Parallel implementation of the equation-of-motion coupled-cluster singles and doubles method and application for radical adducts of cytosine.

The equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) method has been implemented into the massively parallel ACES III program using two alternative strategies: (1) storing the entire EOM Hamiltonian matrix prior to diagonalization and (2) recomputing the four-virtual part of the matrix from integrals in a direct mode. The second is found to be far more efficient. EOM-CC shows virtually ideal scaling from 32 to 256 processors. With basis sets as large as 552 functions, the program was applied to determine vertical excitation energies for five cytosine radical adducts of -OH and -H at three sites C5, C6, and N3. These radicals are considered to play an important role in radiation induced DNA damage. The excitation energy spectrum shows two distinct patterns for the lowest transitions distinguishing the C6-OH, C6-H, and N3-H adducts from the C5-OH and C5-H. The results indicate that the two lowest transitions of the C6-OH isomer should contribute to the experimentally observed absorption maximum at 2.88 eV, while the third and fourth transitions of C6-OH and the two lowest transitions of C5-OH contribute to the 3.65 eV absorption maximum. We also report the CCSD with noniterative triples correction [CCSD(T)] relative energies of the C5-OH and C6-OH adducts using 1000 processors.