6.12.6 Size-Consistent Brillouin-Wigner Second-Order Approximate Coupled Cluster Singles and Doubles (BWs-CC2)

Choosing to approximate the Hamiltonian in the CCSD equations using the unperturbed Møller-Plesset Hamiltonian is a popular, but not unique choice. Instead, it is possible to use the unperturbed size-consistent Brillouin-Wigner Hamiltonian ¹⁹⁵ Carter-Fenk K., Head-Gordon M.
J. Chem. Phys.
(2023), 158, pp. 234108. Link ^{, 201} Carter-Fenk K., Shee J., Head-Gordon M.
J. Chem. Phys.
(2023), 159, pp. 171104. Link during evaluation of the Doubles equation. For a definition of this Hamiltonian, see Section 6.8.

Compared to regular (MP-)CC2, BWs-CC2 is a regular theory, i. e. for positive regularisation strength, it does not diverge for vanishing HOMO/LUMO gaps. It can therefore be applied to dissociated bonds and few-reference systems whose dominating determinants are coupled by double excitations. ³³⁷ Dittmer L. B., Tkachenko N. V., Head-Gordon M.
J. Chem. Phys.
(2025), 163, pp. 124113. Link

In order to invoke this method, set METHOD = RIBWS2 in the REM section, and EXC_METHOD = CC2 in the $bws2 section. BWs-CC2 is available for all partitions, recommended is the use of either BW-s2 or MPR-BWs2. Currently, only the electronic ground state is implemented. For general purposes it is recommended to employ MPR-BWs2 with $A_0=2$ and $B_0=0$ in BWs-CC2 calculations, though the optimal parameter set is dependent on the system and chemical quantity of interest.