Active-space coupled-cluster methods through connected quadruple excitations

Coupled-cluster methods that include just a subset of all connected triple, quadruple, or both excitation amplitudes, according to the ansatz of and Adamowicz co-workers Int. Rev. Phys. Chem. 12, 339 (1993); J. Chem. Phys. 99, 1875 (1993); 100, 5792 (1994)] and Piecuch et al. J. Chem. Phys. 110, 6103 (1999)], have been implemented into parallel execution programs. They are applicable to closed- and open-shell species and they take advantage of real Abelian point-group symmetry. A symbol manipulation program has been invoked to automate the implementation. These methods have been applied to the singlet-triplet separations of five triatomic hydrides (CH2, NH2+, SiH2, PH2+, and AsH2+) with consideration of scalar relativistic effects. They have been shown to be remarkably effective with errors arising from the use of a very small subset of higher-order excitations being no more than a few tenths of 1 kcal/mol.