the potential energy surface (PES) for the [NH3 + HCl] system are presented using several standard ab initio methods such as Hartree-Fock (HF), second-order Møller-Plesset perturbation theory (MP2), coupled cluster (CC), complete active space self-consistent-field (CASSCF), density functional theory (DFT), and less traditional ab initio approaches such as Dirac-Fock four-components and the use of effective Hamiltonian techniques, such as the recently proposed K functional. All calculations predict a single minimum for the complex, corresponding to a hydrogen-bonded structure, confirming early studies. The dynamical and nondynamical contributions to the correlation energy are discussed for different cuts of the PES, involving different N(SINGLE BOND)Cl distances. The complex has also been characterized by performing a full geometry optimization within the HF and DFT schemes; with the latter we have performed also the vibrational analysis. The predicted binding energies and infrared (IR) spectrum are compared with other theoretical and experimental results. For the gas phase, we propose a binding energy of −5.3 ± 0.5 kcal/mol, thus revising the experimental value of −8.0 ± 2.8 kcal/mol; for the minimum, the predicted N(SINGLE BOND)H and H(SINGLE BOND)Cl distances are 5.91 ± 0.05 and 2.46 ± 0.05 a.u., respectively. When the computation is done with approximate inclusion of solvent effects (Onsager reaction field), the minimum is shifted and it corresponds to the ion pair NH+4·Cl− structure, similar to Mulliken's outer complex. Since the first ab initio computation for the NH4Cl complex is the pioneer work in 1967 by E. Clementi, the present work provides us with an opportunity to comment on some aspects of the evolution in computational chemistry, particularly for energy determinations. We have concluded our comments with the invitation to use four-components Fock-Dirac for molecules both with high and low Z atoms, rather than the traditional Hartree-Fock and related methods. In other words, we are of the opinion that the time is ready in quantum chemistry to switch from the Schrödinger to the Dirac representation, due to new developments in computer hardware and software. In addition, the use of effective Hamiltonians, like the recently proposed “K functional,” seems to deserve attention, because of their computational simplicity and physical reliability in predicting correlation corrections. © 1996 John Wiley & Sons, Inc.
International Journal of Quantum Chemistry, 1996
International Journal of Heat and Mass Transfer, 2008
In this paper, the equations governing the transport of gas mixtures through porous media in 1D g... more In this paper, the equations governing the transport of gas mixtures through porous media in 1D geometry and in absence of mass sources are examined. When the mass fluxes are determined by external conditions, the transport equations can be solved to find the variations of gas composition through the media. For this class of problems, we show that the convective transport mechanism can be, in many cases, neglected, regardless of the physical properties of the porous media and of the flux intensities. An estimate of the maximum error made in neglecting the convective term is provided. A matrix analytical solution of the diffusion equations is given for the general case of N-components gas mixtures. Explicit analytical solutions are derived in two particular cases.
Computer Physics Communications, 2000
Abstract A new, simple estimate of the diffraction coefficients, to be used in a ray tracing calc... more Abstract A new, simple estimate of the diffraction coefficients, to be used in a ray tracing calculation, is derived applying the stationary phase technique to the boundary wave integral. This approach is shown to be the last step (0-dimensions) of a dimensionality reduction procedure which begins with the Kirchhoff's integral method (2-dimensions) and proceeds with the boundary wave approach (1-dimension). While the reduction – from three to two – of the dimensionality, requires the physical optics approximation and the reduction from two to one holds only for perfectly conducting solids with flat faces and spherical or plane incident waves, no additional price is paid to pass from one to zero dimensions. The Zero-Dimensional (ZD) approach is validated through numerical comparison with higher dimensionality methods and analytical (exact) results in a number of test cases.
Collana seminari interni 2012, Number 20120307.Viene presentato un modello geometrico, che lega l... more Collana seminari interni 2012, Number 20120307.Viene presentato un modello geometrico, che lega le proprietà di trasporto in un mezzo poroso alla sua porosità e ad un fattore di forma che ne caratterizza la struttura. Al variare del fattore di forma, si riesce a descrivere una gran varietà di mezzi porosi, da quelli non "tortuosi" a quelli molto tortuosi con limiti di percolazione, con innumerevoli applicazioni
In this work, a geometrical model is used to evaluate the sun radiation reflected from the helios... more In this work, a geometrical model is used to evaluate the sun radiation reflected from the heliostats toward the aim point on the tower top, by taking into account the shading and blocking between neighboring heliostats. This results in an analytical expression for the heliostats efficiency, which can provide very useful informations for the optimization of the heliostat field. In particular, we obtain a very simple and exact expression of the maximal energy collectable by the solar field and present effective strategies to reach such maximum.
Solar Energy, 2019
This paper presents an analysis of the effect of the land cost on the size and shape of the solar... more This paper presents an analysis of the effect of the land cost on the size and shape of the solar field. A parametric study has been performed, where the effect of the land cost is considered through the introduction of a parameter k, representing the ratio between the land and heliostats surface cost. Three different field layouts and two different geographical sites have been taken into consideration. The selected field layouts have been studied for several k-values within a relevant range. Results show that introducing a cost ratio k different from 0 has a strong influence on the shape of the resulting fields, significantly compressing the heliostats fields also for low land costs. For example, for , the area is reduced, with respect to = k 0, from 28% to 49%, following the considered cases; on the same time, the yearly collection efficiency is reduced by only about 1% in all cases.
AIP Conference Proceedings, 2018
This paper summarizes the collaborative work done by CENER, Fraunhofer ISE and CRS4 within the sc... more This paper summarizes the collaborative work done by CENER, Fraunhofer ISE and CRS4 within the scope of the STAGE-STE project related to heliostat field generation algorithms and their application to small heliostats (<10 m 2). Radially staggered heliostat field layouts have been commonly applied to real solar fields with known results. However, their use in conjunction with small heliostats or the use of brand new ways for heliostat allocation, such natural pattern based algorithms, is still unknown. Herein, the most promising heliostat field generation algorithms are studied and compared, in annual optical efficiency terms, for different scenarios. These efficiencies correspond to the best heliostat layouts found by the optimization process that each generation algorithm can create, for three scenarios. Results show that the fields from the selected algorithms lead to similar optical efficiencies. The slight differences are not enough evidence to nominate one of the algorithms as the best choice, taking into account the inherent error of the simulation tools, the optimization process and further requirements needed in commercial applications (e.g. access paths) not coped in this study.
Http Www Theses Fr, 1997
Le premier chapitre decrit les fondements physiques de la chimie quantique relativiste a partir d... more Le premier chapitre decrit les fondements physiques de la chimie quantique relativiste a partir de l'equation de dirac pour un electron dans un champ electromagnetique. Ensuite nous decrivons le calcul des proprietes des systemes comprenant plusieurs electrons dans l'approximation orbitale (dirac-fock), et enfin nous abordons le probleme de la correlation electronique en decrivant une methode originale pour l'estimation de la correlation dynamique, le k-functional. Dans le deuxieme chapitre nous decrivons en detail le programme morel pour le calcul moleculaire relativiste. Nous commencons par presenter la formulation originale choisie pour l'implementation des equations de dirac-fock dans un programme de calcul. Ensuite, nous decrivons quelques uns des aspects plus importantes du calcul comme le traitement des couches ouvertes, l'emploi d'une base contractee, la convergence du procede scf et la parallelisation. Les deux chapitres suivants presentent deux applications du code morel. La premiere application porte sur l'etude des hydrides des atomes du sixieme groupe. La deuxieme application s'interesse a l'etude de la reaction de l'ammoniac avec l'acide chlorhydrique.
Current Environmental Issues and Challenges, 2014
In this chapter, the key role of fuel cells and regenerative fuel cell technologies, in the futur... more In this chapter, the key role of fuel cells and regenerative fuel cell technologies, in the future energetic scenarios based on renewable energy sources, is described. The fuel cell technology is reviewed; its working principles are summarized and mathematically described by means of a simple analytical model. It is found that the limits of fuel cell competitiveness can be overcome only through a specific design of its key component materials: the electrolyte and the electrocatalyst. Examples of material design research achievements are reported for both fuel cells and regenerative fuel cells.
Encyclopedia of Electrochemical Power Sources, 2009
Proton-exchange membrane fuel cells are categorized into two classes defined by their specific el... more Proton-exchange membrane fuel cells are categorized into two classes defined by their specific electrical connection architecture that is used to increase the output voltage within application requirement ranges. For automotive and stationary applications, separate unit cells are stacked together in series to form an integrated three-dimensional structure that includes connections and manifolds to distribute reactants and coolant. For smaller portable applications, the serial connection is achieved on a mostly two-dimensional structure based preferably on a single proton-exchange membrane for manufacturing ease. Functions, materials, and designs for the prevalent components are discussed and include the bipolar plate, membrane/electrode assembly, and seal and compression mechanism. The advent of portable devices using printed circuit board or microelectromechanical system technologies has led to the appearance of components fulfilling many of the functions traditionally attributed to components designed for larger automotive and stationary applications.
Transport in Porous Media, 2011
In this article, we derive a simple expression for the tortuosity of porous media as a function o... more In this article, we derive a simple expression for the tortuosity of porous media as a function of porosity and of a single parameter characterizing the shape of the porous medium components. Following its value, a very large range of porous materials is described, from non-tortuous to high tortuosity ones with percolation limits. The proposed relation is compared with a widely used expression derived from percolation theory, and its predictive power is demonstrated through comparison with numerical simulations of diffusion phenomena. Application to the tortuosity of hydrated polymeric membranes is shown.
Journal of Power Sources, 2009
In this work, the conductivity limits of sulfonated membranes are investigated through a model an... more In this work, the conductivity limits of sulfonated membranes are investigated through a model analysis. A recent analytical conductivity model has been modified by reducing the number of variables to only three parameters, representing the hydration level, the ion exchange capacity and the morphology of the membrane. The effects of these parameters on the conductivity are investigated through a parametric
Journal of The Electrochemical Society, 2007
This work presents the development of an analytical model for the simulation of solid oxide fuel ... more This work presents the development of an analytical model for the simulation of solid oxide fuel cells (SOFCs). The unique features of the analytical code include Stefan-Maxwell-Knudsen diffusion of five-component gas mixtures (H 2 , CO, CO 2 , H 2 O, and CH 4 ) as fuel and of air as comburent, microdiffusion to the reaction sites, reaction kinetics described accurately beyond the linear limit, internal reforming reaction, shift reaction equilibrium, thermal sources evaluation. This results in a fast and accurate computer code, which can be used both as a stand-alone tool for one-dimensional simulations and as a user-supplied source term in the framework of fluid-dynamics three-dimensional platforms. The one-dimensional simulation model is validated against literature experimental data of SOFCs operating with hydrogen-carbon monoxide mixtures. A performance analysis on the effect of SOFC geometry variation has been reported. Analysis of the results indicates that the analytical model is an effective tool for SOFC design and optimization.
Journal of Computational Chemistry, 1994
A new code has been written to perform relativistic Dirac-Fock self-consistent field (SCF) calcul... more A new code has been written to perform relativistic Dirac-Fock self-consistent field (SCF) calculations on closed-shell molecules of any symmetry. The choice of the basis set allows us to work at different levels of approximation depending on the precision required. Calculations on the H2Po molecule show that accurate results on specific problems like geometry optimization can be obtained by evaluating the two-electron integrals on half the basis spinors. 0 1994 by John Wiley & Sons, Inc.
The Journal of Chemical Physics, 1994
Dirac–Fock (DF) and Hartree–Fock (HF) calculations have been performed for the ground state confi... more Dirac–Fock (DF) and Hartree–Fock (HF) calculations have been performed for the ground state configuration of the H2O, H2S, H2Se, H2Te, and H2Po molecules. Equilibrium geometries, atomization energies, and molecular orbitals energies are evaluated with both methods, compared and discussed with the help of population analysis and atomic orbital energies. Particular attention has been given to a qualitative understanding of the relativistic effects. Molecular spin–orbits corrections appear to be essential to a description of some in the sixth group hydrides set. A description of the relativistic computer program is presented elsewhere [L. Pisani and E. Clementi, J. Comput. Chem. (in press)].
The Journal of Chemical Physics, 1995
Dirac–Fock and Hartree–Fock calculations have been performed for the ground state of the HO, HS, ... more Dirac–Fock and Hartree–Fock calculations have been performed for the ground state of the HO, HS, HSe, HTe, and HPo molecules. Equilibrium geometries, atomization energies, and ionization potentials, with both methods, are evaluated, compared, and discussed. Calculations on the molecules H2M (M=O, S, Se, Te, and Po) have been already published [L. Pisani and E. Clementi, J. Chem. Phys. 101, 3079 (1994)], therefore, the results of the two series of molecules are compared. The effects of electronic correlation have been estimated by using the k-functional technique [L. Pisani, L. De Windt, and E. Clementi, Int. J. Quantum Chem. (in press)]. The agreement with the experimental data, available for low Z, is satisfactory.
Journal of Chemical Education, 1993
International Journal of Quantum Chemistry, 1995
The Coulomb-Hole-Hartree-Fock method introduced by E. Clementi in the early 1960s and reparametri... more The Coulomb-Hole-Hartree-Fock method introduced by E. Clementi in the early 1960s and reparametrized more recently by S. Chakraworty and E, Clementi to compute the correlated electronic energy in atomic systems, is here extended to compute molecules. The new parametrization is obtained empirically by fitting first and second atomic ionization potentials from He to Ca and a few diatomic molecules. The
International Journal of Quantum Chemistry, 1996
ABSTRACT Theoretical calculations of the potential energy surface (PES) for the [NH3 + HCl] syste... more ABSTRACT Theoretical calculations of the potential energy surface (PES) for the [NH3 + HCl] system are presented using several standard ab initio methods such as Hartree-Fock (HF), second-order Møller-Plesset perturbation theory (MP2), coupled cluster (CC), complete active space self-consistent-field (CASSCF), density functional theory (DFT), and less traditional ab initio approaches such as Dirac-Fock four-components and the use of effective Hamiltonian techniques, such as the recently proposed K functional. All calculations predict a single minimum for the complex, corresponding to a hydrogen-bonded structure, confirming early studies. The dynamical and nondynamical contributions to the correlation energy are discussed for different cuts of the PES, involving different N(SINGLE BOND)Cl distances. The complex has also been characterized by performing a full geometry optimization within the HF and DFT schemes; with the latter we have performed also the vibrational analysis. The predicted binding energies and infrared (IR) spectrum are compared with other theoretical and experimental results. For the gas phase, we propose a binding energy of −5.3 ± 0.5 kcal/mol, thus revising the experimental value of −8.0 ± 2.8 kcal/mol; for the minimum, the predicted N(SINGLE BOND)H and H(SINGLE BOND)Cl distances are 5.91 ± 0.05 and 2.46 ± 0.05 a.u., respectively. When the computation is done with approximate inclusion of solvent effects (Onsager reaction field), the minimum is shifted and it corresponds to the ion pair NH+4·Cl− structure, similar to Mulliken's outer complex. Since the first ab initio computation for the NH4Cl complex is the pioneer work in 1967 by E. Clementi, the present work provides us with an opportunity to comment on some aspects of the evolution in computational chemistry, particularly for energy determinations. We have concluded our comments with the invitation to use four-components Fock-Dirac for molecules both with high and low Z atoms, rather than the traditional Hartree-Fock and related methods. In other words, we are of the opinion that the time is ready in quantum chemistry to switch from the Schrödinger to the Dirac representation, due to new developments in computer hardware and software. In addition, the use of effective Hamiltonians, like the recently proposed “K functional,” seems to deserve attention, because of their computational simplicity and physical reliability in predicting correlation corrections. © 1996 John Wiley & Sons, Inc.
International Journal of Quantum Chemistry, 1996
International Journal of Heat and Mass Transfer, 2008
In this paper, the equations governing the transport of gas mixtures through porous media in 1D g... more In this paper, the equations governing the transport of gas mixtures through porous media in 1D geometry and in absence of mass sources are examined. When the mass fluxes are determined by external conditions, the transport equations can be solved to find the variations of gas composition through the media. For this class of problems, we show that the convective transport mechanism can be, in many cases, neglected, regardless of the physical properties of the porous media and of the flux intensities. An estimate of the maximum error made in neglecting the convective term is provided. A matrix analytical solution of the diffusion equations is given for the general case of N-components gas mixtures. Explicit analytical solutions are derived in two particular cases.
Computer Physics Communications, 2000
Abstract A new, simple estimate of the diffraction coefficients, to be used in a ray tracing calc... more Abstract A new, simple estimate of the diffraction coefficients, to be used in a ray tracing calculation, is derived applying the stationary phase technique to the boundary wave integral. This approach is shown to be the last step (0-dimensions) of a dimensionality reduction procedure which begins with the Kirchhoff's integral method (2-dimensions) and proceeds with the boundary wave approach (1-dimension). While the reduction – from three to two – of the dimensionality, requires the physical optics approximation and the reduction from two to one holds only for perfectly conducting solids with flat faces and spherical or plane incident waves, no additional price is paid to pass from one to zero dimensions. The Zero-Dimensional (ZD) approach is validated through numerical comparison with higher dimensionality methods and analytical (exact) results in a number of test cases.