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Papers by Mario Raimondi

Research paper thumbnail of On the role of different spin bases within spin-coupled theory

Molecular Physics, May 1, 1993

Research paper thumbnail of Spin-Coupled Valence Bond Study of the Reaction between Benzene and a Methyl Cation

Journal of Physical Chemistry A, Apr 1, 1997

We present spin-coupled valence bond calculations on the reaction pathway for the insertion of a ... more We present spin-coupled valence bond calculations on the reaction pathway for the insertion of a methyl cation onto the aromatic system of benzene, leading to the Wheland intermediate C 6 H 6 CH 3 +. Simultaneously with the geometrical rehybridization of the two carbons forming the new substrate-electrophile bond (from sp 2 to sp 3), we observe the crossing of two spin-coupled potential energy curves; at large separations, these correspond to C 6 H 6 + CH 3 + and to C 6 H 6 + + CH 3. In the neighborhood of the crossing point, the spincoupled valence bond wave function obtained by mixing these two orbital configurations switches continuously from the former to the latter.

Research paper thumbnail of The Lowest Singlet and Triplet States of<i>o</i>-Benzyne: Spin-Coupled Interpretation of the Electronic Structure at CAS SCF Equilibrium Geometries

Israel Journal of Chemistry, 1993

The electronic structure of the X̃1A1 and ã3B2states of o‐benzyne is analyzed by means of spin‐co... more The electronic structure of the X̃1A1 and ã3B2states of o‐benzyne is analyzed by means of spin‐coupled (SC) wave functions involving eight active orbitals: two at the dehydro centers, in the plane of the ring, and six π orbitals, inherited from benzene. The equilibrium geometries of the two states, at which all calculations have been carried out, result from separate full geometry optimizations employing ‘8 in 8’ complete active space self‐consistent field (CAS SCF) wave functions incorporating an analogous core‐valence partitioning. The diradical character of the ground state of o‐benzyne, as approximated by the singlet SC wave function, is reflected in the low value of the overlap (0.46) between the two in‐plane sp2‐like SC orbitals at the dehydro centers, which are just 1.274 Å apart, and in the nonnegligible extent of triplet coupling between the spins of the electrons in these orbitals — ca. 6.4% of the total spin function for the valence electrons (expressed in the Serber spin basis) is made up of spin eigenfunctions, in which these electrons enter as a triplet pair. The geometry of the ground state is found to have an aryne‐like character, which is further confirmed by the pattern of alternation of the overlaps between neighboring 71 valence orbitals, and by the domination of the total spin function for the valence electrons (expressed in the Rumer spin basis) by the spin‐coupling patterns corresponding to an arynic structure.

Research paper thumbnail of Aromatic electrophilic substitution. A modern valence bond study

Journal of the Chemical Society, Faraday Transactions, 1995

ABSTRACT

Research paper thumbnail of Spin-Coupled Study of the Electronic Structure of Polyenyl Radicals C3H5-C9H11

Journal of the American Chemical Society, Mar 1, 1994

Spin-coupled (SC) theory is applied to the description of the electronic structure of polyenyl ra... more Spin-coupled (SC) theory is applied to the description of the electronic structure of polyenyl radicals allyl (CsHs), c5H.1, C~H Q , and CsH,,. A SC wave function treating nil seven electrons involved in the carbon-carbon bonds in C3H5 as active is found to reproduce the correct C, geometry for allyl in its ground state, as well as the proper spatial and spin symmetry of this state (2A2). Although no u-A separation is imposed a priori upon the seven-electron active space, the optimized SC orbitals display this symmetry and exhibit no tendency toward bent-bond formation and/or symmetry breaking. The most important correlation effects are shown to be included in the A space: The energy of the 7-only three-electron SC wave function for C3H5 coincides with that of the corresponding '3 in 3" completeactive-space self-consistent-field (CAS SCF) wave function. The SC model for the A electrons in allyl confirms the utility of the antipair concept introduced with the SC treatment of antiaromatic systems. The SC approach is demonstrated to yield highly correlated valence-bond-style A-active-space wave functions of the appropriate spatial and spin symmetry (2Az and 2B1) for the subseries of polyenyl radicals, involving 4n + 3 and 4n + 1 carbon atoms, respectively. Except for allyl, the 7 electrons are accommodated in tightly localized SC orbitals, which resemble distorted C(2p,) atomic orbitals. [4n + 31 chains (with the exception of the C3H5) are best described by an antiresonance between equivalent KekulC structures. The SC wave functions for the other ([4n + 11) series are dominated by a single symmetry-adapted KekulC structure which is in conventional resonance with less important pairs of equivalent structures.

Research paper thumbnail of On the role of different spin bases within spin-coupled theory

Molecular Physics, May 1, 1993

Research paper thumbnail of Spin-Coupled Valence Bond Study of the Reaction between Benzene and a Methyl Cation

Journal of Physical Chemistry A, Apr 1, 1997

We present spin-coupled valence bond calculations on the reaction pathway for the insertion of a ... more We present spin-coupled valence bond calculations on the reaction pathway for the insertion of a methyl cation onto the aromatic system of benzene, leading to the Wheland intermediate C 6 H 6 CH 3 +. Simultaneously with the geometrical rehybridization of the two carbons forming the new substrate-electrophile bond (from sp 2 to sp 3), we observe the crossing of two spin-coupled potential energy curves; at large separations, these correspond to C 6 H 6 + CH 3 + and to C 6 H 6 + + CH 3. In the neighborhood of the crossing point, the spincoupled valence bond wave function obtained by mixing these two orbital configurations switches continuously from the former to the latter.

Research paper thumbnail of The Lowest Singlet and Triplet States of<i>o</i>-Benzyne: Spin-Coupled Interpretation of the Electronic Structure at CAS SCF Equilibrium Geometries

Israel Journal of Chemistry, 1993

The electronic structure of the X̃1A1 and ã3B2states of o‐benzyne is analyzed by means of spin‐co... more The electronic structure of the X̃1A1 and ã3B2states of o‐benzyne is analyzed by means of spin‐coupled (SC) wave functions involving eight active orbitals: two at the dehydro centers, in the plane of the ring, and six π orbitals, inherited from benzene. The equilibrium geometries of the two states, at which all calculations have been carried out, result from separate full geometry optimizations employing ‘8 in 8’ complete active space self‐consistent field (CAS SCF) wave functions incorporating an analogous core‐valence partitioning. The diradical character of the ground state of o‐benzyne, as approximated by the singlet SC wave function, is reflected in the low value of the overlap (0.46) between the two in‐plane sp2‐like SC orbitals at the dehydro centers, which are just 1.274 Å apart, and in the nonnegligible extent of triplet coupling between the spins of the electrons in these orbitals — ca. 6.4% of the total spin function for the valence electrons (expressed in the Serber spin basis) is made up of spin eigenfunctions, in which these electrons enter as a triplet pair. The geometry of the ground state is found to have an aryne‐like character, which is further confirmed by the pattern of alternation of the overlaps between neighboring 71 valence orbitals, and by the domination of the total spin function for the valence electrons (expressed in the Rumer spin basis) by the spin‐coupling patterns corresponding to an arynic structure.

Research paper thumbnail of Aromatic electrophilic substitution. A modern valence bond study

Journal of the Chemical Society, Faraday Transactions, 1995

ABSTRACT

Research paper thumbnail of Spin-Coupled Study of the Electronic Structure of Polyenyl Radicals C3H5-C9H11

Journal of the American Chemical Society, Mar 1, 1994

Spin-coupled (SC) theory is applied to the description of the electronic structure of polyenyl ra... more Spin-coupled (SC) theory is applied to the description of the electronic structure of polyenyl radicals allyl (CsHs), c5H.1, C~H Q , and CsH,,. A SC wave function treating nil seven electrons involved in the carbon-carbon bonds in C3H5 as active is found to reproduce the correct C, geometry for allyl in its ground state, as well as the proper spatial and spin symmetry of this state (2A2). Although no u-A separation is imposed a priori upon the seven-electron active space, the optimized SC orbitals display this symmetry and exhibit no tendency toward bent-bond formation and/or symmetry breaking. The most important correlation effects are shown to be included in the A space: The energy of the 7-only three-electron SC wave function for C3H5 coincides with that of the corresponding '3 in 3" completeactive-space self-consistent-field (CAS SCF) wave function. The SC model for the A electrons in allyl confirms the utility of the antipair concept introduced with the SC treatment of antiaromatic systems. The SC approach is demonstrated to yield highly correlated valence-bond-style A-active-space wave functions of the appropriate spatial and spin symmetry (2Az and 2B1) for the subseries of polyenyl radicals, involving 4n + 3 and 4n + 1 carbon atoms, respectively. Except for allyl, the 7 electrons are accommodated in tightly localized SC orbitals, which resemble distorted C(2p,) atomic orbitals. [4n + 31 chains (with the exception of the C3H5) are best described by an antiresonance between equivalent KekulC structures. The SC wave functions for the other ([4n + 11) series are dominated by a single symmetry-adapted KekulC structure which is in conventional resonance with less important pairs of equivalent structures.

Research paper thumbnail of Spin-coupled VB theory of molecular electronic structure. Ground and low-lying 1Σ+ states of CH+

Journal of Chemical Physics, Feb 15, 1985

The spin-coupled VB theory is applied to the ground and low-lying 1Σ+ states of CH+. Using 500 sp... more The spin-coupled VB theory is applied to the ground and low-lying 1Σ+ states of CH+. Using 500 spin-coupled structures, the calculated values of the spectrosopic constants for the ground states De(eV)=4.14 (4.26), ωe(cm−1)=2845 (2858), Re(Å) =1.135 (1.131); experimental results are given in parentheses. The form of the ground state potential energy curve is very close to the MCSCF-CI curve of Green et al. [Phys. Rev. A 5, 1614 (1972)], and similarly the excited states are very similar to those obtaind by MCSCF-CI methods by Saxon et al. [J. Chem. Phys. 73, 1873 (1980)]. The only exception is in the 4 1Σ+ state below R=3.5a0 where there is an avoided intersection with a Rydberg state. At least for small systems of this kind, the spin-coupled VB approach is capable of the same level of accuracy as the best MCSCF-CI methods, but uses an order of magnitude fewer functions. Each state in the spin-coupled VB calculation is dominated by one or two structures. This provides the wave function with much visuality, but without the usual attendant loss in accuracy.

Research paper thumbnail of From biomass to energy: hydrogen-based technology by bio-ethanol reforming

From biomass to energy: hydrogen-based technology by bio-ethanol reforming/I. Rossetti, C. Biffi,... more From biomass to energy: hydrogen-based technology by bio-ethanol reforming/I. Rossetti, C. Biffi, G. Faita, GF Tantardini, M. Raimondi, L. Forni.-2009. International Workshop on Distributed Energy Systems: The Role of Chemical Sciences and ...

Research paper thumbnail of From biomass to energy: H2 based technology from bioethanol

From research to …, 2009

From biomass to energy: H2 based technology from bioethanol / I. Rossetti, C. Biffi, G. Faita, M.... more From biomass to energy: H2 based technology from bioethanol / I. Rossetti, C. Biffi, G. Faita, M. Raimondi, GF Tantardini, L. Forni - In: From research to industry and markets : 17. European Biomass Conference & Exhibition : proceedings of the international conference held in ...

Research paper thumbnail of Da biomassa ad energia : tecnologia ad idrogeno da bioetanolo

Seminario della Facoltà di Scienze MFN : “L’energia di domani”, 2008

Research paper thumbnail of A Symposium on Molecular Electronic-Structure Calculations - Methods and Applications - General Discussion

Research paper thumbnail of The charge transfer of N with atomic hydrogen

Research paper thumbnail of Study of the mechanism of formation of CH + 2 ( 2 A 1 , 2 B 1 ) from C + ( 2 P ) + H 2 using spin-coupled valence bond theory

Molecular Physics, 1987

The spin-coupled valence bond method is used to study the formation of CH+2 in its lowest 2A1 and... more The spin-coupled valence bond method is used to study the formation of CH+2 in its lowest 2A1 and 2B1 states for perpendicular approaches by C+(2P) to H2. The potential surfaces for these two states have been extensively mapped out at the spin-coupled level. These wave functions show that in both states there is a sudden switch-over from C+(2P) + H2

Research paper thumbnail of Application of valence-bond techniques to the study of weakly bound complexes. The potential energy surface of the Ne?CH4 system

Physical Chemistry Chemical Physics, 2007

We present a comprehensive survey of the Molecular Orbital-Valence Bond (MO-VB) method, a theoret... more We present a comprehensive survey of the Molecular Orbital-Valence Bond (MO-VB) method, a theoretical scheme developed within the framework of the Valence Bond theory to deal with weakly bound intermolecular complexes. According to the MO-VB, the wavefunction of the system is expressed as a truncated non-orthogonal Configuration Interaction expansion, which is size extensive and a priori free of basis set superposition error. We report on the recent developments of the method, which extend the range of application of the MO-VB to intermolecular complexes with a quite large number of correlated electrons, showing that VB-based methods are nowadays a valid alternative to Molecular Orbital approaches also in this field. The MO-VB has been applied to study extensively the Ne-CH(4) complex, and compared with the more standard MP4 and CCSD(T) results. We determined two analytical Potential Energy Surfaces (PES) for this system, computed at MO-VB and MP4 level, which represent the first ones coming entirely from ab initio computations. The features of our potentials are discussed, and compared to the single analytical potential which includes the anisotropy available in the literature, determined about twenty years ago by Udo Buck and co-workers using a semiempirical approach [U. Buck, A. Kolhase, D. Secrest, T. Phillips, G. Scoles and F. Grein, Mol. Phys., 1985, 55, 1233]. The differences among the three PES are quite relevant, and are due to play a relevant role in the theoretical simulations of the dynamical properties of the Ne-CH(4) system.

Research paper thumbnail of The spin-coupled description of aromatic, antiaromatic and nonaromatic systems

Theoretical and Computational Chemistry, 1999

The correlated motion of π electrons in a range of aromatic, antiaromatic and nonaromatic systems... more The correlated motion of π electrons in a range of aromatic, antiaromatic and nonaromatic systems is investigated using modern spin-coupled theory. In the particular case of benzene, the resulting wavefunction, which is superior to the simple MO description, resembles closely the classical VB picture, except for small (but crucial) deformations of the orbitals. It is argued, by reference to a multiconfiguration approach which subsumes both of the simple MO and VB models, that we should feel comfortable with switching between the two modes of description, according to which leads most directly to correct predictions for the particular problem at hand. The bonding in benzene is contrasted with that in cyclobutadiene and cyclooctatetraene, at various geometries. In the spin-coupled model, the distinguishing feature of antiaromatic species is the occurrence of triplet coupling of electron pairs. Of course, the singlet ground states of both of these molecules distort from the idealized high-symmetry geometries to nonaromatic situations with olefinic bonds.

Research paper thumbnail of An MO−VB Approach for the Determination of Intermolecular Forces. Theory and Calculations on the He 2 , He−CH 4 , and He−H 2 O Systems

The Journal of Physical Chemistry A, 2002

We present the improvement of a previously developed strategy for the evaluation of intermolecula... more We present the improvement of a previously developed strategy for the evaluation of intermolecular forces. The approach defines a variational VB (valence bond) wave function, consisting of single and double excitations from the SCF-MI (self-consistent field for molecular interactions) determinant. The central idea of the method is the determination of optimal virtual orbitals, to contract the virtual space spanned by all singly and doubly excited localized configurations, by means of an iterative optimization procedure. The performance of the strategy is tested by comparison with results where the full virtual space is considered, and the entire approach is also compared with more conventional quantum chemical methods. Test calculations on three weakly interacting complexes, namely, He 2 , He-CH 4 , and He-H 2 O, are presented. Whatever the system studied, we found an overall agreement between VB, MP4, and CCSD(T) results. The VB well depths estimates are somewhat larger than MP4 and CCSD(T) ones.

Research paper thumbnail of On the role of different spin bases within spin-coupled theory

Molecular Physics, May 1, 1993

Research paper thumbnail of Spin-Coupled Valence Bond Study of the Reaction between Benzene and a Methyl Cation

Journal of Physical Chemistry A, Apr 1, 1997

We present spin-coupled valence bond calculations on the reaction pathway for the insertion of a ... more We present spin-coupled valence bond calculations on the reaction pathway for the insertion of a methyl cation onto the aromatic system of benzene, leading to the Wheland intermediate C 6 H 6 CH 3 +. Simultaneously with the geometrical rehybridization of the two carbons forming the new substrate-electrophile bond (from sp 2 to sp 3), we observe the crossing of two spin-coupled potential energy curves; at large separations, these correspond to C 6 H 6 + CH 3 + and to C 6 H 6 + + CH 3. In the neighborhood of the crossing point, the spincoupled valence bond wave function obtained by mixing these two orbital configurations switches continuously from the former to the latter.

Research paper thumbnail of The Lowest Singlet and Triplet States of<i>o</i>-Benzyne: Spin-Coupled Interpretation of the Electronic Structure at CAS SCF Equilibrium Geometries

Israel Journal of Chemistry, 1993

The electronic structure of the X̃1A1 and ã3B2states of o‐benzyne is analyzed by means of spin‐co... more The electronic structure of the X̃1A1 and ã3B2states of o‐benzyne is analyzed by means of spin‐coupled (SC) wave functions involving eight active orbitals: two at the dehydro centers, in the plane of the ring, and six π orbitals, inherited from benzene. The equilibrium geometries of the two states, at which all calculations have been carried out, result from separate full geometry optimizations employing ‘8 in 8’ complete active space self‐consistent field (CAS SCF) wave functions incorporating an analogous core‐valence partitioning. The diradical character of the ground state of o‐benzyne, as approximated by the singlet SC wave function, is reflected in the low value of the overlap (0.46) between the two in‐plane sp2‐like SC orbitals at the dehydro centers, which are just 1.274 Å apart, and in the nonnegligible extent of triplet coupling between the spins of the electrons in these orbitals — ca. 6.4% of the total spin function for the valence electrons (expressed in the Serber spin basis) is made up of spin eigenfunctions, in which these electrons enter as a triplet pair. The geometry of the ground state is found to have an aryne‐like character, which is further confirmed by the pattern of alternation of the overlaps between neighboring 71 valence orbitals, and by the domination of the total spin function for the valence electrons (expressed in the Rumer spin basis) by the spin‐coupling patterns corresponding to an arynic structure.

Research paper thumbnail of Aromatic electrophilic substitution. A modern valence bond study

Journal of the Chemical Society, Faraday Transactions, 1995

ABSTRACT

Research paper thumbnail of Spin-Coupled Study of the Electronic Structure of Polyenyl Radicals C3H5-C9H11

Journal of the American Chemical Society, Mar 1, 1994

Spin-coupled (SC) theory is applied to the description of the electronic structure of polyenyl ra... more Spin-coupled (SC) theory is applied to the description of the electronic structure of polyenyl radicals allyl (CsHs), c5H.1, C~H Q , and CsH,,. A SC wave function treating nil seven electrons involved in the carbon-carbon bonds in C3H5 as active is found to reproduce the correct C, geometry for allyl in its ground state, as well as the proper spatial and spin symmetry of this state (2A2). Although no u-A separation is imposed a priori upon the seven-electron active space, the optimized SC orbitals display this symmetry and exhibit no tendency toward bent-bond formation and/or symmetry breaking. The most important correlation effects are shown to be included in the A space: The energy of the 7-only three-electron SC wave function for C3H5 coincides with that of the corresponding '3 in 3" completeactive-space self-consistent-field (CAS SCF) wave function. The SC model for the A electrons in allyl confirms the utility of the antipair concept introduced with the SC treatment of antiaromatic systems. The SC approach is demonstrated to yield highly correlated valence-bond-style A-active-space wave functions of the appropriate spatial and spin symmetry (2Az and 2B1) for the subseries of polyenyl radicals, involving 4n + 3 and 4n + 1 carbon atoms, respectively. Except for allyl, the 7 electrons are accommodated in tightly localized SC orbitals, which resemble distorted C(2p,) atomic orbitals. [4n + 31 chains (with the exception of the C3H5) are best described by an antiresonance between equivalent KekulC structures. The SC wave functions for the other ([4n + 11) series are dominated by a single symmetry-adapted KekulC structure which is in conventional resonance with less important pairs of equivalent structures.

Research paper thumbnail of On the role of different spin bases within spin-coupled theory

Molecular Physics, May 1, 1993

Research paper thumbnail of Spin-Coupled Valence Bond Study of the Reaction between Benzene and a Methyl Cation

Journal of Physical Chemistry A, Apr 1, 1997

We present spin-coupled valence bond calculations on the reaction pathway for the insertion of a ... more We present spin-coupled valence bond calculations on the reaction pathway for the insertion of a methyl cation onto the aromatic system of benzene, leading to the Wheland intermediate C 6 H 6 CH 3 +. Simultaneously with the geometrical rehybridization of the two carbons forming the new substrate-electrophile bond (from sp 2 to sp 3), we observe the crossing of two spin-coupled potential energy curves; at large separations, these correspond to C 6 H 6 + CH 3 + and to C 6 H 6 + + CH 3. In the neighborhood of the crossing point, the spincoupled valence bond wave function obtained by mixing these two orbital configurations switches continuously from the former to the latter.

Research paper thumbnail of The Lowest Singlet and Triplet States of<i>o</i>-Benzyne: Spin-Coupled Interpretation of the Electronic Structure at CAS SCF Equilibrium Geometries

Israel Journal of Chemistry, 1993

The electronic structure of the X̃1A1 and ã3B2states of o‐benzyne is analyzed by means of spin‐co... more The electronic structure of the X̃1A1 and ã3B2states of o‐benzyne is analyzed by means of spin‐coupled (SC) wave functions involving eight active orbitals: two at the dehydro centers, in the plane of the ring, and six π orbitals, inherited from benzene. The equilibrium geometries of the two states, at which all calculations have been carried out, result from separate full geometry optimizations employing ‘8 in 8’ complete active space self‐consistent field (CAS SCF) wave functions incorporating an analogous core‐valence partitioning. The diradical character of the ground state of o‐benzyne, as approximated by the singlet SC wave function, is reflected in the low value of the overlap (0.46) between the two in‐plane sp2‐like SC orbitals at the dehydro centers, which are just 1.274 Å apart, and in the nonnegligible extent of triplet coupling between the spins of the electrons in these orbitals — ca. 6.4% of the total spin function for the valence electrons (expressed in the Serber spin basis) is made up of spin eigenfunctions, in which these electrons enter as a triplet pair. The geometry of the ground state is found to have an aryne‐like character, which is further confirmed by the pattern of alternation of the overlaps between neighboring 71 valence orbitals, and by the domination of the total spin function for the valence electrons (expressed in the Rumer spin basis) by the spin‐coupling patterns corresponding to an arynic structure.

Research paper thumbnail of Aromatic electrophilic substitution. A modern valence bond study

Journal of the Chemical Society, Faraday Transactions, 1995

ABSTRACT

Research paper thumbnail of Spin-Coupled Study of the Electronic Structure of Polyenyl Radicals C3H5-C9H11

Journal of the American Chemical Society, Mar 1, 1994

Spin-coupled (SC) theory is applied to the description of the electronic structure of polyenyl ra... more Spin-coupled (SC) theory is applied to the description of the electronic structure of polyenyl radicals allyl (CsHs), c5H.1, C~H Q , and CsH,,. A SC wave function treating nil seven electrons involved in the carbon-carbon bonds in C3H5 as active is found to reproduce the correct C, geometry for allyl in its ground state, as well as the proper spatial and spin symmetry of this state (2A2). Although no u-A separation is imposed a priori upon the seven-electron active space, the optimized SC orbitals display this symmetry and exhibit no tendency toward bent-bond formation and/or symmetry breaking. The most important correlation effects are shown to be included in the A space: The energy of the 7-only three-electron SC wave function for C3H5 coincides with that of the corresponding '3 in 3" completeactive-space self-consistent-field (CAS SCF) wave function. The SC model for the A electrons in allyl confirms the utility of the antipair concept introduced with the SC treatment of antiaromatic systems. The SC approach is demonstrated to yield highly correlated valence-bond-style A-active-space wave functions of the appropriate spatial and spin symmetry (2Az and 2B1) for the subseries of polyenyl radicals, involving 4n + 3 and 4n + 1 carbon atoms, respectively. Except for allyl, the 7 electrons are accommodated in tightly localized SC orbitals, which resemble distorted C(2p,) atomic orbitals. [4n + 31 chains (with the exception of the C3H5) are best described by an antiresonance between equivalent KekulC structures. The SC wave functions for the other ([4n + 11) series are dominated by a single symmetry-adapted KekulC structure which is in conventional resonance with less important pairs of equivalent structures.

Research paper thumbnail of Spin-coupled VB theory of molecular electronic structure. Ground and low-lying 1Σ+ states of CH+

Journal of Chemical Physics, Feb 15, 1985

The spin-coupled VB theory is applied to the ground and low-lying 1Σ+ states of CH+. Using 500 sp... more The spin-coupled VB theory is applied to the ground and low-lying 1Σ+ states of CH+. Using 500 spin-coupled structures, the calculated values of the spectrosopic constants for the ground states De(eV)=4.14 (4.26), ωe(cm−1)=2845 (2858), Re(Å) =1.135 (1.131); experimental results are given in parentheses. The form of the ground state potential energy curve is very close to the MCSCF-CI curve of Green et al. [Phys. Rev. A 5, 1614 (1972)], and similarly the excited states are very similar to those obtaind by MCSCF-CI methods by Saxon et al. [J. Chem. Phys. 73, 1873 (1980)]. The only exception is in the 4 1Σ+ state below R=3.5a0 where there is an avoided intersection with a Rydberg state. At least for small systems of this kind, the spin-coupled VB approach is capable of the same level of accuracy as the best MCSCF-CI methods, but uses an order of magnitude fewer functions. Each state in the spin-coupled VB calculation is dominated by one or two structures. This provides the wave function with much visuality, but without the usual attendant loss in accuracy.

Research paper thumbnail of From biomass to energy: hydrogen-based technology by bio-ethanol reforming

From biomass to energy: hydrogen-based technology by bio-ethanol reforming/I. Rossetti, C. Biffi,... more From biomass to energy: hydrogen-based technology by bio-ethanol reforming/I. Rossetti, C. Biffi, G. Faita, GF Tantardini, M. Raimondi, L. Forni.-2009. International Workshop on Distributed Energy Systems: The Role of Chemical Sciences and ...

Research paper thumbnail of From biomass to energy: H2 based technology from bioethanol

From research to …, 2009

From biomass to energy: H2 based technology from bioethanol / I. Rossetti, C. Biffi, G. Faita, M.... more From biomass to energy: H2 based technology from bioethanol / I. Rossetti, C. Biffi, G. Faita, M. Raimondi, GF Tantardini, L. Forni - In: From research to industry and markets : 17. European Biomass Conference & Exhibition : proceedings of the international conference held in ...

Research paper thumbnail of Da biomassa ad energia : tecnologia ad idrogeno da bioetanolo

Seminario della Facoltà di Scienze MFN : “L’energia di domani”, 2008

Research paper thumbnail of A Symposium on Molecular Electronic-Structure Calculations - Methods and Applications - General Discussion

Research paper thumbnail of The charge transfer of N with atomic hydrogen

Research paper thumbnail of Study of the mechanism of formation of CH + 2 ( 2 A 1 , 2 B 1 ) from C + ( 2 P ) + H 2 using spin-coupled valence bond theory

Molecular Physics, 1987

The spin-coupled valence bond method is used to study the formation of CH+2 in its lowest 2A1 and... more The spin-coupled valence bond method is used to study the formation of CH+2 in its lowest 2A1 and 2B1 states for perpendicular approaches by C+(2P) to H2. The potential surfaces for these two states have been extensively mapped out at the spin-coupled level. These wave functions show that in both states there is a sudden switch-over from C+(2P) + H2

Research paper thumbnail of Application of valence-bond techniques to the study of weakly bound complexes. The potential energy surface of the Ne?CH4 system

Physical Chemistry Chemical Physics, 2007

We present a comprehensive survey of the Molecular Orbital-Valence Bond (MO-VB) method, a theoret... more We present a comprehensive survey of the Molecular Orbital-Valence Bond (MO-VB) method, a theoretical scheme developed within the framework of the Valence Bond theory to deal with weakly bound intermolecular complexes. According to the MO-VB, the wavefunction of the system is expressed as a truncated non-orthogonal Configuration Interaction expansion, which is size extensive and a priori free of basis set superposition error. We report on the recent developments of the method, which extend the range of application of the MO-VB to intermolecular complexes with a quite large number of correlated electrons, showing that VB-based methods are nowadays a valid alternative to Molecular Orbital approaches also in this field. The MO-VB has been applied to study extensively the Ne-CH(4) complex, and compared with the more standard MP4 and CCSD(T) results. We determined two analytical Potential Energy Surfaces (PES) for this system, computed at MO-VB and MP4 level, which represent the first ones coming entirely from ab initio computations. The features of our potentials are discussed, and compared to the single analytical potential which includes the anisotropy available in the literature, determined about twenty years ago by Udo Buck and co-workers using a semiempirical approach [U. Buck, A. Kolhase, D. Secrest, T. Phillips, G. Scoles and F. Grein, Mol. Phys., 1985, 55, 1233]. The differences among the three PES are quite relevant, and are due to play a relevant role in the theoretical simulations of the dynamical properties of the Ne-CH(4) system.

Research paper thumbnail of The spin-coupled description of aromatic, antiaromatic and nonaromatic systems

Theoretical and Computational Chemistry, 1999

The correlated motion of π electrons in a range of aromatic, antiaromatic and nonaromatic systems... more The correlated motion of π electrons in a range of aromatic, antiaromatic and nonaromatic systems is investigated using modern spin-coupled theory. In the particular case of benzene, the resulting wavefunction, which is superior to the simple MO description, resembles closely the classical VB picture, except for small (but crucial) deformations of the orbitals. It is argued, by reference to a multiconfiguration approach which subsumes both of the simple MO and VB models, that we should feel comfortable with switching between the two modes of description, according to which leads most directly to correct predictions for the particular problem at hand. The bonding in benzene is contrasted with that in cyclobutadiene and cyclooctatetraene, at various geometries. In the spin-coupled model, the distinguishing feature of antiaromatic species is the occurrence of triplet coupling of electron pairs. Of course, the singlet ground states of both of these molecules distort from the idealized high-symmetry geometries to nonaromatic situations with olefinic bonds.

Research paper thumbnail of An MO−VB Approach for the Determination of Intermolecular Forces. Theory and Calculations on the He 2 , He−CH 4 , and He−H 2 O Systems

The Journal of Physical Chemistry A, 2002

We present the improvement of a previously developed strategy for the evaluation of intermolecula... more We present the improvement of a previously developed strategy for the evaluation of intermolecular forces. The approach defines a variational VB (valence bond) wave function, consisting of single and double excitations from the SCF-MI (self-consistent field for molecular interactions) determinant. The central idea of the method is the determination of optimal virtual orbitals, to contract the virtual space spanned by all singly and doubly excited localized configurations, by means of an iterative optimization procedure. The performance of the strategy is tested by comparison with results where the full virtual space is considered, and the entire approach is also compared with more conventional quantum chemical methods. Test calculations on three weakly interacting complexes, namely, He 2 , He-CH 4 , and He-H 2 O, are presented. Whatever the system studied, we found an overall agreement between VB, MP4, and CCSD(T) results. The VB well depths estimates are somewhat larger than MP4 and CCSD(T) ones.