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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry
  • Theoretical Chemistry

Background:

  • The internally contracted multireference coupled-cluster (icMRCCSD) method is crucial for accurate electronic structure calculations.
  • Previous implementations faced limitations in computational efficiency and scope.

Purpose of the Study:

  • To present a new, efficient implementation of the icMRCCSD method.
  • To extend the applicability of icMRCCSD to larger systems and more complex problems.
  • To benchmark the performance against existing methods.

Main Methods:

  • Developed a new code featuring an efficient tensor contraction kernel.
  • Implemented a strategy to avoid full four-external integral transformations.
  • Supported spin-adapted doublet and triplet coupled-cluster wavefunctions for two active electrons in two orbitals.

Main Results:

  • Linearized icMRCCSD variants show run times comparable to single-reference coupled-cluster singles and doubles (CCSD) and internally contracted multireference configuration interaction (icMRCI).
  • Non-linear terms in more complete icMRCCSD variants increase computation time by an order of magnitude.
  • The new code enables larger-scale computations with reduced memory and disk-space requirements.

Conclusions:

  • The new icMRCCSD implementation significantly enhances computational efficiency and expands the method's applicability.
  • The code facilitates accurate calculations for molecular structure optimizations, force fields, and electronic properties of various systems.
  • A new commutator approximation ensures consistency between single-reference and multireference coupled-cluster methods.