CCSD

CCSD, or Coupled-Cluster Singles and Doubles, is a post-Hartree-Fock method used in quantum chemistry to calculate the electronic structure of molecules. It is based on coupled-cluster theory and uses an exponential wavefunction ansatz that aims to capture electron correlation with high accuracy.

In CCSD, the many-electron wavefunction is written as Ψ = exp(T) Φ0, where Φ0 is a single-determinant reference state (typically from a Hartree-Fock calculation) and T = T1 + T2 comprises single and double excitation operators. This approach effectively sums certain classes of electronic excitations to infinite order, making the method size-extensive and well-suited for dynamic correlation in systems with a reasonably well-behaved single-reference state. CCSD is most reliable for molecules that do not exhibit strong multi-reference character.

Computationally, solving the CCSD equations scales roughly as the sixth power of the number of basis functions, placing it between some cheaper methods and more demanding approaches that include triple or higher excitations. Variants and extensions include CCSD(T), which adds a perturbative treatment of triple excitations and is often regarded as a benchmark in quantum chemistry for reaction energetics. Other developments include EOM-CCSD for excited states and CCSD-F12 methods to accelerate convergence with respect to the basis set.

Applications of CCSD span geometry optimizations, reaction energy profiles, and the prediction of spectroscopic properties for a wide range of molecular systems, particularly organic and inorganic compounds. Limitations arise for systems with strong static correlation or near-degenerate configurations, where single-reference CCSD can fail or require more sophisticated, multi-reference approaches. See also coupled-cluster theory and post-Hartree-Fock methods.