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Chemical Shift: Internal References and Solvent Effects01:17

Chemical Shift: Internal References and Solvent Effects

In an NMR sample, precise measurement of the absolute absorption frequencies of nuclei is difficult. A standard internal reference compound is added, and the frequency difference between the reference signal and sample signals is measured.
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Accurate ab initio density fitting for multiconfigurational self-consistent field methods.

Francesco Aquilante1, Thomas Bondo Pedersen, Roland Lindh

  • 1Department of Theoretical Chemistry, Chemical Center, University of Lund, P.O. Box 124, S-221 00 Lund, Sweden. francesco.aquilante@teokem.lu.se

The Journal of Chemical Physics
|July 16, 2008
PubMed
Summary
This summary is machine-generated.

This study presents a new computational chemistry method for large-scale applications. The Cholesky decomposition and density fitting approximations enable accurate calculations on bigger molecules using the complete active space self-consistent field (CASSCF) method.

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Area of Science:

  • Computational Chemistry
  • Quantum Chemistry
  • Theoretical Chemistry

Background:

  • The complete active space self-consistent field (CASSCF) method is crucial for accurately describing electronic structures of molecules with strong electron correlation.
  • Scaling CASSCF calculations to large systems is computationally demanding due to the complexity of electron repulsion integrals.

Purpose of the Study:

  • To develop and implement an efficient CASSCF method suitable for large-scale computational chemistry applications.
  • To leverage Cholesky decomposition and density fitting for approximating electron repulsion integrals, thereby reducing computational cost.

Main Methods:

  • Implementation of the CASSCF method incorporating Cholesky decomposition for integral approximation.
  • Utilization of density fitting with auxiliary basis sets derived from Cholesky decomposition to further enhance efficiency.
  • Application of the developed method to model systems like benzene, diaquo-tetra-mu-acetato-dicopper(II), and diuraniumendofullerene.

Main Results:

  • The Cholesky and density fitting approximations enable the treatment of larger basis sets and molecular systems at the CASSCF level.
  • Controllable accuracy is maintained while significantly reducing computational resources.
  • Strict error control is inherent in the Cholesky approximation, and errors in density fitting are managed through specialized auxiliary basis sets.

Conclusions:

  • The presented CASSCF implementation with Cholesky decomposition and density fitting offers a computationally feasible approach for studying large and complex molecular systems.
  • This method provides a balance between accuracy and efficiency, expanding the scope of CASSCF applications in computational chemistry.