Journal ArticleDOI
Molecular MC–SCF calculations
TLDR
In this article, a method for finding multi-configurational SCF wave functions is proposed, where the basic equation is equivalent to the Brillouin theorem; comparison with the usual SCF equations obtained through effective hamiltonians gives an interpretation of the offdiagonal Lagrange multipliers.Abstract:
A practical method for finding multi-configurational SCF wave functions is proposed The basic equation is equivalent to the Brillouin theorem; comparison with the usual SCF equations obtained through effective hamiltonians gives an interpretation of the offdiagonal Lagrange multipliers Numerical applications to Formaldehyde in a minimum Slater-type orbital basis with four different variational wave functions are reported The molecular orbitals found in these calculations are localized on the chemical bonds The largest contributions to the energy are obtained from π-π and dispersion-type σ-π correlationread more
Citations
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Book ChapterDOI
The Complete Active Space Self‐Consistent Field Method and its Applications in Electronic Structure Calculations
Journal ArticleDOI
The complete active space SCF (CASSCF) method in a Newton–Raphson formulation with application to the HNO molecule
Abstract: The complete active space (CAS) SCF method is presented in detail with special emphasis on computational aspects. The CASSCF wave function is formed from a complete distribution of a number of active electrons in a set of active orbitals, which in general constitute a subset of the total occupied space. In contrast to other MCSCF schemes, a CASSCF calculation involves no selection of individual configurations, and the wave function therefore typically consists of a large number of terms. The largest case treated here includes 10 416 spin and space adapted configurations. To be able to treat such large CI expansions, a density‐matrix oriented formalism is used. The Newton–Raphson scheme is applied to calculate the orbital rotations, and the secular problem is solved with recent developments of CI techniques. The applicability of the method is demonstrated in calculations on the HNO molecule in ground and excited states, using a triple‐zeta basis and different sizes of the active space. With a reasonable choice of active space, the calculations converge in 6–10 iterations. This is true also for states which are not the lowest state of the symmetry in question. The equilibrium geometry for the ground state is RNO=1.215(1.212) A, RNH =1.079(1.063) A, ϑHNO=108.8(108.6) °, the experimental values given in parenthesis for comparison. The best estimates for the transition energies to the lowest 3A″ and 1A″ states are 0.67(0.85) eV and 1.52(1.63) eV, respectively. The results obtained indicate that the choice of active space may be crucial for the convergence properties of CASSCF calculations.
Journal ArticleDOI
PNO–CI Studies of electron correlation effects. I. Configuration expansion by means of nonorthogonal orbitals, and application to the ground state and ionized states of methane
TL;DR: In this paper, the convergence of the configuration expansion of a many-electron wave function was improved without significantly complicating the energy matrix elements by using partially non-orthogonal orbitals.
Journal ArticleDOI
Principles for a direct SCF approach to LICAO–MOab-initio calculations
TL;DR: In this article, the principles and structure of an LCAO-MO ab-initio computer program which recalculates all two-electron integrals needed in each SCF iteration are formulated and discussed.
Journal ArticleDOI
The construction and interpretation of MCSCF wavefunctions.
TL;DR: The multiconfiguration self-consistent field method offers the most general approach to the computation of chemical reactions and multiple electronic states and localized orbitals are convenient both for selection of the appropriate active space and for understanding the computed results.
References
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Journal ArticleDOI
Quantum Theory of Many-Particle Systems. I. Physical Interpretations by Means of Density Matrices, Natural Spin-Orbitals, and Convergence Problems in the Method of Configurational Interaction
TL;DR: In this article, the authors define a set of generalized density matrices for the Hermitean density matrix of order $k, which is further antisymmetric in each set of these indices.
Journal ArticleDOI
Self-Consistent Field Theory for Open Shells of Electronic Systems
Journal ArticleDOI
Canonical Configurational Interaction Procedure
J. M. Foster,Samuel Francis Boys +1 more
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