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CCSD

The CCSD method adopts an exponential wavefunction ansatz:

\[\psi=e^{T_1+T_2}|\psi_0>\]

where \(\psi_0\) is a Hartree-Fock wavefunction (single determinant), \(T_1\) is composed of single excitation operators and amplitudes, and \(T_2\) is composed of double excitation operators and amplitudes:

\[T_1 = \sum_{ia} t_a^i \hat a^{\dagger}_a \hat a_i \]
\[T_2 = \sum_{ijab} t_{ab}^{ij} \hat a^{\dagger}_a \hat a^{\dagger}_b \hat a_j \hat a_i \]

where \(t_a^i\) and \(t_{ab}^{ij}\) are the amplitudes that need to be determined. The primary advantage of coupled cluster methods is their ability to describe electron correlation while remaining rigorously size-extensive. The primary disadvantage is that they are (usually) restricted to a single determinant reference. Therefore, they can perform poorly when static electron correlation is important (for example, when breaking covalent bonds).

TeraChem implements conventional CCSD as well as tensor-hypercontracted (THC-CCSD) and rank-reduced (RR-CCSD and RR-THC-CCSD) forms. It also implements equation-of-motion CCSD (EOM-CCSD) for excited electronic states. Only energies are currently implemented for CCSD methods (i.e., analytic gradients are not available). All coupled-cluster methods implemented in TeraChem assume a closed-shell reference, i.e. a preceding restricted Hartree-Fock calculation. Unrestricted Hartree-Fock (UHF) references are not allowed, nor are open-shell (ROHF) references supported.

To run a CCSD calculation in TeraChem, one needs to include 

ccbox yes
ccbox_ccsd yes

in the input file.

Summary of relevant keywords

Keyword Type Default Description
ccbox_ccsd_maxiter integer 100 Maximum number of iterations in the CCSD equations
ccbox_ccsd_diisvecs integer 5 Maximum number of DIIS vectors when solving the CCSD equations
ccbox_ccsd_r_convthre float 1.0e-6 Convergence threshold for amplitudes
ccbox_ccsd_e_convthre float 1.0e-6 Convergence threshold for CCSD energy
ccbox_ccsdt boolean no Compute perturbative triples correction, i.e. CCSD(T)?

References

  1. B. S. Fales, E. R. Curtis, K. G. Johnson, D. Lahana, S. Seritan, Y. Wang, H. Weir, T. J. Martinez and E. G. Hohenstein, Performance of Coupled-Cluster Singles and Doubles on Modern Stream Processing Architectures, J. Chem. Theory Comput. 16 4021 (2020). ↩