MCSCF Calculations Research Articles

Numerical MCSCF study of the total (electronic and nuclear) parallel polarizability and hyperpolarizability for the H2, HD and D2molecules

We present numerical MCSCF calculations of the electronic and nuclear components of the parallel molecular polarizabilities (α zz ) and the second hyperpolarizabilities (γ zzzz ) of H2, HD and D2. Contrary to the H+ 2 case, where the vibrational contribution to γ is fifty times larger than the pure electronic γ, the vibrational γ and the electronic γ for H2 have the same magnitude. The electronic correlation has a stronger effect on α and γ for vibrationally-excited states than for the ground state.

Electronic structure of phenyl cation by MC SCF ab initio calculations

In order to investigate the mechanistic aspects of gas-phase ion-molecule reactions of phenyl cation, C 6H 5 +, formed by nuclear decay of tritiated benzene, with nucleophilic species, the electronic structure of this ion has been examined by ab initio methods. In particular, SCF and MC SCF calculations have been performed on the two most stable electronic states of phenyl cation, 1A 1 and 3B 1. The results obtained indicate that 1A 1 is the ground electronic state, 3B 1 being ≈ 5 (15) kcal mol −1 higher in the 6-31G*//STO-3G (4-31G//STO-3G) MC SCF level of calculation.

A quantum chemical study on the conformations of nitrosomethanol

Two rotameric conformations, cis and trans, of the recently produced molecule, nitrosomethanol H 2C (OH) (NO) have been studied. The results of SCF MO with MP2 calculation of the optimized geometries using RHF and UHF ab initio methods with 3-21G basis set confirm that the singlets are the ground states for the two isomers of the molecule. The CAS MCSCF calculation with STO-3G basis set supports the conclusion made by MP2. It is also concluded that the singlet state of the cis form is more stable than that of the trans while the reverse order of stability occurs for the triplet states of the molecule.

Development of a program for MCSCF calculations with large basis sets

AbstractA new program named JASON2 for ab initio MCSCF calculations with large basis sets has been developed. In this program, the density‐formulated super‐CI CASSCF method proposed by Roos et al. was adopted in order to overcome difficult problems caused by large basis sets. The sparsity of integrals and vectorization on supercomputers were utilized as much as possible. With JASON2, we performed CASSCF calculations on a Fe–porphine complex with 232 CGTFs (contracted Gaussian‐type functions). Our algorithms and timing data of the above calculations are reported.

Application of the multiconfiguration time-dependent Hartree–Fock method to CH+: The auxiliary active space

The multiconfigurational time-dependent Hartree–Fock (MCTDHF) method is used to study the excitation energies and transition moments of the CH+ ion. A consistent approach to the selection of configurations based on the complete active space (CAS) model is successfully applied. In this approach a large set of orbitals is included in the active space without a substantial increase in the number of configurations, since only for a smaller subset of orbitals is the configuration expansion complete and the remaining orbitals active are considered an auxiliary set. For CH+ the active space includes the valence and additional orbitals, with the valence orbitals constituting the complete subset. Extra configurations involving single and double excitations from the valence to the additional (auxiliary) orbitals are included both for the MCSCF reference state and in the MCTDHF. It is demonstrated that this active space accurately mimics MCSCF and MCTDHF calculations with an enlarged complete active space which includes both the original CAS and the auxiliary orbitals. The calculations for the 1 Π and 1 Σ+ excited states at the equilibrium geometry show that accurate excitation energies and transition moments are obtained in this way. For most of these states for which there are other good ab initio results available, we obtain comparable excitation energies and transition moments. In addition to 1 Π and 1 Σ+, low lying states of other symmetries are obtained and discussed.

Numerical Hartree-Fock and MCSCF calculations on diatomic copper: calibration of basis sets

NHF and NMCSCF results for Cu2 are compared with calculations employing basis set expansions. We find that nearly all previous SCF calculations using Gaussian basis sets have underestimated the bond length by about the same amount (0.03 A) as that attributed to the unlinked cluster and relativistic corrections. The error is shown to be due to deficiencies in the 3d primitive set which yield sizable basis set superposition errors.

A theoretical study of hydridocobalt carbonyls. II. Interdependence of geometry and electronic structure

The electronic structure of hydridocolbalt tri- and tetra-carbonyls was investigated by means of ab initio methods in two different Gaussian basis sets. Hartree–Fock, configuration-interaction, and MCSCF calculations were performed on a number of conformations and electronic states of both compounds. Pyramidal distortions of the symmetric C3v forms of HCo(CO)3 were investigated, and Jahn–Teller distortions of the triplet states were described at the MCSCF level of theory. The interdependence of correlation effects and molecular geometry was examined in both basis sets. It is concluded that dispersion forces play a large role in metal–ligand bonding and need to be included in the qualitative model.

UHF natural orbitals for defining and starting MC-SCF calculations

A simple way of generating starting orbitals for multiconfigurational SCF calculations (particularly of the CAS type) is to use the natural orbitals of the unrestricted Hartree–Fock wave function. Significant fractional occupancy of a UHF natural orbital indicates that the orbital should be included in the active space; this is illustrated for symmetrically stretched water, for NO, N2O4, and for the transition state on the Me3CO→Me2CO+CH3 reaction surface. Average natural orbitals should be used if there are several UHF solutions for the same state; this restores symmetry broken at the the UHF level. Configuration selection based on fractional occupation appears to be the basis for a reliable automated procedure. The UHF natural orbitals provide good starting orbitals for the two-configuration SCF and for the 4×4 CAS wave function in stretched F2, H2O2, and C2H6, methane, water, in twisted ethylene, in ozone, and for various CAS wave functions in the first asynchronous transition state of the HCNO+H2C2 1,3-cycloaddition. Comparison is made with the extended Hartree–Fock method.

The electronic and molecular structure of silyl nitrene

FORS MC SCF calculations performed with the 6-31G d basis set have been used to predict the geometries of the lowest singlet ( 1E) and triplet ( 3A 2) states of silyl nitrene. Both states are found to have C 3v symmetry at this computational level. Single- second-order configuration interaction calculations that is, all single and double excitations from the FORS MC SCF reference configuration) at the MC geometries predict the triplet ground state to be 42.3 kcal/mol below 1E and only 8.2 kcal/mol above the singlet ground state of silaimine.

Reaction of singlet oxygen with organic sulfides. A theoretical study

The intermediates in the reactions of singlet oxygen with H/sub 2/S and Me/sub 2/S have been calculated at Hartree-Fock, Moeller-Plesset, and multiconfiguration levels of theory. A peroxy sulfoxide structure, which has been proposed as an intermediate in the photooxidation of organic sulfides, was found to be a stable intermediate, but the barrier for dissociation to reactants is calculated to be only a few kilocalories per mole. The infrared frequencies and intensities of this intermediate were calculated for the purpose of possible spectroscopic identification. A cyclic thiadioxirane, which also has been suggested as a possible intermediate, was found not to be a minimum on the potential energy surface. Large-scale MCSCF calculation shows that the peroxy sulfoxide is best described by a zwitterionic, not a biradical, structure. Calculated activation energies and overall reaction energies are found to require a very large basis set in order to give reasonable agreement with experimental results.

Second-order Mo/ller–Plesset perturbation theory as a configuration and orbital generator in multiconfiguration self-consistent field calculations

It is shown that the magnitude of the natural orbital (NO) occupation numbers of second-order Mo/ller–Plesset (MP2) perturbation theory can be used to select physically reasonable configuration spaces for ground state MCSCF calculations. When the MP2 NO’s are used as an initial guess for the orbitals, a second-order Newton–Raphson MCSCF calculation is in the local region from the first iteration. Fast convergence is therefore ensured to a stationary point with orbitals of similar structure as the MP2 NO’s, thereby reducing significantly the risk of converging to undesired stationary points.

Electron shake-up and correlation satellites and continuum shake-off distributions in X-Ray photoelectron spectra of the rare gas atoms

The rare gases H, He, Ne, Ar, Kr and Xe have been studied in the gas phase at low pressure by means of high resolution photoelectron Spectroscopy. Monochromatized Al Kα radiation at 90° was used to excite the photoelectrons. Extended energy regions of all valence electron spectra and the most interesting core electron spectra have been recorded. The spectra contain a large number of correlation satellites and shake-up as well as double shake-up satellites which originate from unknown highly excited, singly ionized states. The energies and intensities of these discrete satellites have been determined and several states are identified. The assignments rely partly on results from relativistic MCSCF calculations, and partly on comparisons with optical data and recent Auger electron data. Relativistic MCSCF energies for a large number of excited states in Kr + and Xe + are reported. Extended shake-off continua in both the valence region and in the core electron regions have been recorded. The shake-off continua of Ne and Ar reveal several Fano-like resonance states. The resonances occur when shake-up photoionization channels, involving inner valence electron excitation or multiple valence excitation, have the same energies and symmetries as photoionization channels for outer valence electron shake-off. A theoretical model for this type of resonance is presented and some numerical results are obtained and compared with experimentally determined Fano parameters for the most prominent resonance in the Nels shake-off spectrum.

ChemInform Abstract: Spin‐Orbit Coupling in Biradicals. Ab initio MCSCF Calculations on Trimethylene and the Methyl‐Methyl Radical Pair.

ChemInform Abstract: Spin‐Orbit Coupling in Biradicals. Ab initio MCSCF Calculations on Trimethylene and the Methyl‐Methyl Radical Pair.

Single and double shake-up processes in the Ne 1s photoelectron spectrum

The core electron satellite spectrum accompanying a 1s ionisation in atomic Ne is studied using monochromatised X-ray excited (1487 eV photon energy) photoelectron spectroscopy. Several new 2s-3s satellites are observed and assigned. The shake-off limits for the 2p-np and 2s-ns series are determined and also the continuum strength is measured to 37% of the main 1s line intensity. Fano-type resonances are detected both in the 2p-np and 2s-ns single shake-up regions and among the double shake-up lines. In the double shake-up region 18 lines are observed and a preliminary assignment is made using theoretical MCSCF calculations.

CAS SCF/CI calculations of potential energy surfaces of He 3+ and He 2+

Complete active space MC SCF (CAS SCF) calculations followed by second-order configuration interaction (SOCI) calculations are carried out on the potential energy surfaces (bending surface, linear surfaces) of the 2Σ g + ground state of He 3 +. The potential minimum for the 2Σ g + state occurs at a linear geometry with HeHe bond length of 1.248 Å. The binding energy of He 3 + with respect to He + He + + He was calculated to be 2.47 eV at the SOCI level. The energy required to dissociate He 3 + ( 2Σ g +) into He 2 + ( 2Σ u +) and He( 1S) is calculated to be 0.14 eV. The same level of SOCI calculations of He 2 + yield a D e value of 2.36 eV.

Frozen core and virtual orbitals in the MC SCF theory

An efficient approximation in the multiconfigurational self-consistent field (MC SCF) method is presented for calculations on large systems. In this method, the orbitals are spanned by adequate orthonormal orbitals, such as Hartree-Fock orbitals, instead of by the entire set of basis functions, and certain of these are not included in the orbital optimization. These include the core orbitals and high-energy virtual orbitals selected from the Hartree-Fock manifold. The computational time for MC SCF calculations dramatically decreases without significant loss of accuracy. This saving in computer time is primarily due to the reduction in the size of the integral transformation. Test calculations are performed for Sc 2, CO, and acetylene, and the accuracy of the energy and the Hellmann-Feynman force are checked.

Theoretical investigation of the electronic states of helium tetramer(1+)

Complete active space MCSCF calculations followed by second-order configuration interaction calculations (SOCI) are carried out on the /sup 2/B/sub 2/ and /sup 2/A/sub 1/ electronic states of the He/sub 4//sup +/ ion with a large Gaussian basis set. A number of geometries such as linear, rhombus, square, and tetrahedron are considered for these electronic states. Two nearly degenerate lowest electronic states are found. The ground state is found to be a linear /sup 2/..sigma../sub u//sup +/ electronic state. Another electronic state of /sup 2/B/sub 2u/ (D/sub 2h/) symmetry with an equilibrium geometry of rhombus is only 2.2 kcal/mol above the linear /sup 2/..sigma../sub u//sup +/ state. The acute angle of the rhombus structure is 26.6/sup 0/.

Spin-orbit coupling in biradicals. Ab initio MCSCF calculations on trimethylene and the methyl-methyl radical pair

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSpin-orbit coupling in biradicals. Ab initio MCSCF calculations on trimethylene and the methyl-methyl radical pairLouis Carlacci, Charles Doubleday Jr., Thomas R. Furlani, Harry F. King, and James W. McIver Jr.Cite this: J. Am. Chem. Soc. 1987, 109, 18, 5323–5329Publication Date (Print):September 1, 1987Publication History Published online1 May 2002Published inissue 1 September 1987https://doi.org/10.1021/ja00252a004RIGHTS & PERMISSIONSArticle Views511Altmetric-Citations70LEARN 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 (860 KB) Get e-Alerts Get e-Alerts

ChemInform Abstract: Dynamic Stability of Carbonyl Ylides

The non- and fluorine-substituted singlet carbonyl ylides are studied by using ab initio MCSCF calculations. The thermodynamic stability of the carbonyl ylides and the intramolecular stability to isomerization or fragmentation reaction coordinates is demonstrated in terms of the topological structure of the ab initio potential energy surfaces. The allylic resonance is found to be dynamically unstable, considering out-of-plane vibrational mode. The instability is studied by the symmetries of the low-lying excitations out of the MCSCF wave function.

Theoretical study of the energetics of the O 3(X̃ 1A 1) → O 2(X 3Σ −g) + O( 3P) dissociation process

The dissociation energy ( D e) for the O 3(X̃ 1A 1) → O 2(X 3Σ − g) + O( 3P) process is computed using MC SCF, CI, MBPT, and CCD methods. A full-valence MC SCF calculation utilizing a [9s5p3d1f/5s3p2d1f] basis set yields a D e value of 0.43 eV, far below the experimental value of 1.13 eV, demonstrating the importance of correlation effects involving non-valence orbitals. A CI calculation in the same basis set allowing for all single and double excitations from three-reference configuration yields a D e value of 0.72 eV. This value is increased to 1.06 eV when the Davidson correction is included. When the number of reference configurations is increased to eight, the resulting CI calculation gives a D e value of 0.82 eV prior to the Davidson correction and 1.10 eV after this correction.