Q. What is the difference between EOM-CC, CC-LRT, and SAC-CI?

A. The EOM-CC(1989,1993)[1] and the CC-LRT methods(1979,1990)[2] are theoretically equivalent to the SAC-CI method(1978). Some numerical proofs showing the equivalence of these methods are given in the "I-E. Equivalence of EOM-CC and CC-LRT to SAC-CI" of SAC-CI GUIDE. We performed the SAC-CI SD-R calculations for CH₂ and CH⁺ for which the EOM-CCSD[4] and CCSD-LRT[5] calculations were reported. We also show that the approximation of neglecting the unimportant unlinked terms and the perturbation selection of the linked operators are both accurate and useful. It should be noted that the program system MEG/EX-MEG[6] coded in 1985 can perform both SAC-CI SD-R and general-R calculations as special cases of more general MEG/EX-MEG calculations [7-10] without introducing any approximations.

A policy of the SAC/SAC-CI program is that we calculate only important terms, and we neglect terms if their effect are certainly negligibly small. By doing so, we can make the computations more efficient, so that we can calculate larger and more complex systems, and the physics and chemistry of the calculated results become clearer than otherwise. It is by this virtue that we could have been able to apply our SAC-CI method to the porphyrin systems and to the reaction center of photosynthesis of Rhodopseudomonas viridis. For this purpose, we may introduce the following approximations. (1) Some classes of unimportant unlinked terms are neglected from the beginning. (2) The perturbation selection of the linked operators and further the selection among the unlinked terms composed of the selected linked terms are performed.

[1] J.Geertsen, M. Rittby, R. J. Batlett, Chem. Phys. Lett. 164, 57 (1989); J.F. Stanton and R.J. Bartlett, J. Chem. Phys. 98, 7029 (1993).
[2] D. Mukherjee and P.K. Mukherjee, Chem. Phys. 39, 325 (1979); H. Koch and P. Jorgensen, J. Chem. Phys. 93, 3333 (1990).
[3] H. Nakatsuji, Chem. Phys. Lett., 59, 362 (1998); ibid, 67, 329, 334 (1979).
[4] S. Hirata, M. Nooijen, R.J. Bartlett Chem. Phys. Lett. 326, 255 (2000).
[5] H. Koch, H. J. Aa. Jensen, P. Jorgensen, T. Helgaker, J. Chem. Phys. 93, 3345 (1990).
[6] H. Nakatsuji, Program system for the MEG/EX-MEG methods applied molecluar ground, excited, ionized and electron attached states, 1985.
[7] H. Nakatsuji, J. Chem. Phys. 83, 713, 5743 (1985).
[8] H. Nakatsuji, Theoret. Chim. Acta, 71, 201 (1987).
[9] H. Nakatsuji, J. Chem. Phys. 94, 6716 (1991).
[10]H. Nakatsuji, J. Chem. Phys. 95, 4296 (1991).