MCSCF Calculations Research Articles

Ion-impact-induced fragmentation of water molecules

The multiple ionization and fragmentation of H 2O by fast H +, He +, O 6+ and O 7+ ions was studied utilizing a position-and time-sensitive multi-particle detector which allows the coincident measurement of the momenta of correlated fragments. Thereby, the fragmentation energy and angular correlations for each individual event can be derived. Of special interest are the processes H 2O → H + + H + + O q+ , where a simple Coulomb-explosion model is insufficient to explain the measured energy and angular spectra. Better agreement was achieved with ab initio MCSCF calculations which take into account several molecular states of the fragmenting highly charged H 2O ( q+2)+ ion.

Determination of contracted atomic basis sets for the study of CN −; application to the MCSCF calculations of its spectroscopic constants

Contracted atomic basis sets have been constructed from ANOs in order to describe the ground and excited states of C and N and their anions. Application is given with the MCSCF calculation of the vibration frequency of CN − and of other spectroscopic constants. A procedure is proposed to do MCSCF calculations when some problems of convergence arise.

MCSCF calculation of response properties of Argon

MCSCF calculation of response properties of Argon

Parallel algorithm for integral transformations and GUGA MCSCF

An algorithm for the parallelization of the atomic to molecular integral transformation and the subsequent steps in a GUGA based MCSCF calculation is presented. Timing data shows that the transformation and diagonalization steps are well parallelized and that several of the other portions of the MCSCF code are moderately parallel. Remaining sequential bottlenecks are identified.

Linear response calculation of potential energy curves of BH

AbstractThe multiconfiguration time‐dependent Hartree–Fock (MC TDHF) approximation is applied to study the excited states of the BH molecule. The potential energy curves and spectroscopic properties of the excited states are analyzed. The MC TDHF results are compared to similar MC SCF calculations for individual excited states. © 1994 John Wiley & Sons, Inc.

MCSCF and CASPT2N Calculations on the Excited States of 1,2,4,5-Tetramethylenebenzene. The UV-Vis Spectrum Observed Belongs to the Singlet State of the Diradical

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTMCSCF and CASPT2N Calculations on the Excited States of 1,2,4,5-Tetramethylenebenzene. The UV-Vis Spectrum Observed Belongs to the Singlet State of the DiradicalDavid A. Hrovat and Weston Thatcher BordenCite this: J. Am. Chem. Soc. 1994, 116, 14, 6327–6331Publication Date (Print):July 1, 1994Publication History Published online1 May 2002Published inissue 1 July 1994https://doi.org/10.1021/ja00093a036RIGHTS & PERMISSIONSArticle Views176Altmetric-Citations23LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-AlertsSupporting Info (2)»Supporting Information Supporting Information Get e-Alerts

Biradical Thermochemistry from Collision-Induced Dissociation Threshold Energy Measurements. Absolute Heats of Formation of ortho-, meta-, and para-Benzyne

The absolute heats of formation of 1,2-, 1,3-, and 1,4-dehydrobenzene (ortho, meta-, and para-benzyne) have been determined from measurements of the threshold energies for collision-induced dissociation (CID) of ortho, meta-, and para-chlorophenyl anions in a flowing afterglow-triple quadrupole apparatus. The 298 K heats of formation for ortho-, meta-, and para-benzyne derived in this manner are 106.6 [+-] 3.0, 122.0 [+-] 3.1, and 137.3 [+-] 3.3 kcal/mol, respectively. The values for meta- and para-benzyne are higher than those reported previously but are in excellent agreement with recently reported MCSCF and CI calculations. Several control experiments are described which demonstrate that the earlier results for meta- and para-benzyne suffered from an acid-catalyzed isomerization of the reactant chlorophenyl anions in the flowing afterglow prior to CID threshold analysis. 70 refs., 5 figs., 4 tabs.

MCSCF calculations of Verdet constants

Quadratic response function theory for self-consistent field and multiconfiguration self-consistent field wavefunctions have been used to calculate the Verdet constants for the nitrogen and acetylene molecules. The correlation contribution is oppositely directed in these two molecules and is important in obtaining close agreement with experimental results.

Potential energy surfaces of carbon dioxide

Several bent valence states of CO2 are characterized by means of full-valence-space MCSCF calculations. The ground state potential energy surface exhibits a double well corresponding to a ring minimum, with C2vsymmetry (1A1) and a 73.1° OCO angle, in addition to the linear (1σ) global minimum. The transition state for the ring opening process, which has a barrier of 12.1 kcal/mole with respect to the ring minimum, is however found to have Cs symmetry. Double minima are also shown to exist for the 1A2, 1B1 and 1B2 excited states. However, in these cases all minima are bent. Cross sections through the ground state potential energy surface corresponding to the two collinear exchange reactions O(1D) + CO(1σ+) → OC(1σ+) + O(1D) C(3P) + O2(3σ) → CO(1σ+) + O(1D) are also calculated and their energy contour maps are reported. The latter reveals the existence of a stable linear intermediate with the structure COO. © 1994 John Wiley & Sons, Inc.

Structure and harmonic vibrational frequencies of cyclopentadiene in the lowest singlet states

MCSCF calculations are reported for the electronic ground state and the two lowest excited singlet states of cyclopentadiene. Molecular geometries have been optimized for each state individually and the complete harmonic force field calculated. Similar to the situation found for cis- and ( trans-butadiene the CC double bond length significantly increases on electron excitation and the CC single bond length within the cis-butadiene substructure decreases. The changes in the vibrational frequencies and in the character of the vibrational modes on excitation is documented in detail. Whereas the electronic ground state and the 1 1B 2 state have C 2v symmetry (with the carbon ring system and the CH bonds in one plane) the 2 1A 1 state is predicted to have a lower symmetry with a very shallow minimum for the respective torsional modes.

On the choice of the active space and the basis set for MCSCF calculations on small molecules and reactive surfaces

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTOn the choice of the active space and the basis set for MCSCF calculations on small molecules and reactive surfacesLeif A. Eriksson, Xabier Lopez, and Russell J. BoydCite this: J. Phys. Chem. 1993, 97, 46, 11969–11973Publication Date (Print):November 1, 1993Publication History Published online1 May 2002Published inissue 1 November 1993https://doi.org/10.1021/j100148a021RIGHTS & PERMISSIONSArticle Views195Altmetric-Citations2LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (630 KB) Get e-Alerts

Ab initio study of bond stretching: Implications in force‐field parametrization for molecular mechanics and dynamics

AbstractWe report a theoretical study of the stretching of chemical bonds and its implications on the force‐field parametrization. Computations were performed at the SCF and MCSCF levels by using minimal, split‐valence, and large extended and polarized basis sets. The stretching energy profiles were determined considering up to 25 perturbed geometries of 11 different bonds (6 singles, 2 doubles, and 3 triples). The energy profiles and stretching parameters are compared with the experimental data compiled in the most popular force fields. MCSCF stretching energy profiles are mainly anharmonic and can be only roughly reproduced by quadratic equations. The use of Allinger's MM2 quasiharmonic expression appears as the best choice because it fits with reasonable accuracy a large percentage of the stretching profile without increasing the complexity of the formalism and of the parametrization procedure. MCSCF computations are needed to obtain reliable stretching force parameters. In this respect, MCSCF calculations considering as active space only the bonded and nonbonded orbitals of the perturbed bond seems to be the best strategy to obtain good results at a minimum computational cost, especially if small split‐valence basis sets like the 3‐21G are used. Results obtained at this level of sophistication are completely comparable to stretching parameters compiled on empirical force fields. © 1993 John Wiley & Sons, Inc.

Reaction of lithium amide with methane: an ab initio and atoms in molecules study

The reaction of lithium amide with methane was studied with ab initio calculations, and the electron density function was analyzed using the theory of atoms in molecules. The transition state for the reaction is a highly ionic species that can be modeled approximately as an ionic aggregate. The nature and evolution of the bonds were monitored throughout the reaction and related to the accompanying changes in the behavior of the charge density at the bond critical points and in the properties of the individual atoms. The contributions of each atom to the energies of reaction and activation were determined. The HF/6-31G * level of theory was tested by an MCSCF calculation of the transition state and shown to perform adequately in the description of the reaction

Low-lying states of SF6 and SF6?: Electron affinity of SF6 and electron detachment of SF6?

Calculations have been performed on low-lying states of SF6 and SF at the complete active space multiconfiguration self-consistent field (CAS–MCSCF) level. These calculations involved optimization of basis function exponents, particularly the f-functions on sulfur and their effect on the potential energies and the SF6 electron affinity. The CAS–MCSCF calculations for the anion and neutral included 4,863 and 2,115 configuration functions, respectively. Octahedral and selected distorted geometries were considered. The ground-state equilibrium geometries correspond to the octahedral configuration of SF6 (1A1g), R(SF) = 1.56 A, ωe = 776 cm−1, and of the anion SF(2A1g), R(SF) = 1.70 A, ωe = 558 cm−1. The adiabatic and vertical electron affinities are found to be -0.63 and 1.8 eV, respectively. Electron autodetachment phenomena associated with the anion are also discussed and a possible mechanism for this process is presented. © 1993 John Wiley & Sons, Inc.

Transition‐state structures for the reaction between styrene and tbutyl cyanoketene: Theoretical evidence for antarafacial characteristics in a π2 + π2 thermal cycloaddition

AbstractSemiempirical AM1 SCFMO models for the cycloaddition between monosubstituted alkenes (RCH = CH2, R = Me, Ph, CF3) and tbutyl cyanoketene predict the lowest‐energy transition state to be relatively polar and asynchronous in character and to have a cis relationship between the R and the tbutyl groups, in agreement with the experimentally determined product for R = Ph. We interpret this stereochemistry as resulting from the presence of a significant antarafacial component in the transition state, which enables these two bulky groups to adopt pseudoequatorial positions in which steric interactions are minimized. The alternative higher energy trans geometry forces the R and the cyano groups to adopt more sterically hindered pseudoaxial positions. Alternative biradical‐like transition states inferred by others from ab initio MCSCF calculations on the reaction between ketene and ethene do not retain any significant antarafacial component on the ketene and, therefore, form less satisfactory models for this specific reaction. © 1992 John Wiley & Sons, Inc.

The band structure of nickel: ab initio calculations on the Ni 6 cluster

All-electron, ab initio MC SCF calculations have been carried out for an octahedral cluster of six Ni atoms. The d band is found to be narrow, and lower in energy than the broad s band, indicating that the bonding is primarily due to the s electrons. This suggests that d electrons can be represented by a local description, justifying the use of one-electron ECP methods for this and similar systems.

Calculation of barriers to proton transfer using a variety of electron correlation methods

The usefulness of various combinations of MCSCF and CI methods in computing correlated proton transfer potentials is investigated for the systems, HF, H7N, H3O, and H5O. MCSCF calculations can accurately determine proton transfer barriers, provided the correlation is limited to the proton transfer process. The proper correlated space can be obtained more easily if the canonical occupied MOs are first subjected to a localization. Various means are tested of including additional electron correlation into the MCSCF methods. CIS and CISD calculations are performed following MCSCF expansion of the wave function using various different MCSCF reference wave functions. The MCSCF + CISD results are excellent, being fairly independent of choice of virtual Mos, although it is important that the occupied orbitals be balanced between the donor and acceptor. Localizing the occupied MOS prior to the MCSCF part of the calculation again results in a further improvement. These results are compared to CI computations using the canonical orbitals (and which are not preceded by MCSCF preparation of the wave function) and to Møller–Plesset results. © 1992 John Wiley & Sons, Inc.

Interactions of Rydberg and valence states in CO 2

The electronically excited singlet and triplet states of CO 2 lying below 11.4 eV have been investigated by ab initio MCSCF calculations. It has been found that the valence excited states with the electronic configuration (1π 3 g2p2π 1 u form sharp avoided crossings along the symmetric stretching coordinate with the Rydberg states of the same symmetry resulting from the electronic configuration (1π 3 g3p3π 1 u). Hence, the Rydberg states can dissociate into the lowest valence asymptotes O( 3P, 1D) + CO(X 1Σ +), in accordance with experimental findings. In addition, the photodissociation of CO 2 is complicated by the fact that the valence singlet and triplet states 1,3Σ − u and 1,3Δ u form conical intersections with the diffuse 1,3Π g states in the Franck-Condon region of the absorption spectrum.

A critical investigation of XPS predicted proton affinities

A thermochemical model for predicting gas-phase proton affinities from core ionization energies is investigated for a number of test cases. Deviations from experimental values are attributed to alternate sites of protonation, geometrical relaxation effects and chemical effects on the core replacement energies in the equivalent core approximation. The error types are further explored by ab initio MCSCF calculations, including geometry optimizations, using analytical molecular gradients and hessians. For the protonation of ammonia the thermochemical model is simulated by MCSCF calculations, confirming both the predicted proton affinity and the observed discrepancy vs. the experimental value. The importance of including zero-point vibrational energies in both core ionization and proton affinity calculations is stressed. The calculations suggest that the geometry-related error is small for protonation of nitrogen but considerable for protonation of fluorine, ≈ 1 eV.

Bond formation between positively charged species. Non-adiabatic analysis and valence-bond model in the CO 2+ case

Ab initio MCSCF calculations of the potential energy curves of the first low-lying states of the CO 2+ dication have been performed. A localization procedure of MCSCF orbitals allows us to discuss the bond formation in the framework of valence-bond theory. The non-adiabatic curves corresponding to C + + O +, C 2+ + O and C + O 2+ are described using both orthogonal and non-orthogonal quasi-atomic orbitals, allowing us to re-examine the currently proposed models of bond formation between positively charged species.